[intro] - C++14 → C++17

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-# General <a id="intro">[[intro]]</a>
-## Scope <a id="intro.scope">[[intro.scope]]</a>
-This International Standard specifies requirements for implementations
-of the C++programming language. The first such requirement is that they
-implement the language, and so this International Standard also defines
-C++. Other requirements and relaxations of the first requirement appear
-at various places within this International Standard.
-C++is a general purpose programming language based on the C programming
-language as described in ISO/IEC 9899:1999 *Programming languages — C*
-(hereinafter referred to as the *C standard*). In addition to the
-facilities provided by C, C++provides additional data types, classes,
-templates, exceptions, namespaces, operator overloading, function name
-overloading, references, free store management operators, and additional
-library facilities.
-## Normative references <a id="intro.refs">[[intro.refs]]</a>
-The following referenced documents are indispensable for the application
-of this document. For dated references, only the edition cited applies.
-For undated references, the latest edition of the referenced document
-(including any amendments) applies.
-- Ecma International, *ECMAScript Language Specification*, Standard
-  Ecma-262, third edition, 1999.
-- ISO/IEC 2382 (all parts), *Information technology — Vocabulary*
-- ISO/IEC 9899:1999, *Programming languages — C*
-- ISO/IEC 9899:1999/Cor.1:2001(E), *Programming languages — C, Technical
-  Corrigendum 1*
-- ISO/IEC 9899:1999/Cor.2:2004(E), *Programming languages — C, Technical
-  Corrigendum 2*
-- ISO/IEC 9899:1999/Cor.3:2007(E), *Programming languages — C, Technical
-  Corrigendum 3*
-- ISO/IEC 9945:2003, *Information Technology — Portable Operating System
-  Interface (POSIX)*
-- ISO/IEC 10646-1:1993, *Information technology — Universal
-  Multiple-Octet Coded Character Set (UCS) — Part 1: Architecture and
-  Basic Multilingual Plane*
-- ISO/IEC TR 19769:2004, *Information technology — Programming
-  languages, their environments and system software interfaces —
-  Extensions for the programming language C to support new character
-  data types*
-The library described in Clause 7 of ISO/IEC 9899:1999 and Clause 7 of
-ISO/IEC 9899:1999/Cor.1:2001 and Clause 7 of ISO/IEC
-9899:1999/Cor.2:2003 is hereinafter called the *C standard library*.[^1]
-The library described in ISO/IEC TR 19769:2004 is hereinafter called the
-*C Unicode TR*.
-The operating system interface described in ISO/IEC 9945:2003 is
-hereinafter called *POSIX*.
-The ECMAScript Language Specification described in Standard Ecma-262 is
-hereinafter called *ECMA-262*.
-## Terms and definitions <a id="intro.defs">[[intro.defs]]</a>
-For the purposes of this document, the following definitions apply.
-[[definitions]] defines additional terms that are used only in Clauses
-[[library]] through  [[thread]] and Annex  [[depr]].
-Terms that are used only in a small portion of this International
-Standard are defined where they are used and italicized where they are
-defined.
-#### 1 argument <a id="defns.argument">[[defns.argument]]</a>
-\<function call expression\> expression in the comma-separated list
-bounded by the parentheses
-#### 2 argument <a id="defns.argument.macro">[[defns.argument.macro]]</a>
-\<function-like macro\> sequence of preprocessing tokens in the
-comma-separated list bounded by the parentheses
-#### 3 argument <a id="defns.argument.throw">[[defns.argument.throw]]</a>
-\<throw expression\> the operand of `throw`
-#### 4 argument <a id="defns.argument.templ">[[defns.argument.templ]]</a>
-\<template instantiation\> expression, *type-id* or *template-name* in
-the comma-separated list bounded by the angle brackets
-#### 5 conditionally-supported <a id="defns.cond.supp">[[defns.cond.supp]]</a>
-program construct that an implementation is not required to support
-Each implementation documents all conditionally-supported constructs
-that it does not support.
-#### 6 diagnostic message <a id="defns.diagnostic">[[defns.diagnostic]]</a>
-message belonging to an *implementation-defined* subset of the
-implementation’s output messages
-#### 7 dynamic type <a id="defns.dynamic.type">[[defns.dynamic.type]]</a>
-\<glvalue\> type of the most derived object ([[intro.object]]) to which
-the glvalue denoted by a glvalue expression refers
-if a pointer ([[dcl.ptr]]) `p` whose static type is “pointer to class
-`B`” is pointing to an object of class `D`, derived from `B` (Clause
-[[class.derived]]), the dynamic type of the expression `*p` is “`D`.”
-References ([[dcl.ref]]) are treated similarly.
-#### 8 dynamic type <a id="defns.dynamic.type.prvalue">[[defns.dynamic.type.prvalue]]</a>
-\<prvalue\> static type of the prvalue expression
-#### 9 ill-formed program <a id="defns.ill.formed">[[defns.ill.formed]]</a>
-program that is not well formed
-#### 10 implementation-defined behavior <a id="defns.impl.defined">[[defns.impl.defined]]</a>
-behavior, for a well-formed program construct and correct data, that
-depends on the implementation and that each implementation documents
-#### 11 implementation limits <a id="defns.impl.limits">[[defns.impl.limits]]</a>
-restrictions imposed upon programs by the implementation
-#### 12 locale-specific behavior <a id="defns.locale.specific">[[defns.locale.specific]]</a>
-behavior that depends on local conventions of nationality, culture, and
-language that each implementation documents
-#### 13 multibyte character <a id="defns.multibyte">[[defns.multibyte]]</a>
-sequence of one or more bytes representing a member of the extended
-character set of either the source or the execution environment
-The extended character set is a superset of the basic character set (
-[[lex.charset]]).
-#### 14 parameter <a id="defns.parameter">[[defns.parameter]]</a>
-\<function or catch clause\> object or reference declared as part of a
-function declaration or definition or in the catch clause of an
-exception handler that acquires a value on entry to the function or
-handler
-#### 15 parameter <a id="defns.parameter.macro">[[defns.parameter.macro]]</a>
-\<function-like macro\> identifier from the comma-separated list bounded
-by the parentheses immediately following the macro name
-#### 16 parameter <a id="defns.parameter.templ">[[defns.parameter.templ]]</a>
-\<template\> *template-parameter*
-#### 17 signature <a id="defns.signature">[[defns.signature]]</a>
-\<function\> name, parameter type list ([[dcl.fct]]), and enclosing
-namespace (if any)
-Signatures are used as a basis for name mangling and linking.
-#### 18 signature <a id="defns.signature.templ">[[defns.signature.templ]]</a>
-\<function template\> name, parameter type list ([[dcl.fct]]),
-enclosing namespace (if any), return type, and template parameter list
-#### 19 signature <a id="defns.signature.spec">[[defns.signature.spec]]</a>
-\<function template specialization\> signature of the template of which
-it is a specialization and its template arguments (whether explicitly
-specified or deduced)
-#### 20 signature <a id="defns.signature.member">[[defns.signature.member]]</a>
-\<class member function\> name, parameter type list ([[dcl.fct]]),
-class of which the function is a member, cv-qualifiers (if any), and
-*ref-qualifier* (if any)
-#### 21 signature <a id="defns.signature.member.templ">[[defns.signature.member.templ]]</a>
-\<class member function template\> name, parameter type list (
-[[dcl.fct]]), class of which the function is a member, cv-qualifiers (if
-any), *ref-qualifier* (if any), return type, and template parameter list
-#### 22 signature <a id="defns.signature.member.spec">[[defns.signature.member.spec]]</a>
-\<class member function template specialization\> signature of the
-member function template of which it is a specialization and its
-template arguments (whether explicitly specified or deduced)
-#### 23 static type <a id="defns.static.type">[[defns.static.type]]</a>
-type of an expression ([[basic.types]]) resulting from analysis of the
-program without considering execution semantics
-The static type of an expression depends only on the form of the program
-in which the expression appears, and does not change while the program
-is executing.
-#### 24 undefined behavior <a id="defns.undefined">[[defns.undefined]]</a>
-behavior for which this International Standard imposes no requirements
-Undefined behavior may be expected when this International Standard
-omits any explicit definition of behavior or when a program uses an
-erroneous construct or erroneous data. Permissible undefined behavior
-ranges from ignoring the situation completely with unpredictable
-results, to behaving during translation or program execution in a
-documented manner characteristic of the environment (with or without the
-issuance of a diagnostic message), to terminating a translation or
-execution (with the issuance of a diagnostic message). Many erroneous
-program constructs do not engender undefined behavior; they are required
-to be diagnosed.
-#### 25 unspecified behavior <a id="defns.unspecified">[[defns.unspecified]]</a>
-behavior, for a well-formed program construct and correct data, that
-depends on the implementation
-The implementation is not required to document which behavior occurs.
-The range of possible behaviors is usually delineated by this
-International Standard.
-#### 26 well-formed program <a id="defns.well.formed">[[defns.well.formed]]</a>
-C++program constructed according to the syntax rules, diagnosable
-semantic rules, and the One Definition Rule ([[basic.def.odr]]).
 ## Implementation compliance <a id="intro.compliance">[[intro.compliance]]</a>
 The set of *diagnosable rules* consists of all syntactic and semantic
 rules in this International Standard except for those rules containing
 an explicit notation that “no diagnostic is required” or which are
-described as resulting in “undefined behavior.”
 Although this International Standard states only requirements on C++
 implementations, those requirements are often easier to understand if
 they are phrased as requirements on programs, parts of programs, or
 execution of programs. Such requirements have the following meaning:
 - If a program contains no violations of the rules in this International
   Standard, a conforming implementation shall, within its resource
   limits, accept and correctly execute[^2] that program.
 - If a program contains a violation of any diagnosable rule or an
-  occurrence of a construct described in this Standard as
   “conditionally-supported” when the implementation does not support
   that construct, a conforming implementation shall issue at least one
   diagnostic message.
 - If a program contains a violation of a rule for which no diagnostic is
   required, this International Standard places no requirement on
   implementations with respect to that program.
 For classes and class templates, the library Clauses specify partial
 definitions. Private members (Clause  [[class.access]]) are not
 specified, but each implementation shall supply them to complete the
 definitions according to the description in the library Clauses.
@@ -281,45 +64,44 @@ execute such programs.
 Each implementation shall include documentation that identifies all
 conditionally-supported constructs that it does not support and defines
 all locale-specific characteristics.[^3]
-## Structure of this International Standard <a id="intro.structure">[[intro.structure]]</a>
 Clauses  [[lex]] through  [[cpp]] describe the C++programming language.
 That description includes detailed syntactic specifications in a form
 described in  [[syntax]]. For convenience, Annex  [[gram]] repeats all
 such syntactic specifications.
 Clauses  [[language.support]] through  [[thread]] and Annex  [[depr]]
-(the *library clauses*) describe the Standard C++library. That
-description includes detailed descriptions of the templates, classes,
-functions, constants, and macros that constitute the library, in a form
-described in Clause  [[library]].
 Annex  [[implimits]] recommends lower bounds on the capacity of
 conforming implementations.
 Annex  [[diff]] summarizes the evolution of C++since its first published
 description, and explains in detail the differences between C++and C.
