[special] - C++17 → C++20

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@@ -1,37 +1,37 @@
-# Special member functions <a id="special">[[special]]</a>
-The default constructor ([[class.ctor]]), copy constructor and copy
-assignment operator ([[class.copy]]), move constructor and move
-assignment operator ([[class.copy]]), and destructor ([[class.dtor]])
-are *special member functions*.
 [*Note 1*: The implementation will implicitly declare these member
 functions for some class types when the program does not explicitly
 declare them. The implementation will implicitly define them if they are
-odr-used ([[basic.def.odr]]). See  [[class.ctor]], [[class.dtor]] and
-[[class.copy]]. — *end note*]
 An implicitly-declared special member function is declared at the
 closing `}` of the *class-specifier*. Programs shall not define
 implicitly-declared special member functions.
 Programs may explicitly refer to implicitly-declared special member
 functions.
 [*Example 1*:
-A program may explicitly call, take the address of, or form a pointer to
-member to an implicitly-declared special member function.
 ``` cpp
 struct A { };                   // implicitly declared A::operator=
 struct B : A {
   B& operator=(const B &);
 };
 B& B::operator=(const B& s) {
-  this->A::operator=(s);        // well formed
   return *this;
 }
 ```
 — *end example*]
@@ -39,2606 +39,37 @@ B& B::operator=(const B& s) {
 [*Note 2*: The special member functions affect the way objects of class
 type are created, copied, moved, and destroyed, and how values can be
 converted to values of other types. Often such special member functions
 are called implicitly. — *end note*]
-Special member functions obey the usual access rules (Clause
-[[class.access]]).
-[*Example 2*: Declaring a constructor `protected` ensures that only
 derived classes and friends can create objects using
 it. — *end example*]
 For a class, its non-static data members, its non-virtual direct base
-classes, and, if the class is not abstract ([[class.abstract]]), its
 virtual base classes are called its *potentially constructed
 subobjects*.
-## Constructors <a id="class.ctor">[[class.ctor]]</a>
-Constructors do not have names. In a declaration of a constructor, the
-*declarator* is a function declarator ([[dcl.fct]]) of the form
-``` bnf
-ptr-declarator '(' parameter-declaration-clause ')' noexcept-specifierₒₚₜ attribute-specifier-seqₒₚₜ
-```
-where the *ptr-declarator* consists solely of an *id-expression*, an
-optional *attribute-specifier-seq*, and optional surrounding
-parentheses, and the *id-expression* has one of the following forms:
-- in a *member-declaration* that belongs to the *member-specification*
-  of a class but is not a friend declaration ([[class.friend]]), the
-  *id-expression* is the injected-class-name (Clause  [[class]]) of the
-  immediately-enclosing class;
-- in a *member-declaration* that belongs to the *member-specification*
-  of a class template but is not a friend declaration, the
-  *id-expression* is a *class-name* that names the current
-  instantiation ([[temp.dep.type]]) of the immediately-enclosing class
-  template; or
-- in a declaration at namespace scope or in a friend declaration, the
-  *id-expression* is a *qualified-id* that names a constructor (
-  [[class.qual]]).
-The *class-name* shall not be a *typedef-name*. In a constructor
-declaration, each *decl-specifier* in the optional *decl-specifier-seq*
-shall be `friend`, `inline`, `explicit`, or `constexpr`.
-[*Example 1*:
-``` cpp
-struct S {
-  S();              // declares the constructor
-};
-S::S() { }          // defines the constructor
-```
-— *end example*]
-A constructor is used to initialize objects of its class type. Because
-constructors do not have names, they are never found during name lookup;
-however an explicit type conversion using the functional notation (
-[[expr.type.conv]]) will cause a constructor to be called to initialize
-an object.
-[*Note 1*: For initialization of objects of class type see
-[[class.init]]. — *end note*]
-A constructor can be invoked for a `const`, `volatile` or `const`
-`volatile` object. `const` and `volatile` semantics ([[dcl.type.cv]])
-are not applied on an object under construction. They come into effect
-when the constructor for the most derived object ([[intro.object]])
-ends.
-A *default* constructor for a class `X` is a constructor of class `X`
-for which each parameter that is not a function parameter pack has a
-default argument (including the case of a constructor with no
-parameters). If there is no user-declared constructor for class `X`, a
-non-explicit constructor having no parameters is implicitly declared as
-defaulted ([[dcl.fct.def]]). An implicitly-declared default constructor
-is an `inline` `public` member of its class.