 Certain features of C++exist solely for compatibility purposes; Annex
 [[depr]] describes those features.
-Throughout this International Standard, each example is introduced by “”
-and terminated by “”. Each note is introduced by “” and terminated by
-“”. Examples and notes may be nested.
 ## Syntax notation <a id="syntax">[[syntax]]</a>
-In the syntax notation used in this International Standard, syntactic
-categories are indicated by *italic* type, and literal words and
-characters in `constant` `width` type. Alternatives are listed on
-separate lines except in a few cases where a long set of alternatives is
-marked by the phrase “one of.” If the text of an alternative is too long
-to fit on a line, the text is continued on subsequent lines indented
-from the first one. An optional terminal or non-terminal symbol is
-indicated by the subscript “”, so
 ``` bnf
 '{' expressionₒₚₜ '}'
 ```
@@ -333,42 +115,49 @@ the following rules:
 - *X-id* is an identifier with no context-dependent meaning (e.g.,
   *qualified-id*).
 - *X-seq* is one or more *X*’s without intervening delimiters (e.g.,
   *declaration-seq* is a sequence of declarations).
 - *X-list* is one or more *X*’s separated by intervening commas (e.g.,
-  *expression-list* is a sequence of expressions separated by commas).
 ## The C++memory model <a id="intro.memory">[[intro.memory]]</a>
 The fundamental storage unit in the C++memory model is the *byte*. A
 byte is at least large enough to contain any member of the basic
 execution character set ([[lex.charset]]) and the eight-bit code units
 of the Unicode UTF-8 encoding form and is composed of a contiguous
-sequence of bits, the number of which is *implementation-defined*. The
-least significant bit is called the *low-order bit*; the most
 significant bit is called the *high-order bit*. The memory available to
 a C++program consists of one or more sequences of contiguous bytes.
 Every byte has a unique address.
-The representation of types is described in  [[basic.types]].
 A *memory location* is either an object of scalar type or a maximal
-sequence of adjacent bit-fields all having non-zero width. Various
-features of the language, such as references and virtual functions,
-might involve additional memory locations that are not accessible to
-programs but are managed by the implementation. Two or more threads of
-execution ([[intro.multithread]]) can update and access separate memory
-locations without interfering with each other.
-Thus a bit-field and an adjacent non-bit-field are in separate memory
-locations, and therefore can be concurrently updated by two threads of
-execution without interference. The same applies to two bit-fields, if
-one is declared inside a nested struct declaration and the other is not,
-or if the two are separated by a zero-length bit-field declaration, or
-if they are separated by a non-bit-field declaration. It is not safe to
-concurrently update two bit-fields in the same struct if all fields
-between them are also bit-fields of non-zero width.
 A structure declared as
 ``` cpp
 struct {
@@ -386,84 +175,172 @@ contains four separate memory locations: The field `a` and bit-fields
 concurrently without interfering with each other. The bit-fields `b` and
 `c` together constitute the fourth memory location. The bit-fields `b`
 and `c` cannot be concurrently modified, but `b` and `a`, for example,
 can be.
 ## The C++object model <a id="intro.object">[[intro.object]]</a>
 The constructs in a C++program create, destroy, refer to, access, and
-manipulate objects. An *object* is a region of storage. A function is
-not an object, regardless of whether or not it occupies storage in the
-way that objects do. An object is created by a *definition* (
-[[basic.def]]), by a *new-expression* ([[expr.new]]) or by the
-implementation ([[class.temporary]]) when needed. The properties of an
-object are determined when the object is created. An object can have a
-*name* (Clause  [[basic]]). An object has a *storage duration* (
-[[basic.stc]]) which influences its *lifetime* ([[basic.life]]). An
-object has a *type* ([[basic.types]]). The term *object type* refers to
-the type with which the object is created. Some objects are
-*polymorphic* ([[class.virtual]]); the implementation generates
 information associated with each such object that makes it possible to
 determine that object’s type during program execution. For other
 objects, the interpretation of the values found therein is determined by
 the type of the *expression*s (Clause  [[expr]]) used to access them.
 Objects can contain other objects, called *subobjects*. A subobject can
 be a *member subobject* ([[class.mem]]), a *base class subobject*
 (Clause  [[class.derived]]), or an array element. An object that is not
-a subobject of any other object is called a *complete object*.
 For every object `x`, there is some object called the *complete object
 of* `x`, determined as follows:
-- If `x` is a complete object, then `x` is the complete object of `x`.
 - Otherwise, the complete object of `x` is the complete object of the
   (unique) object that contains `x`.
 If a complete object, a data member ([[class.mem]]), or an array
 element is of class type, its type is considered the *most derived
 class*, to distinguish it from the class type of any base class
 subobject; an object of a most derived class type or of a non-class type
 is called a *most derived object*.
 Unless it is a bit-field ([[class.bit]]), a most derived object shall
-have a non-zero size and shall occupy one or more bytes of storage. Base
 class subobjects may have zero size. An object of trivially copyable or
 standard-layout type ([[basic.types]]) shall occupy contiguous bytes of
 storage.
 Unless an object is a bit-field or a base class subobject of zero size,
 the address of that object is the address of the first byte it occupies.
-Two objects that are not bit-fields may have the same address if one is
-a subobject of the other, or if at least one is a base class subobject
-of zero size and they are of different types; otherwise, they shall have
-distinct addresses.[^4]
 ``` cpp
 static const char test1 = 'x';
 static const char test2 = 'x';
 const bool b = &test1 != &test2;      // always true
 ```
-C++provides a variety of fundamental types and several ways of composing
-new types from existing types ([[basic.types]]).
 ## Program execution <a id="intro.execution">[[intro.execution]]</a>
 The semantic descriptions in this International Standard define a
 parameterized nondeterministic abstract machine. This International
 Standard places no requirement on the structure of conforming
 implementations. In particular, they need not copy or emulate the
 structure of the abstract machine. Rather, conforming implementations
 are required to emulate (only) the observable behavior of the abstract
-machine as explained below.[^5]
 Certain aspects and operations of the abstract machine are described in
 this International Standard as implementation-defined (for example,
 `sizeof(int)`). These constitute the parameters of the abstract machine.
 Each implementation shall include documentation describing its
-characteristics and behavior in these respects.[^6] Such documentation
 shall define the instance of the abstract machine that corresponds to
 that implementation (referred to as the “corresponding instance” below).
 Certain other aspects and operations of the abstract machine are
 described in this International Standard as unspecified (for example,
@@ -474,56 +351,56 @@ define the nondeterministic aspects of the abstract machine. An instance
 of the abstract machine can thus have more than one possible execution
 for a given program and a given input.
 Certain other operations are described in this International Standard as
 undefined (for example, the effect of attempting to modify a `const`
-object). This International Standard imposes no requirements on the
-behavior of programs that contain undefined behavior.
 A conforming implementation executing a well-formed program shall
 produce the same observable behavior as one of the possible executions
 of the corresponding instance of the abstract machine with the same
 program and the same input. However, if any such execution contains an
 undefined operation, this International Standard places no requirement
 on the implementation executing that program with that input (not even
 with regard to operations preceding the first undefined operation).
-If a signal handler is executed as a result of a call to the `raise`
-function, then the execution of the handler is sequenced after the
-invocation of the `raise` function and before its return. When a signal
-is received for another reason, the execution of the signal handler is
-usually unsequenced with respect to the rest of the program.
 An instance of each object with automatic storage duration (
 [[basic.stc.auto]]) is associated with each entry into its block. Such
 an object exists and retains its last-stored value during the execution
 of the block and while the block is suspended (by a call of a function
 or receipt of a signal).
 The least requirements on a conforming implementation are:
-- Access to volatile objects are evaluated strictly according to the
-  rules of the abstract machine.
 - At program termination, all data written into files shall be identical
   to one of the possible results that execution of the program according
   to the abstract semantics would have produced.
 - The input and output dynamics of interactive devices shall take place
   in such a fashion that prompting output is actually delivered before a
   program waits for input. What constitutes an interactive device is
   *implementation-defined*.
 These collectively are referred to as the *observable behavior* of the
-program. More stringent correspondences between abstract and actual
-semantics may be defined by each implementation.
 Operators can be regrouped according to the usual mathematical rules
-only where the operators really are associative or commutative.[^7] For
 example, in the following fragment
 ``` cpp
 int a, b;
-/*...*/
 a = a + 32760 + b + 5;
 ```
 the expression statement behaves exactly the same as
@@ -560,184 +437,269 @@ since the values for `a` and `b` might have been, respectively, 4 and -8
 or -17 and 12. However on a machine in which overflows do not produce an
 exception and in which the results of overflows are reversible, the
 above expression statement can be rewritten by the implementation in any
 of the above ways because the same result will occur.
-A *full-expression* is an expression that is not a subexpression of
-another expression. in some contexts, such as unevaluated operands, a
-syntactic subexpression is considered a full-expression (Clause
-[[expr]]). If a language construct is defined to produce an implicit
-call of a function, a use of the language construct is considered to be
-an expression for the purposes of this definition. A call to a
-destructor generated at the end of the lifetime of an object other than
-a temporary object is an implicit full-expression. Conversions applied
-to the result of an expression in order to satisfy the requirements of
-the language construct in which the expression appears are also
-considered to be part of the full-expression.
 ``` cpp
 struct S {
-  S(int i): I(i) { }
   int& v() { return I; }
 private:
   int I;
 };
 S s1(1);                   // full-expression is call of S::S(int)
- S s2 = 2;          // full-expression is call of S::S(int)
 void f() {
   if (S(3).v())            // full-expression includes lvalue-to-rvalue and
                            // int to bool conversions, performed before
                            // temporary is deleted at end of full-expression
   { }
 }
 ```
-The evaluation of a full-expression can include the evaluation of
-subexpressions that are not lexically part of the full-expression. For
-example, subexpressions involved in evaluating default arguments (
-[[dcl.fct.default]]) are considered to be created in the expression that
-calls the function, not the expression that defines the default
-argument.
-Accessing an object designated by a `volatile` glvalue (
-[[basic.lval]]), modifying an object, calling a library I/O function, or
-calling a function that does any of those operations are all *side
-effects*, which are changes in the state of the execution environment.
-*Evaluation* of an expression (or a sub-expression) in general includes
-both value computations (including determining the identity of an object
-for glvalue evaluation and fetching a value previously assigned to an
-object for prvalue evaluation) and initiation of side effects. When a
-call to a library I/O function returns or an access to a `volatile`
-object is evaluated the side effect is considered complete, even though
-some external actions implied by the call (such as the I/O itself) or by
-the `volatile` access may not have completed yet.
 *Sequenced before* is an asymmetric, transitive, pair-wise relation
 between evaluations executed by a single thread (
 [[intro.multithread]]), which induces a partial order among those
 evaluations. Given any two evaluations *A* and *B*, if *A* is sequenced
-before *B*, then the execution of *A* shall precede the execution of
-*B*. If *A* is not sequenced before *B* and *B* is not sequenced before
-*A*, then *A* and *B* are *unsequenced*. The execution of unsequenced
-evaluations can overlap. Evaluations *A* and *B* are *indeterminately
-sequenced* when either *A* is sequenced before *B* or *B* is sequenced
-before *A*, but it is unspecified which. Indeterminately sequenced
-evaluations cannot overlap, but either could be executed first.
 Every value computation and side effect associated with a
 full-expression is sequenced before every value computation and side
-effect associated with the next full-expression to be evaluated.[^8]
 Except where noted, evaluations of operands of individual operators and
-of subexpressions of individual expressions are unsequenced. In an
-expression that is evaluated more than once during the execution of a
-program, unsequenced and indeterminately sequenced evaluations of its
-subexpressions need not be performed consistently in different
-evaluations. The value computations of the operands of an operator are
-sequenced before the value computation of the result of the operator. If
-a side effect on a scalar object is unsequenced relative to either
-another side effect on the same scalar object or a value computation
-using the value of the same scalar object, and they are not potentially
-concurrent ([[intro.multithread]]), the behavior is undefined. The next
-section imposes similar, but more complex restrictions on potentially
-concurrent computations.
 ``` cpp
-void f(int, int);
-void g(int i, int* v) {
-  i = v[i++];         // the behavior is undefined
   i = 7, i++, i++;    // i becomes 9
-  i = i++ + 1;        // the behavior is undefined
   i = i + 1;          // the value of i is incremented
-  f(i = -1, i = -1);  // the behavior is undefined
 }
 ```
 When calling a function (whether or not the function is inline), every
 value computation and side effect associated with any argument
 expression, or with the postfix expression designating the called
 function, is sequenced before execution of every expression or statement
-in the body of the called function. Value computations and side effects
-associated with different argument expressions are unsequenced. Every
-evaluation in the calling function (including other function calls) that
-is not otherwise specifically sequenced before or after the execution of
-the body of the called function is indeterminately sequenced with
-respect to the execution of the called function.[^9] Several contexts in
-C++cause evaluation of a function call, even though no corresponding
-function call syntax appears in the translation unit. Evaluation of a
-`new` expression invokes one or more allocation and constructor
-functions; see  [[expr.new]]. For another example, invocation of a
-conversion function ([[class.conv.fct]]) can arise in contexts in which
-no function call syntax appears. The sequencing constraints on the
-execution of the called function (as described above) are features of
-the function calls as evaluated, whatever the syntax of the expression
-that calls the function might be.
 ## Multi-threaded executions and data races <a id="intro.multithread">[[intro.multithread]]</a>
 A *thread of execution* (also known as a *thread*) is a single flow of
 control within a program, including the initial invocation of a specific
 top-level function, and recursively including every function invocation
-subsequently executed by the thread. When one thread creates another,
-the initial call to the top-level function of the new thread is executed
-by the new thread, not by the creating thread. Every thread in a program
-can potentially access every object and function in a program.[^10]
-Under a hosted implementation, a C++program can have more than one
-thread running concurrently. The execution of each thread proceeds as
-defined by the remainder of this standard. The execution of the entire
-program consists of an execution of all of its threads. Usually the
-execution can be viewed as an interleaving of all its threads. However,
-some kinds of atomic operations, for example, allow executions
-inconsistent with a simple interleaving, as described below. Under a
-freestanding implementation, it is *implementation-defined* whether a
-program can have more than one thread of execution.
-A signal handler that is executed as a result of a call to the `raise`
-function belongs to the same thread of execution as the call to the
-`raise` function. Otherwise it is unspecified which thread of execution
-contains a signal handler invocation.
-Implementations should ensure that all unblocked threads eventually make
-progress. Standard library functions may silently block on I/O or locks.
-Factors in the execution environment, including externally-imposed
-thread priorities, may prevent an implementation from making certain
-guarantees of forward progress.
-Executions of atomic functions that are either defined to be lock-free (
-[[atomics.flag]]) or indicated as lock-free ([[atomics.lockfree]]) are
-*lock-free executions*.
-- If there is only one unblocked thread, a lock-free execution in that
-  thread shall complete. Concurrently executing threads may prevent
-  progress of a lock-free execution. For example, this situation can
-  occur with load-locked store-conditional implementations. This
-  property is sometimes termed obstruction-free.
-- When one or more lock-free executions run concurrently, at least one
-  should complete. It is difficult for some implementations to provide
-  absolute guarantees to this effect, since repeated and particularly
-  inopportune interference from other threads may prevent forward
-  progress, e.g., by repeatedly stealing a cache line for unrelated
-  purposes between load-locked and store-conditional instructions.
-  Implementations should ensure that such effects cannot indefinitely
-  delay progress under expected operating conditions, and that such
-  anomalies can therefore safely be ignored by programmers. Outside this
-  International Standard, this property is sometimes termed lock-free.
 The value of an object visible to a thread *T* at a particular point is
 the initial value of the object, a value assigned to the object by *T*,
 or a value assigned to the object by another thread, according to the
-rules below. In some cases, there may instead be undefined behavior.
-Much of this section is motivated by the desire to support atomic
-operations with explicit and detailed visibility constraints. However,
-it also implicitly supports a simpler view for more restricted programs.
 Two expression evaluations *conflict* if one of them modifies a memory
-location ([[intro.memory]]) and the other one accesses or modifies the
 same memory location.
 The library defines a number of atomic operations (Clause  [[atomics]])
 and operations on mutexes (Clause  [[thread]]) that are specially
 identified as synchronization operations. These operations play a
@@ -747,30 +709,30 @@ consume operation, an acquire operation, a release operation, or both an
 acquire and release operation. A synchronization operation without an
 associated memory location is a fence and can be either an acquire
 fence, a release fence, or both an acquire and release fence. In
 addition, there are relaxed atomic operations, which are not
 synchronization operations, and atomic read-modify-write operations,
-which have special characteristics. For example, a call that acquires a
-mutex will perform an acquire operation on the locations comprising the
-mutex. Correspondingly, a call that releases the same mutex will perform
-a release operation on those same locations. Informally, performing a
 release operation on *A* forces prior side effects on other memory
 locations to become visible to other threads that later perform a
 consume or an acquire operation on *A*. “Relaxed” atomic operations are
 not synchronization operations even though, like synchronization
-operations, they cannot contribute to data races.
 All modifications to a particular atomic object *M* occur in some
-particular total order, called the *modification order* of *M*. If *A*
-and *B* are modifications of an atomic object *M* and *A* happens before
-(as defined below) *B*, then *A* shall precede *B* in the modification
-order of *M*, which is defined below. This states that the modification
-orders must respect the “happens before” relationship. There is a
-separate order for each atomic object. There is no requirement that
-these can be combined into a single total order for all objects. In
-general this will be impossible since different threads may observe
-modifications to different objects in inconsistent orders.
 A *release sequence* headed by a release operation *A* on an atomic
 object *M* is a maximal contiguous sub-sequence of side effects in the
 modification order of *M*, where the first operation is `A`, and every
 subsequent operation
@@ -779,18 +741,21 @@ subsequent operation
 - is an atomic read-modify-write operation.
 Certain library calls *synchronize with* other library calls performed
 by another thread. For example, an atomic store-release synchronizes
 with a load-acquire that takes its value from the store (
-[[atomics.order]]). Except in the specified cases, reading a later value
-does not necessarily ensure visibility as described below. Such a
-requirement would sometimes interfere with efficient implementation. The
-specifications of the synchronization operations define when one reads
-the value written by another. For atomic objects, the definition is
-clear. All operations on a given mutex occur in a single total order.
-Each mutex acquisition “reads the value written” by the last mutex
-release.
 An evaluation *A* *carries a dependency* to an evaluation *B* if
 - the value of *A* is used as an operand of *B*, unless:
   - *B* is an invocation of any specialization of
@@ -807,25 +772,25 @@ An evaluation *A* *carries a dependency* to an evaluation *B* if
 - *A* writes a scalar object or bit-field *M*, *B* reads the value
   written by *A* from *M*, and *A* is sequenced before *B*, or
 - for some evaluation *X*, *A* carries a dependency to *X*, and *X*
   carries a dependency to *B*.
-“Carries a dependency to” is a subset of “is sequenced before”, and is
-similarly strictly intra-thread.
 An evaluation *A* is *dependency-ordered before* an evaluation *B* if
 - *A* performs a release operation on an atomic object *M*, and, in
   another thread, *B* performs a consume operation on *M* and reads a
   value written by any side effect in the release sequence headed by
   *A*, or
 - for some evaluation *X*, *A* is dependency-ordered before *X* and *X*
   carries a dependency to *B*.
-The relation “is dependency-ordered before” is analogous to
 “synchronizes with”, but uses release/consume in place of
-release/acquire.
 An evaluation *A* *inter-thread happens before* an evaluation *B* if
 - *A* synchronizes with *B*, or
 - *A* is dependency-ordered before *B*, or
@@ -834,12 +799,12 @@ An evaluation *A* *inter-thread happens before* an evaluation *B* if
   - *A* is sequenced before *X* and *X* inter-thread happens before *B*,
     or
   - *A* inter-thread happens before *X* and *X* inter-thread happens
     before *B*.
-The “inter-thread happens before” relation describes arbitrary
-concatenations of “sequenced before”, “synchronizes with” and
 “dependency-ordered before” relationships, with two exceptions. The
 first exception is that a concatenation is not permitted to end with
 “dependency-ordered before” followed by “sequenced before”. The reason
 for this limitation is that a consume operation participating in a
 “dependency-ordered before” relationship provides ordering only with
@@ -849,164 +814,340 @@ such a concatenation is that any subsequent release operation will
 provide the required ordering for a prior consume operation. The second
 exception is that a concatenation is not permitted to consist entirely
 of “sequenced before”. The reasons for this limitation are (1) to permit
 “inter-thread happens before” to be transitively closed and (2) the
 “happens before” relation, defined below, provides for relationships
-consisting entirely of “sequenced before”.
-An evaluation *A* *happens before* an evaluation *B* if:
 - *A* is sequenced before *B*, or
 - *A* inter-thread happens before *B*.
 The implementation shall ensure that no program execution demonstrates a
-cycle in the “happens before” relation. This cycle would otherwise be
-possible only through the use of consume operations.
 A *visible side effect* *A* on a scalar object or bit-field *M* with
 respect to a value computation *B* of *M* satisfies the conditions:
 - *A* happens before *B* and
 - there is no other side effect *X* to *M* such that *A* happens before
   *X* and *X* happens before *B*.
 The value of a non-atomic scalar object or bit-field *M*, as determined
 by evaluation *B*, shall be the value stored by the visible side effect
-*A*. If there is ambiguity about which side effect to a non-atomic
-object or bit-field is visible, then the behavior is either unspecified
-or undefined. This states that operations on ordinary objects are not
 visibly reordered. This is not actually detectable without data races,
 but it is necessary to ensure that data races, as defined below, and
 with suitable restrictions on the use of atomics, correspond to data
-races in a simple interleaved (sequentially consistent) execution.
 The value of an atomic object *M*, as determined by evaluation *B*,
 shall be the value stored by some side effect *A* that modifies *M*,
-where *B* does not happen before *A*. The set of such side effects is
-also restricted by the rest of the rules described here, and in
-particular, by the coherence requirements below.