-A defaulted default constructor for class `X` is defined as deleted if:
-- `X` is a union that has a variant member with a non-trivial default
-  constructor and no variant member of `X` has a default member
-  initializer,
-- `X` is a non-union class that has a variant member `M` with a
-  non-trivial default constructor and no variant member of the anonymous
-  union containing `M` has a default member initializer,
-- any non-static data member with no default member initializer (
-  [[class.mem]]) is of reference type,
-- any non-variant non-static data member of const-qualified type (or
-  array thereof) with no *brace-or-equal-initializer* does not have a
-  user-provided default constructor,
-- `X` is a union and all of its variant members are of const-qualified
-  type (or array thereof),
-- `X` is a non-union class and all members of any anonymous union member
-  are of const-qualified type (or array thereof),
-- any potentially constructed subobject, except for a non-static data
-  member with a *brace-or-equal-initializer*, has class type `M` (or
-  array thereof) and either `M` has no default constructor or overload
-  resolution ([[over.match]]) as applied to find `M`’s corresponding
-  constructor results in an ambiguity or in a function that is deleted
-  or inaccessible from the defaulted default constructor, or
-- any potentially constructed subobject has a type with a destructor
-  that is deleted or inaccessible from the defaulted default
-  constructor.
-A default constructor is *trivial* if it is not user-provided and if:
-- its class has no virtual functions ([[class.virtual]]) and no virtual
-  base classes ([[class.mi]]), and
-- no non-static data member of its class has a default member
-  initializer ([[class.mem]]), and
-- all the direct base classes of its class have trivial default
-  constructors, and
-- for all the non-static data members of its class that are of class
-  type (or array thereof), each such class has a trivial default
-  constructor.
-Otherwise, the default constructor is *non-trivial*.
-A default constructor that is defaulted and not defined as deleted is
-*implicitly defined* when it is odr-used ([[basic.def.odr]]) to create
-an object of its class type ([[intro.object]]) or when it is explicitly
-defaulted after its first declaration. The implicitly-defined default
-constructor performs the set of initializations of the class that would
-be performed by a user-written default constructor for that class with
-no *ctor-initializer* ([[class.base.init]]) and an empty
-*compound-statement*. If that user-written default constructor would be
-ill-formed, the program is ill-formed. If that user-written default
-constructor would satisfy the requirements of a constexpr constructor (
-[[dcl.constexpr]]), the implicitly-defined default constructor is
-`constexpr`. Before the defaulted default constructor for a class is
-implicitly defined, all the non-user-provided default constructors for
-its base classes and its non-static data members shall have been
-implicitly defined.
-[*Note 2*: An implicitly-declared default constructor has an exception
-specification ([[except.spec]]). An explicitly-defaulted definition
-might have an implicit exception specification, see
-[[dcl.fct.def]]. — *end note*]
-Default constructors are called implicitly to create class objects of
-static, thread, or automatic storage duration ([[basic.stc.static]],
-[[basic.stc.thread]], [[basic.stc.auto]]) defined without an
-initializer ([[dcl.init]]), are called to create class objects of
-dynamic storage duration ([[basic.stc.dynamic]]) created by a
-*new-expression* in which the *new-initializer* is omitted (
-[[expr.new]]), or are called when the explicit type conversion syntax (
-[[expr.type.conv]]) is used. A program is ill-formed if the default
-constructor for an object is implicitly used and the constructor is not
-accessible (Clause  [[class.access]]).
-[*Note 3*:  [[class.base.init]] describes the order in which
-constructors for base classes and non-static data members are called and
-describes how arguments can be specified for the calls to these
-constructors. — *end note*]
-A `return` statement in the body of a constructor shall not specify a
-return value. The address of a constructor shall not be taken.
-A functional notation type conversion ([[expr.type.conv]]) can be used
-to create new objects of its type.
-[*Note 4*: The syntax looks like an explicit call of the
-constructor. — *end note*]
-[*Example 2*:
-``` cpp
-complex zz = complex(1,2.3);
-cprint( complex(7.8,1.2) );
-```
-— *end example*]
-An object created in this way is unnamed.
-[*Note 5*:  [[class.temporary]] describes the lifetime of temporary
-objects. — *end note*]
-[*Note 6*: Explicit constructor calls do not yield lvalues, see
-[[basic.lval]]. — *end note*]
-[*Note 7*:  Some language constructs have special semantics when used
-during construction; see  [[class.base.init]] and
-[[class.cdtor]]. — *end note*]
-During the construction of an object, if the value of the object or any
-of its subobjects is accessed through a glvalue that is not obtained,
-directly or indirectly, from the constructor’s `this` pointer, the value
-of the object or subobject thus obtained is unspecified.