 If an operation *A* that modifies an atomic object *M* happens before an
 operation *B* that modifies *M*, then *A* shall be earlier than *B* in
-the modification order of *M*. This requirement is known as write-write
-coherence.
 If a value computation *A* of an atomic object *M* happens before a
 value computation *B* of *M*, and *A* takes its value from a side effect
 *X* on *M*, then the value computed by *B* shall either be the value
 stored by *X* or the value stored by a side effect *Y* on *M*, where *Y*
-follows *X* in the modification order of *M*. This requirement is known
-as read-read coherence.
 If a value computation *A* of an atomic object *M* happens before an
 operation *B* that modifies *M*, then *A* shall take its value from a
 side effect *X* on *M*, where *X* precedes *B* in the modification order
-of *M*. This requirement is known as read-write coherence.
 If a side effect *X* on an atomic object *M* happens before a value
 computation *B* of *M*, then the evaluation *B* shall take its value
 from *X* or from a side effect *Y* that follows *X* in the modification
-order of *M*. This requirement is known as write-read coherence.
-The four preceding coherence requirements effectively disallow compiler
-reordering of atomic operations to a single object, even if both
-operations are relaxed loads. This effectively makes the cache coherence
-guarantee provided by most hardware available to C++atomic operations.
-The value observed by a load of an atomic depends on the “happens
-before” relation, which depends on the values observed by loads of
-atomics. The intended reading is that there must exist an association of
-atomic loads with modifications they observe that, together with
 suitably chosen modification orders and the “happens before” relation
 derived as described above, satisfy the resulting constraints as imposed
-here.
 Two actions are *potentially concurrent* if
 - they are performed by different threads, or
-- they are unsequenced, and at least one is performed by a signal
-  handler.
 The execution of a program contains a *data race* if it contains two
 potentially concurrent conflicting actions, at least one of which is not
 atomic, and neither happens before the other, except for the special
 case for signal handlers described below. Any such data race results in
-undefined behavior. It can be shown that programs that correctly use
-mutexes and `memory_order_seq_cst` operations to prevent all data races
-and use no other synchronization operations behave as if the operations
-executed by their constituent threads were simply interleaved, with each
-value computation of an object being taken from the last side effect on
-that object in that interleaving. This is normally referred to as
-“sequential consistency”. However, this applies only to data-race-free
-programs, and data-race-free programs cannot observe most program
-transformations that do not change single-threaded program semantics. In
-fact, most single-threaded program transformations continue to be
-allowed, since any program that behaves differently as a result must
-perform an undefined operation.
-Two accesses to the same object of type `volatile sig_atomic_t` do not
-result in a data race if both occur in the same thread, even if one or
-more occurs in a signal handler. For each signal handler invocation,
 evaluations performed by the thread invoking a signal handler can be
 divided into two groups *A* and *B*, such that no evaluations in *B*
 happen before evaluations in *A*, and the evaluations of such
-`volatile sig_atomic_t` objects take values as though all evaluations in
-*A* happened before the execution of the signal handler and the
-execution of the signal handler happened before all evaluations in *B*.
-Compiler transformations that introduce assignments to a potentially
-shared memory location that would not be modified by the abstract
-machine are generally precluded by this standard, since such an
-assignment might overwrite another assignment by a different thread in
-cases in which an abstract machine execution would not have encountered
-a data race. This includes implementations of data member assignment
-that overwrite adjacent members in separate memory locations. Reordering
-of atomic loads in cases in which the atomics in question may alias is
-also generally precluded, since this may violate the coherence rules.
-Transformations that introduce a speculative read of a potentially
-shared memory location may not preserve the semantics of the C++program
-as defined in this standard, since they potentially introduce a data
-race. However, they are typically valid in the context of an optimizing
-compiler that targets a specific machine with well-defined semantics for
-data races. They would be invalid for a hypothetical machine that is not
-tolerant of races or provides hardware race detection.
 The implementation may assume that any thread will eventually do one of
 the following:
 - terminate,
 - make a call to a library I/O function,
-- access or modify a volatile object, or
 - perform a synchronization operation or an atomic operation.
-This is intended to allow compiler transformations such as removal of
-empty loops, even when termination cannot be proven.
 An implementation should ensure that the last value (in modification
 order) assigned by an atomic or synchronization operation will become
 visible to all other threads in a finite period of time.
 ## Acknowledgments <a id="intro.ack">[[intro.ack]]</a>
-The C++programming language as described in this International Standard
-is based on the language as described in Chapter R (Reference Manual) of
-Stroustrup: *The C++Programming Language* (second edition,
-Addison-Wesley Publishing Company, ISBN 0-201-53992-6, copyright ©1991
-AT&T). That, in turn, is based on the C programming language as
-described in Appendix A of Kernighan and Ritchie: *The C Programming
-Language* (Prentice-Hall, 1978, ISBN 0-13-110163-3, copyright ©1978
-AT&T).
-Portions of the library Clauses of this International Standard are based
-on work by P.J. Plauger, which was published as *The Draft Standard
-C++Library* (Prentice-Hall, ISBN 0-13-117003-1, copyright ©1995 P.J.
-Plauger).
 POSIX® is a registered trademark of the Institute of Electrical and
 Electronic Engineers, Inc.
 All rights in these originals are reserved.
 <!-- Link reference definitions -->
 [atomics]: atomics.md#atomics
 [atomics.flag]: atomics.md#atomics.flag
@@ -1018,95 +1159,118 @@ All rights in these originals are reserved.
 [basic.def.odr]: basic.md#basic.def.odr
 [basic.life]: basic.md#basic.life
 [basic.link]: basic.md#basic.link
 [basic.lval]: basic.md#basic.lval
 [basic.namespace]: dcl.md#basic.namespace
 [basic.stc]: basic.md#basic.stc
 [basic.stc.auto]: basic.md#basic.stc.auto
 [basic.types]: basic.md#basic.types
 [class.access]: class.md#class.access
 [class.bit]: class.md#class.bit
 [class.conv.fct]: special.md#class.conv.fct
 [class.derived]: class.md#class.derived
 [class.mem]: class.md#class.mem
 [class.temporary]: special.md#class.temporary
 [class.virtual]: class.md#class.virtual
 [compliance]: library.md#compliance
 [cpp]: cpp.md#cpp
 [cpp.include]: cpp.md#cpp.include
 [dcl.fct]: dcl.md#dcl.fct
 [dcl.fct.default]: dcl.md#dcl.fct.default
 [dcl.ptr]: dcl.md#dcl.ptr
 [dcl.ref]: dcl.md#dcl.ref
 [definitions]: library.md#definitions
 [depr]: future.md#depr
 [diff]: compatibility.md#diff
 [diff.library]: compatibility.md#diff.library
 [expr]: expr.md#expr
 [expr.comma]: expr.md#expr.comma
 [expr.cond]: expr.md#expr.cond
 [expr.log.and]: expr.md#expr.log.and
 [expr.log.or]: expr.md#expr.log.or
 [expr.new]: expr.md#expr.new
 [gram]: grammar.md#gram
 [implimits]: limits.md#implimits
 [intro]: #intro
 [intro.ack]: #intro.ack
 [intro.compliance]: #intro.compliance
 [intro.defs]: #intro.defs
 [intro.execution]: #intro.execution
 [intro.memory]: #intro.memory
 [intro.multithread]: #intro.multithread
 [intro.object]: #intro.object
 [intro.refs]: #intro.refs
 [intro.scope]: #intro.scope
 [intro.structure]: #intro.structure
 [language.support]: language.md#language.support
 [lex]: lex.md#lex
 [lex.charset]: lex.md#lex.charset
 [lex.phases]: lex.md#lex.phases
 [library]: library.md#library
 [syntax]: #syntax
 [thread]: thread.md#thread
 [^1]: With the qualifications noted in Clauses  [[language.support]]
     through  [[thread]] and in  [[diff.library]], the C standard library
     is a subset of the C++standard library.
 [^2]: “Correct execution” can include undefined behavior, depending on
-    the data being processed; see  [[intro.defs]] and
     [[intro.execution]].
 [^3]: This documentation also defines implementation-defined behavior;
     see  [[intro.execution]].
-[^4]: Under the “as-if” rule an implementation is allowed to store two
     objects at the same machine address or not store an object at all if
     the program cannot observe the difference ([[intro.execution]]).
-[^5]: This provision is sometimes called the “as-if” rule, because an
     implementation is free to disregard any requirement of this
     International Standard as long as the result is *as if* the
     requirement had been obeyed, as far as can be determined from the
     observable behavior of the program. For instance, an actual
     implementation need not evaluate part of an expression if it can
     deduce that its value is not used and that no side effects affecting
     the observable behavior of the program are produced.
-[^6]: This documentation also includes conditionally-supported
     constructs and locale-specific behavior. See  [[intro.compliance]].
-[^7]: Overloaded operators are never assumed to be associative or
     commutative.
-[^8]: As specified in  [[class.temporary]], after a full-expression is
     evaluated, a sequence of zero or more invocations of destructor
     functions for temporary objects takes place, usually in reverse
     order of the construction of each temporary object.
-[^9]: In other words, function executions do not interleave with each
     other.
-[^10]: An object with automatic or thread storage duration (
     [[basic.stc]]) is associated with one specific thread, and can be
     accessed by a different thread only indirectly through a pointer or
     reference ([[basic.compound]]).

+# General principles <a id="intro">[[intro]]</a>
 ## Implementation compliance <a id="intro.compliance">[[intro.compliance]]</a>
 The set of *diagnosable rules* consists of all syntactic and semantic
 rules in this International Standard except for those rules containing
 an explicit notation that “no diagnostic is required” or which are
+described as resulting in “undefined behavior”.
 Although this International Standard states only requirements on C++
 implementations, those requirements are often easier to understand if
 they are phrased as requirements on programs, parts of programs, or
 execution of programs. Such requirements have the following meaning:
 - If a program contains no violations of the rules in this International
   Standard, a conforming implementation shall, within its resource
   limits, accept and correctly execute[^2] that program.
 - If a program contains a violation of any diagnosable rule or an
+  occurrence of a construct described in this International Standard as
   “conditionally-supported” when the implementation does not support
   that construct, a conforming implementation shall issue at least one
   diagnostic message.
 - If a program contains a violation of a rule for which no diagnostic is
   required, this International Standard places no requirement on
   implementations with respect to that program.
+[*Note 1*: During template argument deduction and substitution, certain
+constructs that in other contexts require a diagnostic are treated
+differently; see  [[temp.deduct]]. — *end note*]
 For classes and class templates, the library Clauses specify partial
 definitions. Private members (Clause  [[class.access]]) are not
 specified, but each implementation shall supply them to complete the
 definitions according to the description in the library Clauses.
 Each implementation shall include documentation that identifies all
 conditionally-supported constructs that it does not support and defines
 all locale-specific characteristics.[^3]
+## Structure of this document <a id="intro.structure">[[intro.structure]]</a>
 Clauses  [[lex]] through  [[cpp]] describe the C++programming language.
 That description includes detailed syntactic specifications in a form
 described in  [[syntax]]. For convenience, Annex  [[gram]] repeats all
 such syntactic specifications.