-[*Example 3*:
-``` cpp
-struct C;
-void no_opt(C*);
-struct C {
-  int c;
-  C() : c(0) { no_opt(this); }
-};
-const C cobj;
-void no_opt(C* cptr) {
-  int i = cobj.c * 100;         // value of cobj.c is unspecified
-  cptr->c = 1;
-  cout << cobj.c * 100          // value of cobj.c is unspecified
-       << '\n';
-}
-extern struct D d;
-struct D {
-  D(int a) : a(a), b(d.a) {}
-  int a, b;
-};
-D d = D(1);                     // value of d.b is unspecified
-```
-— *end example*]
-## Temporary objects <a id="class.temporary">[[class.temporary]]</a>
-Temporary objects are created
-- when a prvalue is materialized so that it can be used as a glvalue (
-  [[conv.rval]]),
-- when needed by the implementation to pass or return an object of
-  trivially-copyable type (see below), and
-- when throwing an exception ([[except.throw]]). \[*Note 1*: The
-  lifetime of exception objects is described in
-  [[except.throw]]. — *end note*]
-Even when the creation of the temporary object is unevaluated (Clause
-[[expr]]), all the semantic restrictions shall be respected as if the
-temporary object had been created and later destroyed.
-[*Note 2*: This includes accessibility (Clause  [[class.access]]) and
-whether it is deleted, for the constructor selected and for the
-destructor. However, in the special case of the operand of a
-*decltype-specifier* ([[expr.call]]), no temporary is introduced, so
-the foregoing does not apply to such a prvalue. — *end note*]
-The materialization of a temporary object is generally delayed as long
-as possible in order to avoid creating unnecessary temporary objects.
-[*Note 3*:
-Temporary objects are materialized:
-- when binding a reference to a prvalue ([[dcl.init.ref]],
-  [[expr.type.conv]], [[expr.dynamic.cast]], [[expr.static.cast]],
-  [[expr.const.cast]], [[expr.cast]]),
-- when performing member access on a class prvalue ([[expr.ref]],
-  [[expr.mptr.oper]]),
-- when performing an array-to-pointer conversion or subscripting on an
-  array prvalue ([[conv.array]], [[expr.sub]]),
-- when initializing an object of type `std::initializer_list<T>` from a
-  *braced-init-list* ([[dcl.init.list]]),
-- for certain unevaluated operands ([[expr.typeid]], [[expr.sizeof]]),
-  and
-- when a prvalue appears as a discarded-value expression (Clause
-  [[expr]]).
-— *end note*]
-[*Example 1*:
-Consider the following code:
-``` cpp
-class X {
-public:
-  X(int);
-  X(const X&);
-  X& operator=(const X&);
-  ~X();
-};
-class Y {
-public:
-  Y(int);
-  Y(Y&&);
-  ~Y();
-};
-X f(X);
-Y g(Y);
-void h() {
-  X a(1);
-  X b = f(X(2));
-  Y c = g(Y(3));
-  a = f(a);
-}
-```
-`X(2)` is constructed in the space used to hold `f()`’s argument and
-`Y(3)` is constructed in the space used to hold `g()`’s argument.
-Likewise, `f()`’s result is constructed directly in `b` and `g()`’s
-result is constructed directly in `c`. On the other hand, the expression
-`a = f(a)` requires a temporary for the result of `f(a)`, which is
-materialized so that the reference parameter of `A::operator=(const A&)`
-can bind to it.
-— *end example*]
-When an object of class type `X` is passed to or returned from a
-function, if each copy constructor, move constructor, and destructor of
-`X` is either trivial or deleted, and `X` has at least one non-deleted
-copy or move constructor, implementations are permitted to create a
-temporary object to hold the function parameter or result object. The
-temporary object is constructed from the function argument or return
-value, respectively, and the function’s parameter or return object is
-initialized as if by using the non-deleted trivial constructor to copy
-the temporary (even if that constructor is inaccessible or would not be
-selected by overload resolution to perform a copy or move of the
-object).
-[*Note 4*: This latitude is granted to allow objects of class type to
-be passed to or returned from functions in registers. — *end note*]
-When an implementation introduces a temporary object of a class that has
-a non-trivial constructor ([[class.ctor]], [[class.copy]]), it shall
-ensure that a constructor is called for the temporary object. Similarly,
-the destructor shall be called for a temporary with a non-trivial
-destructor ([[class.dtor]]). Temporary objects are destroyed as the
-last step in evaluating the full-expression ([[intro.execution]]) that
-(lexically) contains the point where they were created. This is true
-even if that evaluation ends in throwing an exception. The value
-computations and side effects of destroying a temporary object are
-associated only with the full-expression, not with any specific
-subexpression.
-There are three contexts in which temporaries are destroyed at a
-different point than the end of the full-expression. The first context
-is when a default constructor is called to initialize an element of an
-array with no corresponding initializer ([[dcl.init]]). The second
-context is when a copy constructor is called to copy an element of an
-array while the entire array is copied ([[expr.prim.lambda.capture]],
-[[class.copy]]). In either case, if the constructor has one or more
-default arguments, the destruction of every temporary created in a
-default argument is sequenced before the construction of the next array
-element, if any.