 Clauses  [[language.support]] through  [[thread]] and Annex  [[depr]]
+(the *library clauses*) describe the C++standard library. That
+description includes detailed descriptions of the entities and macros
+that constitute the library, in a form described in Clause  [[library]].
 Annex  [[implimits]] recommends lower bounds on the capacity of
 conforming implementations.
 Annex  [[diff]] summarizes the evolution of C++since its first published
 description, and explains in detail the differences between C++and C.
 Certain features of C++exist solely for compatibility purposes; Annex
 [[depr]] describes those features.
+Throughout this document, each example is introduced by “” and
+terminated by “”. Each note is introduced by “” and terminated by “”.
+Examples and notes may be nested.
 ## Syntax notation <a id="syntax">[[syntax]]</a>
+In the syntax notation used in this document, syntactic categories are
+indicated by *italic* type, and literal words and characters in
+`constant` `width` type. Alternatives are listed on separate lines
+except in a few cases where a long set of alternatives is marked by the
+phrase “one of”. If the text of an alternative is too long to fit on a
+line, the text is continued on subsequent lines indented from the first
+one. An optional terminal or non-terminal symbol is indicated by the
+subscript “”, so
 ``` bnf
 '{' expressionₒₚₜ '}'
 ```
 - *X-id* is an identifier with no context-dependent meaning (e.g.,
   *qualified-id*).
 - *X-seq* is one or more *X*’s without intervening delimiters (e.g.,
   *declaration-seq* is a sequence of declarations).
 - *X-list* is one or more *X*’s separated by intervening commas (e.g.,
+  *identifier-list* is a sequence of identifiers separated by commas).
 ## The C++memory model <a id="intro.memory">[[intro.memory]]</a>
 The fundamental storage unit in the C++memory model is the *byte*. A
 byte is at least large enough to contain any member of the basic
 execution character set ([[lex.charset]]) and the eight-bit code units
 of the Unicode UTF-8 encoding form and is composed of a contiguous
+sequence of bits,[^4] the number of which is *implementation-defined*.
+The least significant bit is called the *low-order bit*; the most
 significant bit is called the *high-order bit*. The memory available to
 a C++program consists of one or more sequences of contiguous bytes.
 Every byte has a unique address.
+[*Note 1*: The representation of types is described in
+[[basic.types]]. — *end note*]
 A *memory location* is either an object of scalar type or a maximal
+sequence of adjacent bit-fields all having nonzero width.
+[*Note 2*: Various features of the language, such as references and
+virtual functions, might involve additional memory locations that are
+not accessible to programs but are managed by the
+implementation. — *end note*]
+Two or more threads of execution ([[intro.multithread]]) can access
+separate memory locations without interfering with each other.
+[*Note 3*: Thus a bit-field and an adjacent non-bit-field are in
+separate memory locations, and therefore can be concurrently updated by
+two threads of execution without interference. The same applies to two
+bit-fields, if one is declared inside a nested struct declaration and
+the other is not, or if the two are separated by a zero-length bit-field
+declaration, or if they are separated by a non-bit-field declaration. It
+is not safe to concurrently update two bit-fields in the same struct if
+all fields between them are also bit-fields of nonzero
+width. — *end note*]
+[*Example 1*:
 A structure declared as
 ``` cpp
 struct {
 concurrently without interfering with each other. The bit-fields `b` and
 `c` together constitute the fourth memory location. The bit-fields `b`
 and `c` cannot be concurrently modified, but `b` and `a`, for example,
 can be.
+— *end example*]
 ## The C++object model <a id="intro.object">[[intro.object]]</a>
 The constructs in a C++program create, destroy, refer to, access, and
+manipulate objects. An *object* is created by a definition (
+[[basic.def]]), by a *new-expression* ([[expr.new]]), when implicitly
+changing the active member of a union ([[class.union]]), or when a
+temporary object is created ([[conv.rval]], [[class.temporary]]). An
+object occupies a region of storage in its period of construction (
+[[class.cdtor]]), throughout its lifetime ([[basic.life]]), and in its
+period of destruction ([[class.cdtor]]).
+[*Note 1*: A function is not an object, regardless of whether or not it
+occupies storage in the way that objects do. — *end note*]
+The properties of an object are determined when the object is created.
+An object can have a name (Clause  [[basic]]). An object has a storage
+duration ([[basic.stc]]) which influences its lifetime (
+[[basic.life]]). An object has a type ([[basic.types]]). Some objects
+are polymorphic ([[class.virtual]]); the implementation generates
 information associated with each such object that makes it possible to
 determine that object’s type during program execution. For other
 objects, the interpretation of the values found therein is determined by
 the type of the *expression*s (Clause  [[expr]]) used to access them.
 Objects can contain other objects, called *subobjects*. A subobject can
 be a *member subobject* ([[class.mem]]), a *base class subobject*
 (Clause  [[class.derived]]), or an array element. An object that is not
+a subobject of any other object is called a *complete object*. If an
+object is created in storage associated with a member subobject or array
+element *e* (which may or may not be within its lifetime), the created
+object is a subobject of *e*’s containing object if:
+- the lifetime of *e*’s containing object has begun and not ended, and
+- the storage for the new object exactly overlays the storage location
+  associated with *e*, and
+- the new object is of the same type as *e* (ignoring cv-qualification).
+[*Note 2*: If the subobject contains a reference member or a `const`
+subobject, the name of the original subobject cannot be used to access
+the new object ([[basic.life]]). — *end note*]
+[*Example 1*:
+``` cpp
+struct X { const int n; };
+union U { X x; float f; };
+void tong() {
+  U u = {{ 1 }};
+  u.f = 5.f;                          // OK, creates new subobject of u ([class.union])
+  X *p = new (&u.x) X {2};            // OK, creates new subobject of u
+  assert(p->n == 2);                  // OK
+  assert(*std::launder(&u.x.n) == 2); // OK
+  assert(u.x.n == 2);                 // undefined behavior, u.x does not name new subobject
+}
+```
+— *end example*]
+If a complete object is created ([[expr.new]]) in storage associated
+with another object *e* of type “array of N `unsigned char`” or of type
+“array of N `std::byte`” ([[cstddef.syn]]), that array *provides
+storage* for the created object if:
+- the lifetime of *e* has begun and not ended, and
+- the storage for the new object fits entirely within *e*, and
+- there is no smaller array object that satisfies these constraints.
+[*Note 3*: If that portion of the array previously provided storage for
+another object, the lifetime of that object ends because its storage was
+reused ([[basic.life]]). — *end note*]
+[*Example 2*:
+``` cpp
+template<typename ...T>
+struct AlignedUnion {
+  alignas(T...) unsigned char data[max(sizeof(T)...)];
+};
+int f() {
+  AlignedUnion<int, char> au;
+  int *p = new (au.data) int;     // OK, au.data provides storage
+  char *c = new (au.data) char(); // OK, ends lifetime of *p
+  char *d = new (au.data + 1) char();
+  return *c + *d; // OK
+}
+struct A { unsigned char a[32]; };
+struct B { unsigned char b[16]; };
+A a;
+B *b = new (a.a + 8) B;      // a.a provides storage for *b
+int *p = new (b->b + 4) int; // b->b provides storage for *p
+                             // a.a does not provide storage for *p (directly),
+                             // but *p is nested within a (see below)
+```
+— *end example*]
+An object *a* is *nested within* another object *b* if:
+- *a* is a subobject of *b*, or
+- *b* provides storage for *a*, or
+- there exists an object *c* where *a* is nested within *c*, and *c* is
+  nested within *b*.
 For every object `x`, there is some object called the *complete object
 of* `x`, determined as follows:
+- If `x` is a complete object, then the complete object of `x` is
+  itself.
 - Otherwise, the complete object of `x` is the complete object of the
   (unique) object that contains `x`.
 If a complete object, a data member ([[class.mem]]), or an array
 element is of class type, its type is considered the *most derived
 class*, to distinguish it from the class type of any base class
 subobject; an object of a most derived class type or of a non-class type
 is called a *most derived object*.
 Unless it is a bit-field ([[class.bit]]), a most derived object shall
+have a nonzero size and shall occupy one or more bytes of storage. Base
 class subobjects may have zero size. An object of trivially copyable or
 standard-layout type ([[basic.types]]) shall occupy contiguous bytes of
 storage.
 Unless an object is a bit-field or a base class subobject of zero size,
 the address of that object is the address of the first byte it occupies.
+Two objects *a* and *b* with overlapping lifetimes that are not
+bit-fields may have the same address if one is nested within the other,
+or if at least one is a base class subobject of zero size and they are
+of different types; otherwise, they have distinct addresses.[^5]
+[*Example 3*:
 ``` cpp
 static const char test1 = 'x';
 static const char test2 = 'x';
 const bool b = &test1 != &test2;      // always true
 ```
+— *end example*]
+[*Note 4*: C++provides a variety of fundamental types and several ways
+of composing new types from existing types (
+[[basic.types]]). — *end note*]
 ## Program execution <a id="intro.execution">[[intro.execution]]</a>
 The semantic descriptions in this International Standard define a
 parameterized nondeterministic abstract machine. This International
 Standard places no requirement on the structure of conforming
 implementations. In particular, they need not copy or emulate the
 structure of the abstract machine. Rather, conforming implementations
 are required to emulate (only) the observable behavior of the abstract
+machine as explained below.[^6]
 Certain aspects and operations of the abstract machine are described in
 this International Standard as implementation-defined (for example,
 `sizeof(int)`). These constitute the parameters of the abstract machine.
 Each implementation shall include documentation describing its
+characteristics and behavior in these respects.[^7] Such documentation
 shall define the instance of the abstract machine that corresponds to
 that implementation (referred to as the “corresponding instance” below).
 Certain other aspects and operations of the abstract machine are
 described in this International Standard as unspecified (for example,
 of the abstract machine can thus have more than one possible execution
 for a given program and a given input.
 Certain other operations are described in this International Standard as
 undefined (for example, the effect of attempting to modify a `const`
+object).
+[*Note 1*: This International Standard imposes no requirements on the
+behavior of programs that contain undefined behavior. — *end note*]
 A conforming implementation executing a well-formed program shall
 produce the same observable behavior as one of the possible executions
 of the corresponding instance of the abstract machine with the same
 program and the same input. However, if any such execution contains an
 undefined operation, this International Standard places no requirement
 on the implementation executing that program with that input (not even
 with regard to operations preceding the first undefined operation).
 An instance of each object with automatic storage duration (
 [[basic.stc.auto]]) is associated with each entry into its block. Such
 an object exists and retains its last-stored value during the execution
 of the block and while the block is suspended (by a call of a function
 or receipt of a signal).
 The least requirements on a conforming implementation are:
+- Accesses through volatile glvalues are evaluated strictly according to
+ the rules of the abstract machine.
 - At program termination, all data written into files shall be identical
   to one of the possible results that execution of the program according
   to the abstract semantics would have produced.
 - The input and output dynamics of interactive devices shall take place
   in such a fashion that prompting output is actually delivered before a
   program waits for input. What constitutes an interactive device is
   *implementation-defined*.