-The third context is when a reference is bound to a temporary.[^1] The
-temporary to which the reference is bound or the temporary that is the
-complete object of a subobject to which the reference is bound persists
-for the lifetime of the reference except:
-- A temporary object bound to a reference parameter in a function call (
-  [[expr.call]]) persists until the completion of the full-expression
-  containing the call.
-- The lifetime of a temporary bound to the returned value in a function
-  return statement ([[stmt.return]]) is not extended; the temporary is
-  destroyed at the end of the full-expression in the return statement.
-- A temporary bound to a reference in a *new-initializer* (
-  [[expr.new]]) persists until the completion of the full-expression
-  containing the *new-initializer*.
-  \[*Example 2*:
-  ``` cpp
-  struct S { int mi; const std::pair<int,int>& mp; };
-  S a { 1, {2,3} };
-  S* p = new S{ 1, {2,3} };   // Creates dangling reference
-  ```
-  — *end example*]
-  \[*Note 5*: This may introduce a dangling reference, and
-  implementations are encouraged to issue a warning in such a
-  case. — *end note*]
-The destruction of a temporary whose lifetime is not extended by being
-bound to a reference is sequenced before the destruction of every
-temporary which is constructed earlier in the same full-expression. If
-the lifetime of two or more temporaries to which references are bound
-ends at the same point, these temporaries are destroyed at that point in
-the reverse order of the completion of their construction. In addition,
-the destruction of temporaries bound to references shall take into
-account the ordering of destruction of objects with static, thread, or
-automatic storage duration ([[basic.stc.static]], [[basic.stc.thread]],
-[[basic.stc.auto]]); that is, if `obj1` is an object with the same
-storage duration as the temporary and created before the temporary is
-created the temporary shall be destroyed before `obj1` is destroyed; if
-`obj2` is an object with the same storage duration as the temporary and
-created after the temporary is created the temporary shall be destroyed
-after `obj2` is destroyed.
-[*Example 3*:
-``` cpp
-struct S {
-  S();
-  S(int);
-  friend S operator+(const S&, const S&);
-  ~S();
-};
-S obj1;
-const S& cr = S(16)+S(23);
-S obj2;
-```
-the expression `S(16) + S(23)` creates three temporaries: a first
-temporary `T1` to hold the result of the expression `S(16)`, a second
-temporary `T2` to hold the result of the expression `S(23)`, and a third
-temporary `T3` to hold the result of the addition of these two
-expressions. The temporary `T3` is then bound to the reference `cr`. It
-is unspecified whether `T1` or `T2` is created first. On an
-implementation where `T1` is created before `T2`, `T2` shall be
-destroyed before `T1`. The temporaries `T1` and `T2` are bound to the
-reference parameters of `operator+`; these temporaries are destroyed at
-the end of the full-expression containing the call to `operator+`. The
-temporary `T3` bound to the reference `cr` is destroyed at the end of
-`cr`’s lifetime, that is, at the end of the program. In addition, the
-order in which `T3` is destroyed takes into account the destruction
-order of other objects with static storage duration. That is, because
-`obj1` is constructed before `T3`, and `T3` is constructed before
-`obj2`, `obj2` shall be destroyed before `T3`, and `T3` shall be
-destroyed before `obj1`.
-— *end example*]
-## Conversions <a id="class.conv">[[class.conv]]</a>
-Type conversions of class objects can be specified by constructors and
-by conversion functions. These conversions are called *user-defined
-conversions* and are used for implicit type conversions (Clause
-[[conv]]), for initialization ([[dcl.init]]), and for explicit type
-conversions ([[expr.cast]], [[expr.static.cast]]).
-User-defined conversions are applied only where they are unambiguous (
-[[class.member.lookup]], [[class.conv.fct]]). Conversions obey the
-access control rules (Clause  [[class.access]]). Access control is
-applied after ambiguity resolution ([[basic.lookup]]).
-[*Note 1*: See  [[over.match]] for a discussion of the use of
-conversions in function calls as well as examples below. — *end note*]
-At most one user-defined conversion (constructor or conversion function)
-is implicitly applied to a single value.
-[*Example 1*:
-``` cpp
-struct X {
-  operator int();
-};
-struct Y {
-  operator X();
-};
-Y a;
-int b = a;          // error, a.operator X().operator int() not tried
-int c = X(a);       // OK: a.operator X().operator int()
-```
-— *end example*]
-User-defined conversions are used implicitly only if they are
-unambiguous. A conversion function in a derived class does not hide a
-conversion function in a base class unless the two functions convert to
-the same type. Function overload resolution ([[over.match.best]])
-selects the best conversion function to perform the conversion.
-[*Example 2*:
-``` cpp
-struct X {
-  operator int();
-};
-struct Y : X {
-    operator char();
-};
-void f(Y& a) {
-  if (a) {          // ill-formed: X::operator int() or Y::operator char()
-  }
-}
-```
-— *end example*]
-### Conversion by constructor <a id="class.conv.ctor">[[class.conv.ctor]]</a>
-A constructor declared without the *function-specifier* `explicit`
-specifies a conversion from the types of its parameters (if any) to the
-type of its class. Such a constructor is called a *converting
-constructor*.
-[*Example 1*:
-``` cpp
-struct X {
-    X(int);
-    X(const char*, int =0);
-    X(int, int);
-};
-void f(X arg) {
-  X a = 1;          // a = X(1)
-  X b = "Jessie";   // b = X("Jessie",0)
-  a = 2;            // a = X(2)
-  f(3);             // f(X(3))
-  f({1, 2});        // f(X(1,2))
-}
-```
-— *end example*]
-[*Note 1*:
-An explicit constructor constructs objects just like non-explicit
-constructors, but does so only where the direct-initialization syntax (
-[[dcl.init]]) or where casts ([[expr.static.cast]], [[expr.cast]]) are
-explicitly used; see also  [[over.match.copy]]. A default constructor
-may be an explicit constructor; such a constructor will be used to
-perform default-initialization or value-initialization ([[dcl.init]]).
-[*Example 2*:
-``` cpp
-struct Z {
-  explicit Z();
-  explicit Z(int);
-  explicit Z(int, int);
-};
-Z a;                            // OK: default-initialization performed
-Z b{};                          // OK: direct initialization syntax used
-Z c = {};                       // error: copy-list-initialization
-Z a1 = 1;                       // error: no implicit conversion
-Z a3 = Z(1);                    // OK: direct initialization syntax used
-Z a2(1);                        // OK: direct initialization syntax used
-Z* p = new Z(1);                // OK: direct initialization syntax used
-Z a4 = (Z)1;                    // OK: explicit cast used
-Z a5 = static_cast<Z>(1);       // OK: explicit cast used
-Z a6 = { 3, 4 };                // error: no implicit conversion
-```
-— *end example*]
-— *end note*]
-A non-explicit copy/move constructor ([[class.copy]]) is a converting
-constructor.
-[*Note 2*: An implicitly-declared copy/move constructor is not an
-explicit constructor; it may be called for implicit type
-conversions. — *end note*]
-### Conversion functions <a id="class.conv.fct">[[class.conv.fct]]</a>
-A member function of a class `X` having no parameters with a name of the
-form
-``` bnf
-conversion-function-id:
-    'operator' conversion-type-id
-```
-``` bnf
-conversion-type-id:
-    type-specifier-seq conversion-declaratorₒₚₜ
-```
-``` bnf
-conversion-declarator:
-    ptr-operator conversion-declaratorₒₚₜ
-```
-specifies a conversion from `X` to the type specified by the
-*conversion-type-id*. Such functions are called *conversion functions*.
-A *decl-specifier* in the *decl-specifier-seq* of a conversion function
-(if any) shall be neither a *defining-type-specifier* nor `static`. The
-type of the conversion function ([[dcl.fct]]) is “function taking no
-parameter returning *conversion-type-id*”. A conversion function is
-never used to convert a (possibly cv-qualified) object to the (possibly
-cv-qualified) same object type (or a reference to it), to a (possibly
-cv-qualified) base class of that type (or a reference to it), or to
-(possibly cv-qualified) void.[^2]
-[*Example 1*:
-``` cpp
-struct X {
-  operator int();
-  operator auto() -> short;     // error: trailing return type
-};
-void f(X a) {
-  int i = int(a);
-  i = (int)a;
-  i = a;
-}
-```
-In all three cases the value assigned will be converted by
-`X::operator int()`.
-— *end example*]
-A conversion function may be explicit ([[dcl.fct.spec]]), in which case
-it is only considered as a user-defined conversion for
-direct-initialization ([[dcl.init]]). Otherwise, user-defined
-conversions are not restricted to use in assignments and
-initializations.
-[*Example 2*:
-``` cpp
-class Y { };
-struct Z {
-  explicit operator Y() const;
-};
-void h(Z z) {
-  Y y1(z);          // OK: direct-initialization
-  Y y2 = z;         // ill-formed: copy-initialization
-  Y y3 = (Y)z;      // OK: cast notation
-}
-void g(X a, X b) {
-  int i = (a) ? 1+a : 0;
-  int j = (a&&b) ? a+b : i;
-  if (a) {
-  }
-}
-```
-— *end example*]
-The *conversion-type-id* shall not represent a function type nor an
-array type. The *conversion-type-id* in a *conversion-function-id* is
-the longest sequence of tokens that could possibly form a
-*conversion-type-id*.