 These collectively are referred to as the *observable behavior* of the
+program.
+[*Note 2*: More stringent correspondences between abstract and actual
+semantics may be defined by each implementation. — *end note*]
+[*Note 3*:
 Operators can be regrouped according to the usual mathematical rules
+only where the operators really are associative or commutative.[^8] For
 example, in the following fragment
 ``` cpp
 int a, b;
+...
 a = a + 32760 + b + 5;
 ```
 the expression statement behaves exactly the same as
 or -17 and 12. However on a machine in which overflows do not produce an
 exception and in which the results of overflows are reversible, the
 above expression statement can be rewritten by the implementation in any
 of the above ways because the same result will occur.
+— *end note*]
+A *constituent expression* is defined as follows:
+- The constituent expression of an expression is that expression.
+- The constituent expressions of a *braced-init-list* or of a (possibly
+  parenthesized) *expression-list* are the constituent expressions of
+  the elements of the respective list.
+- The constituent expressions of a *brace-or-equal-initializer* of the
+  form `=` *initializer-clause* are the constituent expressions of the
+  *initializer-clause*.
+[*Example 1*:
+``` cpp
+struct A { int x; };
+struct B { int y; struct A a; };
+B b = { 5, { 1+1 } };
+```
+The constituent expressions of the *initializer* used for the
+initialization of `b` are `5` and `1+1`.
+— *end example*]
+The *immediate subexpressions* of an expression `e` are
+- the constituent expressions of `e`’s operands (Clause [[expr]]),
+- any function call that `e` implicitly invokes,
+- if `e` is a *lambda-expression* ([[expr.prim.lambda]]), the
+  initialization of the entities captured by copy and the constituent
+  expressions of the *initializer* of the *init-capture*s,
+- if `e` is a function call ([[expr.call]]) or implicitly invokes a
+  function, the constituent expressions of each default argument (
+  [[dcl.fct.default]]) used in the call, or
+- if `e` creates an aggregate object ([[dcl.init.aggr]]), the
+  constituent expressions of each default member initializer (
+  [[class.mem]]) used in the initialization.
+A *subexpression* of an expression `e` is an immediate subexpression of
+`e` or a subexpression of an immediate subexpression of `e`.
+[*Note 4*: Expressions appearing in the *compound-statement* of a
+*lambda-expression* are not subexpressions of the
+*lambda-expression*. — *end note*]
+A *full-expression* is
+- an unevaluated operand (Clause [[expr]]),
+- a *constant-expression* ([[expr.const]]),
+- an *init-declarator* (Clause [[dcl.decl]]) or a *mem-initializer* (
+  [[class.base.init]]), including the constituent expressions of the
+  initializer,
+- an invocation of a destructor generated at the end of the lifetime of
+  an object other than a temporary object ([[class.temporary]]), or
+- an expression that is not a subexpression of another expression and
+  that is not otherwise part of a full-expression.
+If a language construct is defined to produce an implicit call of a
+function, a use of the language construct is considered to be an
+expression for the purposes of this definition. Conversions applied to
+the result of an expression in order to satisfy the requirements of the
+language construct in which the expression appears are also considered
+to be part of the full-expression. For an initializer, performing the
+initialization of the entity (including evaluating default member
+initializers of an aggregate) is also considered part of the
+full-expression.
+[*Example 2*:
 ``` cpp
 struct S {
+  S(int i): I(i) { }       // full-expression is initialization of I
   int& v() { return I; }
+  ~S() noexcept(false) { }
 private:
   int I;
 };
 S s1(1);                   // full-expression is call of S::S(int)
 void f() {
+  S s2 = 2;                // full-expression is call of S::S(int)
   if (S(3).v())            // full-expression includes lvalue-to-rvalue and
                            // int to bool conversions, performed before
                            // temporary is deleted at end of full-expression
   { }
+  bool b = noexcept(S());  // exception specification of destructor of S
+                           // considered for noexcept
+  // full-expression is destruction of s2 at end of block
 }
+struct B {
+      B(S = S(0));
+   };
+   B b[2] = { B(), B() };  // full-expression is the entire initialization
+                           // including the destruction of temporaries
 ```
+— *end example*]
+[*Note 5*: The evaluation of a full-expression can include the
+evaluation of subexpressions that are not lexically part of the
+full-expression. For example, subexpressions involved in evaluating
+default arguments ([[dcl.fct.default]]) are considered to be created in
+the expression that calls the function, not the expression that defines
+the default argument. — *end note*]
+Reading an object designated by a `volatile` glvalue ([[basic.lval]]),
+modifying an object, calling a library I/O function, or calling a
+function that does any of those operations are all *side effects*, which
+are changes in the state of the execution environment. *Evaluation* of
+an expression (or a subexpression) in general includes both value
+computations (including determining the identity of an object for
+glvalue evaluation and fetching a value previously assigned to an object
+for prvalue evaluation) and initiation of side effects. When a call to a
+library I/O function returns or an access through a volatile glvalue is
+evaluated the side effect is considered complete, even though some
+external actions implied by the call (such as the I/O itself) or by the
+`volatile` access may not have completed yet.
 *Sequenced before* is an asymmetric, transitive, pair-wise relation
 between evaluations executed by a single thread (
 [[intro.multithread]]), which induces a partial order among those
 evaluations. Given any two evaluations *A* and *B*, if *A* is sequenced
+before *B* (or, equivalently, *B* is *sequenced after* *A*), then the
+execution of *A* shall precede the execution of *B*. If *A* is not
+sequenced before *B* and *B* is not sequenced before *A*, then *A* and
+*B* are *unsequenced*.
+[*Note 6*: The execution of unsequenced evaluations can
+overlap. — *end note*]
+Evaluations *A* and *B* are *indeterminately sequenced* when either *A*
+is sequenced before *B* or *B* is sequenced before *A*, but it is
+unspecified which.
+[*Note 7*: Indeterminately sequenced evaluations cannot overlap, but
+either could be executed first. — *end note*]
+An expression *X* is said to be sequenced before an expression *Y* if
+every value computation and every side effect associated with the
+expression *X* is sequenced before every value computation and every
+side effect associated with the expression *Y*.
 Every value computation and side effect associated with a
 full-expression is sequenced before every value computation and side
+effect associated with the next full-expression to be evaluated.[^9]
 Except where noted, evaluations of operands of individual operators and
+of subexpressions of individual expressions are unsequenced.
+[*Note 8*: In an expression that is evaluated more than once during the
+execution of a program, unsequenced and indeterminately sequenced
+evaluations of its subexpressions need not be performed consistently in
+different evaluations. — *end note*]
+The value computations of the operands of an operator are sequenced
+before the value computation of the result of the operator. If a side
+effect on a memory location ([[intro.memory]]) is unsequenced relative
+to either another side effect on the same memory location or a value
+computation using the value of any object in the same memory location,
+and they are not potentially concurrent ([[intro.multithread]]), the
+behavior is undefined.
+[*Note 9*: The next section imposes similar, but more complex
+restrictions on potentially concurrent computations. — *end note*]
+[*Example 3*:
 ``` cpp
+void g(int i) {
   i = 7, i++, i++;    // i becomes 9
+  i = i++ + 1;        // the value of i is incremented
+  i = i++ + i;        // the behavior is undefined
   i = i + 1;          // the value of i is incremented
 }
 ```
+— *end example*]
 When calling a function (whether or not the function is inline), every
 value computation and side effect associated with any argument
 expression, or with the postfix expression designating the called
 function, is sequenced before execution of every expression or statement
+in the body of the called function. For each function invocation *F*,
+for every evaluation *A* that occurs within *F* and every evaluation *B*
+that does not occur within *F* but is evaluated on the same thread and
+as part of the same signal handler (if any), either *A* is sequenced
+before *B* or *B* is sequenced before *A*.[^10]
+[*Note 10*: If *A* and *B* would not otherwise be sequenced then they
+are indeterminately sequenced. — *end note*]
+Several contexts in C++cause evaluation of a function call, even though
+no corresponding function call syntax appears in the translation unit.
+[*Example 4*: Evaluation of a *new-expression* invokes one or more
+allocation and constructor functions; see  [[expr.new]]. For another
+example, invocation of a conversion function ([[class.conv.fct]]) can
+arise in contexts in which no function call syntax
+appears. — *end example*]
+The sequencing constraints on the execution of the called function (as
+described above) are features of the function calls as evaluated,
+whatever the syntax of the expression that calls the function might be.
+If a signal handler is executed as a result of a call to the
+`std::raise` function, then the execution of the handler is sequenced
+after the invocation of the `std::raise` function and before its return.
+[*Note 11*: When a signal is received for another reason, the execution
+of the signal handler is usually unsequenced with respect to the rest of
+the program. — *end note*]
 ## Multi-threaded executions and data races <a id="intro.multithread">[[intro.multithread]]</a>
 A *thread of execution* (also known as a *thread*) is a single flow of
 control within a program, including the initial invocation of a specific
 top-level function, and recursively including every function invocation
+subsequently executed by the thread.
+[*Note 1*: When one thread creates another, the initial call to the
+top-level function of the new thread is executed by the new thread, not
+by the creating thread. — *end note*]
+Every thread in a program can potentially access every object and
+function in a program.[^11] Under a hosted implementation, a C++program
+can have more than one thread running concurrently. The execution of
+each thread proceeds as defined by the remainder of this International
+Standard. The execution of the entire program consists of an execution
+of all of its threads.
+[*Note 2*: Usually the execution can be viewed as an interleaving of
+all its threads. However, some kinds of atomic operations, for example,
+allow executions inconsistent with a simple interleaving, as described
+below. — *end note*]
+Under a freestanding implementation, it is *implementation-defined*
+whether a program can have more than one thread of execution.
+For a signal handler that is not executed as a result of a call to the
+`std::raise` function, it is unspecified which thread of execution
+contains the signal handler invocation.
+### Data races <a id="intro.races">[[intro.races]]</a>
 The value of an object visible to a thread *T* at a particular point is
 the initial value of the object, a value assigned to the object by *T*,
 or a value assigned to the object by another thread, according to the
+rules below.
+[*Note 1*: In some cases, there may instead be undefined behavior. Much
+of this section is motivated by the desire to support atomic operations
+with explicit and detailed visibility constraints. However, it also
+implicitly supports a simpler view for more restricted
+programs. — *end note*]
 Two expression evaluations *conflict* if one of them modifies a memory
+location ([[intro.memory]]) and the other one reads or modifies the
 same memory location.
 The library defines a number of atomic operations (Clause  [[atomics]])
 and operations on mutexes (Clause  [[thread]]) that are specially
 identified as synchronization operations. These operations play a
 acquire and release operation. A synchronization operation without an
 associated memory location is a fence and can be either an acquire
 fence, a release fence, or both an acquire and release fence. In
 addition, there are relaxed atomic operations, which are not
 synchronization operations, and atomic read-modify-write operations,
+which have special characteristics.
+[*Note 2*: For example, a call that acquires a mutex will perform an
+acquire operation on the locations comprising the mutex.