-[*Note 1*:
-This prevents ambiguities between the declarator operator `*` and its
-expression counterparts.
-[*Example 3*:
-``` cpp
-&ac.operator int*i; // syntax error:
-                    // parsed as: &(ac.operator int *)i
-                    // not as: &(ac.operator int)*i
-```
-The `*` is the pointer declarator and not the multiplication operator.
-— *end example*]
-This rule also prevents ambiguities for attributes.
-[*Example 4*:
-``` cpp
-operator int [[noreturn]] ();   // error: noreturn attribute applied to a type
-```
-— *end example*]
-— *end note*]
-Conversion functions are inherited.
-Conversion functions can be virtual.
-A conversion function template shall not have a deduced return type (
-[[dcl.spec.auto]]).
-[*Example 5*:
-``` cpp
-struct S {
-  operator auto() const { return 10; }      // OK
-  template<class T>
-  operator auto() const { return 1.2; }     // error: conversion function template
-};
-```
-— *end example*]
-## Destructors <a id="class.dtor">[[class.dtor]]</a>
-In a declaration of a destructor, the *declarator* is a function
-declarator ([[dcl.fct]]) of the form
-``` bnf
-ptr-declarator '(' parameter-declaration-clause ')' noexcept-specifierₒₚₜ attribute-specifier-seqₒₚₜ
-```
-where the *ptr-declarator* consists solely of an *id-expression*, an
-optional *attribute-specifier-seq*, and optional surrounding
-parentheses, and the *id-expression* has one of the following forms:
-- in a *member-declaration* that belongs to the *member-specification*
-  of a class but is not a friend declaration ([[class.friend]]), the
-  *id-expression* is `~`*class-name* and the *class-name* is the
-  injected-class-name (Clause  [[class]]) of the immediately-enclosing
-  class;
-- in a *member-declaration* that belongs to the *member-specification*
-  of a class template but is not a friend declaration, the
-  *id-expression* is `~`*class-name* and the *class-name* names the
-  current instantiation ([[temp.dep.type]]) of the
-  immediately-enclosing class template; or
-- in a declaration at namespace scope or in a friend declaration, the
-  *id-expression* is *nested-name-specifier* `~`*class-name* and the
-  *class-name* names the same class as the *nested-name-specifier*.
-The *class-name* shall not be a *typedef-name*. A destructor shall take
-no arguments ([[dcl.fct]]). Each *decl-specifier* of the
-*decl-specifier-seq* of a destructor declaration (if any) shall be
-`friend`, `inline`, or `virtual`.
-A destructor is used to destroy objects of its class type. The address
-of a destructor shall not be taken. A destructor can be invoked for a
-`const`, `volatile` or `const` `volatile` object. `const` and `volatile`
-semantics ([[dcl.type.cv]]) are not applied on an object under
-destruction. They stop being in effect when the destructor for the most
-derived object ([[intro.object]]) starts.
-[*Note 1*: A declaration of a destructor that does not have a
-*noexcept-specifier* has the same exception specification as if had been
-implicitly declared ([[except.spec]]). — *end note*]
-If a class has no user-declared destructor, a destructor is implicitly
-declared as defaulted ([[dcl.fct.def]]). An implicitly-declared
-destructor is an `inline` `public` member of its class.
-A defaulted destructor for a class `X` is defined as deleted if:
-- `X` is a union-like class that has a variant member with a non-trivial
-  destructor,
-- any potentially constructed subobject has class type `M` (or array
-  thereof) and `M` has a deleted destructor or a destructor that is
-  inaccessible from the defaulted destructor,
-- or, for a virtual destructor, lookup of the non-array deallocation
-  function results in an ambiguity or in a function that is deleted or
-  inaccessible from the defaulted destructor.
-A destructor is trivial if it is not user-provided and if:
-- the destructor is not `virtual`,
-- all of the direct base classes of its class have trivial destructors,
-  and
-- for all of the non-static data members of its class that are of class
-  type (or array thereof), each such class has a trivial destructor.
-Otherwise, the destructor is *non-trivial*.
-A destructor that is defaulted and not defined as deleted is *implicitly
-defined* when it is odr-used ([[basic.def.odr]]) or when it is
-explicitly defaulted after its first declaration.
-Before the defaulted destructor for a class is implicitly defined, all
-the non-user-provided destructors for its base classes and its
-non-static data members shall have been implicitly defined.