+Correspondingly, a call that releases the same mutex will perform a
+release operation on those same locations. Informally, performing a
 release operation on *A* forces prior side effects on other memory
 locations to become visible to other threads that later perform a
 consume or an acquire operation on *A*. “Relaxed” atomic operations are
 not synchronization operations even though, like synchronization
+operations, they cannot contribute to data races. — *end note*]
 All modifications to a particular atomic object *M* occur in some
+particular total order, called the *modification order* of *M*.
+[*Note 3*: There is a separate order for each atomic object. There is
+no requirement that these can be combined into a single total order for
+all objects. In general this will be impossible since different threads
+may observe modifications to different objects in inconsistent
+orders. — *end note*]
 A *release sequence* headed by a release operation *A* on an atomic
 object *M* is a maximal contiguous sub-sequence of side effects in the
 modification order of *M*, where the first operation is `A`, and every
 subsequent operation
 - is an atomic read-modify-write operation.
 Certain library calls *synchronize with* other library calls performed
 by another thread. For example, an atomic store-release synchronizes
 with a load-acquire that takes its value from the store (
+[[atomics.order]]).
+[*Note 4*: Except in the specified cases, reading a later value does
+not necessarily ensure visibility as described below. Such a requirement
+would sometimes interfere with efficient implementation. — *end note*]
+[*Note 5*: The specifications of the synchronization operations define
+when one reads the value written by another. For atomic objects, the
+definition is clear. All operations on a given mutex occur in a single
+total order. Each mutex acquisition “reads the value written” by the
+last mutex release. — *end note*]
 An evaluation *A* *carries a dependency* to an evaluation *B* if
 - the value of *A* is used as an operand of *B*, unless:
   - *B* is an invocation of any specialization of
 - *A* writes a scalar object or bit-field *M*, *B* reads the value
   written by *A* from *M*, and *A* is sequenced before *B*, or
 - for some evaluation *X*, *A* carries a dependency to *X*, and *X*
   carries a dependency to *B*.
+[*Note 6*: “Carries a dependency to” is a subset of “is sequenced
+before”, and is similarly strictly intra-thread. — *end note*]
 An evaluation *A* is *dependency-ordered before* an evaluation *B* if
 - *A* performs a release operation on an atomic object *M*, and, in
   another thread, *B* performs a consume operation on *M* and reads a
   value written by any side effect in the release sequence headed by
   *A*, or
 - for some evaluation *X*, *A* is dependency-ordered before *X* and *X*
   carries a dependency to *B*.
+[*Note 7*: The relation “is dependency-ordered before” is analogous to
 “synchronizes with”, but uses release/consume in place of
+release/acquire. — *end note*]
 An evaluation *A* *inter-thread happens before* an evaluation *B* if
 - *A* synchronizes with *B*, or
 - *A* is dependency-ordered before *B*, or
   - *A* is sequenced before *X* and *X* inter-thread happens before *B*,
     or
   - *A* inter-thread happens before *X* and *X* inter-thread happens
     before *B*.
+[*Note 8*: The “inter-thread happens before” relation describes
+arbitrary concatenations of “sequenced before”, “synchronizes with” and
 “dependency-ordered before” relationships, with two exceptions. The
 first exception is that a concatenation is not permitted to end with
 “dependency-ordered before” followed by “sequenced before”. The reason
 for this limitation is that a consume operation participating in a
 “dependency-ordered before” relationship provides ordering only with
 provide the required ordering for a prior consume operation. The second
 exception is that a concatenation is not permitted to consist entirely
 of “sequenced before”. The reasons for this limitation are (1) to permit
 “inter-thread happens before” to be transitively closed and (2) the
 “happens before” relation, defined below, provides for relationships
+consisting entirely of “sequenced before”. — *end note*]
+An evaluation *A* *happens before* an evaluation *B* (or, equivalently,
+*B* *happens after* *A*) if:
 - *A* is sequenced before *B*, or
 - *A* inter-thread happens before *B*.
 The implementation shall ensure that no program execution demonstrates a
+cycle in the “happens before” relation.
+[*Note 9*: This cycle would otherwise be possible only through the use
+of consume operations. — *end note*]
+An evaluation *A* *strongly happens before* an evaluation *B* if either
+- *A* is sequenced before *B*, or
+- *A* synchronizes with *B*, or
+- *A* strongly happens before *X* and *X* strongly happens before *B*.
+[*Note 10*: In the absence of consume operations, the happens before
+and strongly happens before relations are identical. Strongly happens
+before essentially excludes consume operations. — *end note*]
 A *visible side effect* *A* on a scalar object or bit-field *M* with
 respect to a value computation *B* of *M* satisfies the conditions:
 - *A* happens before *B* and
 - there is no other side effect *X* to *M* such that *A* happens before
   *X* and *X* happens before *B*.
 The value of a non-atomic scalar object or bit-field *M*, as determined
 by evaluation *B*, shall be the value stored by the visible side effect
+*A*.
+[*Note 11*: If there is ambiguity about which side effect to a
+non-atomic object or bit-field is visible, then the behavior is either
+unspecified or undefined. — *end note*]
+[*Note 12*: This states that operations on ordinary objects are not
 visibly reordered. This is not actually detectable without data races,
 but it is necessary to ensure that data races, as defined below, and
 with suitable restrictions on the use of atomics, correspond to data
+races in a simple interleaved (sequentially consistent)
+execution. — *end note*]
 The value of an atomic object *M*, as determined by evaluation *B*,
 shall be the value stored by some side effect *A* that modifies *M*,
+where *B* does not happen before *A*.
+[*Note 13*: The set of such side effects is also restricted by the rest
+of the rules described here, and in particular, by the coherence
+requirements below. — *end note*]
 If an operation *A* that modifies an atomic object *M* happens before an
 operation *B* that modifies *M*, then *A* shall be earlier than *B* in
+the modification order of *M*.
+[*Note 14*: This requirement is known as write-write
+coherence. — *end note*]
 If a value computation *A* of an atomic object *M* happens before a
 value computation *B* of *M*, and *A* takes its value from a side effect
 *X* on *M*, then the value computed by *B* shall either be the value
 stored by *X* or the value stored by a side effect *Y* on *M*, where *Y*
+follows *X* in the modification order of *M*.
+[*Note 15*: This requirement is known as read-read
+coherence. — *end note*]
 If a value computation *A* of an atomic object *M* happens before an
 operation *B* that modifies *M*, then *A* shall take its value from a
 side effect *X* on *M*, where *X* precedes *B* in the modification order
+of *M*.
+[*Note 16*: This requirement is known as read-write
+coherence. — *end note*]
 If a side effect *X* on an atomic object *M* happens before a value
 computation *B* of *M*, then the evaluation *B* shall take its value
 from *X* or from a side effect *Y* that follows *X* in the modification
+order of *M*.
+[*Note 17*: This requirement is known as write-read
+coherence. — *end note*]
+[*Note 18*: The four preceding coherence requirements effectively
+disallow compiler reordering of atomic operations to a single object,
+even if both operations are relaxed loads. This effectively makes the
+cache coherence guarantee provided by most hardware available to
+C++atomic operations. — *end note*]
+[*Note 19*: The value observed by a load of an atomic depends on the
+“happens before” relation, which depends on the values observed by loads
+of atomics. The intended reading is that there must exist an association
+of atomic loads with modifications they observe that, together with
 suitably chosen modification orders and the “happens before” relation
 derived as described above, satisfy the resulting constraints as imposed
+here. — *end note*]
 Two actions are *potentially concurrent* if
 - they are performed by different threads, or
+- they are unsequenced, at least one is performed by a signal handler,
+ and they are not both performed by the same signal handler invocation.
 The execution of a program contains a *data race* if it contains two
 potentially concurrent conflicting actions, at least one of which is not
 atomic, and neither happens before the other, except for the special
 case for signal handlers described below. Any such data race results in
+undefined behavior.
+[*Note 20*: It can be shown that programs that correctly use mutexes
+and `memory_order_seq_cst` operations to prevent all data races and use
+no other synchronization operations behave as if the operations executed
+by their constituent threads were simply interleaved, with each value
+computation of an object being taken from the last side effect on that
+object in that interleaving. This is normally referred to as “sequential
+consistency”. However, this applies only to data-race-free programs, and
+data-race-free programs cannot observe most program transformations that
+do not change single-threaded program semantics. In fact, most
+single-threaded program transformations continue to be allowed, since
+any program that behaves differently as a result must perform an
+undefined operation. — *end note*]
+Two accesses to the same object of type `volatile std::sig_atomic_t` do
+not result in a data race if both occur in the same thread, even if one
+or more occurs in a signal handler. For each signal handler invocation,
 evaluations performed by the thread invoking a signal handler can be
 divided into two groups *A* and *B*, such that no evaluations in *B*
 happen before evaluations in *A*, and the evaluations of such
+`volatile std::sig_atomic_t` objects take values as though all
+evaluations in *A* happened before the execution of the signal handler
+and the execution of the signal handler happened before all evaluations
+in *B*.
+[*Note 21*: Compiler transformations that introduce assignments to a
+potentially shared memory location that would not be modified by the
+abstract machine are generally precluded by this International Standard,
+since such an assignment might overwrite another assignment by a
+different thread in cases in which an abstract machine execution would
+not have encountered a data race. This includes implementations of data
+member assignment that overwrite adjacent members in separate memory
+locations. Reordering of atomic loads in cases in which the atomics in
+question may alias is also generally precluded, since this may violate
+the coherence rules. — *end note*]
+[*Note 22*: Transformations that introduce a speculative read of a
+potentially shared memory location may not preserve the semantics of the
+C++program as defined in this International Standard, since they
+potentially introduce a data race. However, they are typically valid in
+the context of an optimizing compiler that targets a specific machine
+with well-defined semantics for data races. They would be invalid for a
+hypothetical machine that is not tolerant of races or provides hardware
+race detection. — *end note*]
+### Forward progress <a id="intro.progress">[[intro.progress]]</a>
 The implementation may assume that any thread will eventually do one of
 the following:
 - terminate,
 - make a call to a library I/O function,
+- perform an access through a volatile glvalue, or
 - perform a synchronization operation or an atomic operation.
+[*Note 1*: This is intended to allow compiler transformations such as
+removal of empty loops, even when termination cannot be
+proven. — *end note*]
+Executions of atomic functions that are either defined to be lock-free (
+[[atomics.flag]]) or indicated as lock-free ([[atomics.lockfree]]) are
+*lock-free executions*.
+- If there is only one thread that is not blocked ([[defns.block]]) in
+  a standard library function, a lock-free execution in that thread
+  shall complete. \[*Note 2*: Concurrently executing threads may prevent
+  progress of a lock-free execution. For example, this situation can
+  occur with load-locked store-conditional implementations. This
+  property is sometimes termed obstruction-free. — *end note*]
+- When one or more lock-free executions run concurrently, at least one
+  should complete. \[*Note 3*: It is difficult for some implementations
+  to provide absolute guarantees to this effect, since repeated and
+  particularly inopportune interference from other threads may prevent
+  forward progress, e.g., by repeatedly stealing a cache line for
+  unrelated purposes between load-locked and store-conditional
+  instructions. Implementations should ensure that such effects cannot
+  indefinitely delay progress under expected operating conditions, and
+  that such anomalies can therefore safely be ignored by programmers.
+  Outside this document, this property is sometimes termed
+  lock-free. — *end note*]
+During the execution of a thread of execution, each of the following is
+termed an *execution step*:
+- termination of the thread of execution,
+- performing an access through a volatile glvalue, or
+- completion of a call to a library I/O function, a synchronization
+  operation, or an atomic operation.
+An invocation of a standard library function that blocks (
+[[defns.block]]) is considered to continuously execute execution steps
+while waiting for the condition that it blocks on to be satisfied.
+[*Example 1*: A library I/O function that blocks until the I/O
+operation is complete can be considered to continuously check whether
+the operation is complete. Each such check might consist of one or more
+execution steps, for example using observable behavior of the abstract
+machine. — *end example*]
+[*Note 4*: Because of this and the preceding requirement regarding what
+threads of execution have to perform eventually, it follows that no
+thread of execution can execute forever without an execution step
+occurring. — *end note*]
+A thread of execution *makes progress* when an execution step occurs or
+a lock-free execution does not complete because there are other
+concurrent threads that are not blocked in a standard library function
+(see above).
+For a thread of execution providing *concurrent forward progress
+guarantees*, the implementation ensures that the thread will eventually
+make progress for as long as it has not terminated.
+[*Note 5*: This is required regardless of whether or not other threads
+of executions (if any) have been or are making progress. To eventually
+fulfill this requirement means that this will happen in an unspecified
+but finite amount of time. — *end note*]
+It is *implementation-defined* whether the implementation-created thread
+of execution that executes `main` ([[basic.start.main]]) and the
+threads of execution created by `std::thread` ([[thread.thread.class]])
+provide concurrent forward progress guarantees.
+[*Note 6*: General-purpose implementations are encouraged to provide
+these guarantees. — *end note*]
+For a thread of execution providing *parallel forward progress
+guarantees*, the implementation is not required to ensure that the
+thread will eventually make progress if it has not yet executed any
+execution step; once this thread has executed a step, it provides
+concurrent forward progress guarantees.
+[*Note 7*: This does not specify a requirement for when to start this
+thread of execution, which will typically be specified by the entity
+that creates this thread of execution. For example, a thread of
+execution that provides concurrent forward progress guarantees and
+executes tasks from a set of tasks in an arbitrary order, one after the
+other, satisfies the requirements of parallel forward progress for these
+tasks. — *end note*]
+For a thread of execution providing *weakly parallel forward progress
+guarantees*, the implementation does not ensure that the thread will
+eventually make progress.
+[*Note 8*: Threads of execution providing weakly parallel forward
+progress guarantees cannot be expected to make progress regardless of
+whether other threads make progress or not; however, blocking with
+forward progress guarantee delegation, as defined below, can be used to
+ensure that such threads of execution make progress
+eventually. — *end note*]
+Concurrent forward progress guarantees are stronger than parallel
+forward progress guarantees, which in turn are stronger than weakly
+parallel forward progress guarantees.
+[*Note 9*: For example, some kinds of synchronization between threads
+of execution may only make progress if the respective threads of
+execution provide parallel forward progress guarantees, but will fail to
+make progress under weakly parallel guarantees. — *end note*]
+When a thread of execution *P* is specified to *block with forward
+progress guarantee delegation* on the completion of a set *S* of threads
+of execution, then throughout the whole time of *P* being blocked on
+*S*, the implementation shall ensure that the forward progress
+guarantees provided by at least one thread of execution in *S* is at
+least as strong as *P*’s forward progress guarantees.
+[*Note 10*: It is unspecified which thread or threads of execution in
+*S* are chosen and for which number of execution steps. The
+strengthening is not permanent and not necessarily in place for the rest
+of the lifetime of the affected thread of execution. As long as *P* is
+blocked, the implementation has to eventually select and potentially
+strengthen a thread of execution in *S*. — *end note*]
+Once a thread of execution in *S* terminates, it is removed from *S*.
+Once *S* is empty, *P* is unblocked.
+[*Note 11*: A thread of execution *B* thus can temporarily provide an
+effectively stronger forward progress guarantee for a certain amount of
+time, due to a second thread of execution *A* being blocked on it with
+forward progress guarantee delegation. In turn, if *B* then blocks with
+forward progress guarantee delegation on *C*, this may also temporarily
+provide a stronger forward progress guarantee to *C*. — *end note*]
+[*Note 12*: If all threads of execution in *S* finish executing (e.g.,
+they terminate and do not use blocking synchronization incorrectly),
+then *P*’s execution of the operation that blocks with forward progress
+guarantee delegation will not result in *P*’s progress guarantee being
+effectively weakened. — *end note*]
+[*Note 13*: This does not remove any constraints regarding blocking
+synchronization for threads of execution providing parallel or weakly
+parallel forward progress guarantees because the implementation is not
+required to strengthen a particular thread of execution whose too-weak
+progress guarantee is preventing overall progress. — *end note*]
 An implementation should ensure that the last value (in modification
 order) assigned by an atomic or synchronization operation will become
 visible to all other threads in a finite period of time.
 ## Acknowledgments <a id="intro.ack">[[intro.ack]]</a>
+The C++programming language as described in this document is based on
+the language as described in Chapter R (Reference Manual) of Stroustrup:
+*The C++Programming Language* (second edition, Addison-Wesley Publishing
+Company, ISBN 0-201-53992-6, copyright ©1991 AT&T). That, in turn, is
+based on the C programming language as described in Appendix A of
+Kernighan and Ritchie: *The C Programming Language* (Prentice-Hall,
+1978, ISBN 0-13-110163-3, copyright ©1978 AT&T).
+Portions of the library Clauses of this document are based on work by
+P.J. Plauger, which was published as *The Draft Standard C++Library*
+(Prentice-Hall, ISBN 0-13-117003-1, copyright ©1995 P.J. Plauger).
 POSIX® is a registered trademark of the Institute of Electrical and
 Electronic Engineers, Inc.
+ECMAScript® is a registered trademark of Ecma International.
 All rights in these originals are reserved.
 <!-- Link reference definitions -->
 [atomics]: atomics.md#atomics
 [atomics.flag]: atomics.md#atomics.flag
 [basic.def.odr]: basic.md#basic.def.odr
 [basic.life]: basic.md#basic.life
 [basic.link]: basic.md#basic.link
 [basic.lval]: basic.md#basic.lval
 [basic.namespace]: dcl.md#basic.namespace
+[basic.start.main]: basic.md#basic.start.main
 [basic.stc]: basic.md#basic.stc
 [basic.stc.auto]: basic.md#basic.stc.auto
 [basic.types]: basic.md#basic.types
 [class.access]: class.md#class.access
+[class.base.init]: special.md#class.base.init
 [class.bit]: class.md#class.bit
+[class.cdtor]: special.md#class.cdtor
 [class.conv.fct]: special.md#class.conv.fct
 [class.derived]: class.md#class.derived
 [class.mem]: class.md#class.mem
 [class.temporary]: special.md#class.temporary
+[class.union]: class.md#class.union
 [class.virtual]: class.md#class.virtual
 [compliance]: library.md#compliance
+[conv.rval]: conv.md#conv.rval
 [cpp]: cpp.md#cpp
 [cpp.include]: cpp.md#cpp.include
+[cpp.replace]: cpp.md#cpp.replace
+[cstddef.syn]: language.md#cstddef.syn
+[dcl.decl]: dcl.md#dcl.decl
 [dcl.fct]: dcl.md#dcl.fct
 [dcl.fct.default]: dcl.md#dcl.fct.default
+[dcl.init.aggr]: dcl.md#dcl.init.aggr
 [dcl.ptr]: dcl.md#dcl.ptr
 [dcl.ref]: dcl.md#dcl.ref
 [definitions]: library.md#definitions
+[defns.block]: #defns.block
+[defns.well.formed]: #defns.well.formed
 [depr]: future.md#depr
 [diff]: compatibility.md#diff
 [diff.library]: compatibility.md#diff.library
 [expr]: expr.md#expr
+[expr.call]: expr.md#expr.call
 [expr.comma]: expr.md#expr.comma
 [expr.cond]: expr.md#expr.cond
+[expr.const]: expr.md#expr.const
 [expr.log.and]: expr.md#expr.log.and
 [expr.log.or]: expr.md#expr.log.or
 [expr.new]: expr.md#expr.new
+[expr.prim.lambda]: expr.md#expr.prim.lambda
+[expr.throw]: expr.md#expr.throw
 [gram]: grammar.md#gram
 [implimits]: limits.md#implimits
 [intro]: #intro
 [intro.ack]: #intro.ack
 [intro.compliance]: #intro.compliance
 [intro.defs]: #intro.defs
 [intro.execution]: #intro.execution
 [intro.memory]: #intro.memory
 [intro.multithread]: #intro.multithread
 [intro.object]: #intro.object
+[intro.progress]: #intro.progress
+[intro.races]: #intro.races
 [intro.refs]: #intro.refs
 [intro.scope]: #intro.scope
 [intro.structure]: #intro.structure
 [language.support]: language.md#language.support
 [lex]: lex.md#lex
 [lex.charset]: lex.md#lex.charset
 [lex.phases]: lex.md#lex.phases
 [library]: library.md#library
 [syntax]: #syntax
+[temp.arg]: temp.md#temp.arg
+[temp.deduct]: temp.md#temp.deduct
 [thread]: thread.md#thread
+[thread.thread.class]: thread.md#thread.thread.class
 [^1]: With the qualifications noted in Clauses  [[language.support]]
     through  [[thread]] and in  [[diff.library]], the C standard library
     is a subset of the C++standard library.
 [^2]: “Correct execution” can include undefined behavior, depending on
+    the data being processed; see Clause  [[intro.defs]] and
     [[intro.execution]].
 [^3]: This documentation also defines implementation-defined behavior;
     see  [[intro.execution]].
+[^4]: The number of bits in a byte is reported by the macro `CHAR_BIT`
+    in the header `<climits>`.
+[^5]: Under the “as-if” rule an implementation is allowed to store two
     objects at the same machine address or not store an object at all if
     the program cannot observe the difference ([[intro.execution]]).
+[^6]: This provision is sometimes called the “as-if” rule, because an
     implementation is free to disregard any requirement of this
     International Standard as long as the result is *as if* the
     requirement had been obeyed, as far as can be determined from the
     observable behavior of the program. For instance, an actual
     implementation need not evaluate part of an expression if it can
     deduce that its value is not used and that no side effects affecting
     the observable behavior of the program are produced.
+[^7]: This documentation also includes conditionally-supported
     constructs and locale-specific behavior. See  [[intro.compliance]].
+[^8]: Overloaded operators are never assumed to be associative or
     commutative.
+[^9]: As specified in  [[class.temporary]], after a full-expression is
     evaluated, a sequence of zero or more invocations of destructor
     functions for temporary objects takes place, usually in reverse
     order of the construction of each temporary object.
+[^10]: In other words, function executions do not interleave with each
     other.
+[^11]: An object with automatic or thread storage duration (
     [[basic.stc]]) is associated with one specific thread, and can be
     accessed by a different thread only indirectly through a pointer or
     reference ([[basic.compound]]).

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