-After executing the body of the destructor and destroying any automatic
-objects allocated within the body, a destructor for class `X` calls the
-destructors for `X`’s direct non-variant non-static data members, the
-destructors for `X`’s non-virtual direct base classes and, if `X` is the
-type of the most derived class ([[class.base.init]]), its destructor
-calls the destructors for `X`’s virtual base classes. All destructors
-are called as if they were referenced with a qualified name, that is,
-ignoring any possible virtual overriding destructors in more derived
-classes. Bases and members are destroyed in the reverse order of the
-completion of their constructor (see  [[class.base.init]]). A `return`
-statement ([[stmt.return]]) in a destructor might not directly return
-to the caller; before transferring control to the caller, the
-destructors for the members and bases are called. Destructors for
-elements of an array are called in reverse order of their construction
-(see  [[class.init]]).
-A destructor can be declared `virtual` ([[class.virtual]]) or pure
-`virtual` ([[class.abstract]]); if any objects of that class or any
-derived class are created in the program, the destructor shall be
-defined. If a class has a base class with a virtual destructor, its
-destructor (whether user- or implicitly-declared) is virtual.
-[*Note 2*:  Some language constructs have special semantics when used
-during destruction; see  [[class.cdtor]]. — *end note*]
-A destructor is invoked implicitly
-- for a constructed object with static storage duration (
-  [[basic.stc.static]]) at program termination ([[basic.start.term]]),
-- for a constructed object with thread storage duration (
-  [[basic.stc.thread]]) at thread exit,
-- for a constructed object with automatic storage duration (
-  [[basic.stc.auto]]) when the block in which an object is created
-  exits ([[stmt.dcl]]),
-- for a constructed temporary object when its lifetime ends (
-  [[conv.rval]], [[class.temporary]]).
-In each case, the context of the invocation is the context of the
-construction of the object. A destructor is also invoked implicitly
-through use of a *delete-expression* ([[expr.delete]]) for a
-constructed object allocated by a *new-expression* ([[expr.new]]); the
-context of the invocation is the *delete-expression*.
-[*Note 3*: An array of class type contains several subobjects for each
-of which the destructor is invoked. — *end note*]
-A destructor can also be invoked explicitly. A destructor is
-*potentially invoked* if it is invoked or as specified in  [[expr.new]],
-[[class.base.init]], and  [[except.throw]]. A program is ill-formed if a
-destructor that is potentially invoked is deleted or not accessible from
-the context of the invocation.
-At the point of definition of a virtual destructor (including an
-implicit definition ([[class.copy]])), the non-array deallocation
-function is determined as if for the expression `delete this` appearing
-in a non-virtual destructor of the destructor’s class (see
-[[expr.delete]]). If the lookup fails or if the deallocation function
-has a deleted definition ([[dcl.fct.def]]), the program is ill-formed.
-[*Note 4*: This assures that a deallocation function corresponding to
-the dynamic type of an object is available for the *delete-expression* (
-[[class.free]]). — *end note*]
-In an explicit destructor call, the destructor is specified by a `~`
-followed by a *type-name* or *decltype-specifier* that denotes the
-destructor’s class type. The invocation of a destructor is subject to
-the usual rules for member functions ([[class.mfct]]); that is, if the
-object is not of the destructor’s class type and not of a class derived
-from the destructor’s class type (including when the destructor is
-invoked via a null pointer value), the program has undefined behavior.
-[*Note 5*: Invoking `delete` on a null pointer does not call the
-destructor; see [[expr.delete]]. — *end note*]
-[*Example 1*:
-``` cpp
-struct B {
-  virtual ~B() { }
-};
-struct D : B {
-  ~D() { }
-};
-D D_object;
-typedef B B_alias;
-B* B_ptr = &D_object;
-void f() {
-  D_object.B::~B();             // calls B's destructor
-  B_ptr->~B();                  // calls D's destructor
-  B_ptr->~B_alias();            // calls D's destructor
-  B_ptr->B_alias::~B();         // calls B's destructor
-  B_ptr->B_alias::~B_alias();   // calls B's destructor
-}
-```
-— *end example*]
-[*Note 6*: An explicit destructor call must always be written using a
-member access operator ([[expr.ref]]) or a *qualified-id* (
-[[expr.prim]]); in particular, the *unary-expression* `~X()` in a member
-function is not an explicit destructor call (
-[[expr.unary.op]]). — *end note*]
-[*Note 7*:
-Explicit calls of destructors are rarely needed. One use of such calls
-is for objects placed at specific addresses using a placement
-*new-expression*. Such use of explicit placement and destruction of
-objects can be necessary to cope with dedicated hardware resources and
-for writing memory management facilities. For example,
-``` cpp
-void* operator new(std::size_t, void* p) { return p; }
-struct X {
-  X(int);
-  ~X();
-};
-void f(X* p);
-void g() {                      // rare, specialized use:
-  char* buf = new char[sizeof(X)];
-  X* p = new(buf) X(222);       // use buf[] and initialize
-  f(p);
-  p->X::~X();                   // cleanup
-}
-```
-— *end note*]
-Once a destructor is invoked for an object, the object no longer exists;
-the behavior is undefined if the destructor is invoked for an object
-whose lifetime has ended ([[basic.life]]).
-[*Example 2*: If the destructor for an automatic object is explicitly
-invoked, and the block is subsequently left in a manner that would
-ordinarily invoke implicit destruction of the object, the behavior is
-undefined. — *end example*]
-[*Note 8*:
-The notation for explicit call of a destructor can be used for any
-scalar type name ([[expr.pseudo]]). Allowing this makes it possible to
-write code without having to know if a destructor exists for a given
-type. For example:
-``` cpp
-typedef int I;
-I* p;
-p->I::~I();
-```
-— *end note*]
-## Free store <a id="class.free">[[class.free]]</a>
-Any allocation function for a class `T` is a static member (even if not
-explicitly declared `static`).
-[*Example 1*:
-``` cpp
-class Arena;
-struct B {
-  void* operator new(std::size_t, Arena*);
-};
-struct D1 : B {
-};
-Arena*  ap;
-void foo(int i) {
-  new (ap) D1;      // calls B::operator new(std::size_t, Arena*)
-  new D1[i];        // calls ::operator new[](std::size_t)
-  new D1;           // ill-formed: ::operator new(std::size_t) hidden
-}
-```
-— *end example*]
-When an object is deleted with a *delete-expression* ([[expr.delete]]),
-a deallocation function (`operator delete()` for non-array objects or
-`operator delete[]()` for arrays) is (implicitly) called to reclaim the
-storage occupied by the object ([[basic.stc.dynamic.deallocation]]).
-Class-specific deallocation function lookup is a part of general
-deallocation function lookup ([[expr.delete]]) and occurs as follows.
-If the *delete-expression* is used to deallocate a class object whose
-static type has a virtual destructor, the deallocation function is the
-one selected at the point of definition of the dynamic type’s virtual
-destructor ([[class.dtor]]).[^3] Otherwise, if the *delete-expression*
-is used to deallocate an object of class `T` or array thereof, the
-static and dynamic types of the object shall be identical and the
-deallocation function’s name is looked up in the scope of `T`. If this
-lookup fails to find the name, general deallocation function lookup (
-[[expr.delete]]) continues. If the result of the lookup is ambiguous or
-inaccessible, or if the lookup selects a placement deallocation
-function, the program is ill-formed.
-Any deallocation function for a class `X` is a static member (even if
-not explicitly declared `static`).
-[*Example 2*:
-``` cpp
-class X {
-  void operator delete(void*);
-  void operator delete[](void*, std::size_t);
-};
-class Y {
-  void operator delete(void*, std::size_t);
-  void operator delete[](void*);
-};
-```
-— *end example*]
-Since member allocation and deallocation functions are `static` they
-cannot be virtual.
-[*Note 1*:
-However, when the *cast-expression* of a *delete-expression* refers to
-an object of class type, because the deallocation function actually
-called is looked up in the scope of the class that is the dynamic type
-of the object, if the destructor is virtual, the effect is the same. For
-example,
-``` cpp
-struct B {
-  virtual ~B();
-  void operator delete(void*, std::size_t);
-};
-struct D : B {
-  void operator delete(void*);
-};
-void f() {
-  B* bp = new D;
-  delete bp;        // 1: uses D::operator delete(void*)
-}
-```
-Here, storage for the non-array object of class `D` is deallocated by
-`D::operator delete()`, due to the virtual destructor.
-— *end note*]
-[*Note 2*:
-Virtual destructors have no effect on the deallocation function actually
-called when the *cast-expression* of a *delete-expression* refers to an
-array of objects of class type. For example,
-``` cpp
-struct B {
-  virtual ~B();
-  void operator delete[](void*, std::size_t);
-};
-struct D : B {
-  void operator delete[](void*, std::size_t);
-};
-void f(int i) {
-  D* dp = new D[i];
-  delete [] dp;     // uses D::operator delete[](void*, std::size_t)
-  B* bp = new D[i];
-  delete[] bp;      // undefined behavior
-}
-```
-— *end note*]
-Access to the deallocation function is checked statically. Hence, even
-though a different one might actually be executed, the statically
-visible deallocation function is required to be accessible.
-[*Example 3*: For the call on line “// 1” above, if
-`B::operator delete()` had been `private`, the delete expression would
-have been ill-formed. — *end example*]
-[*Note 3*: If a deallocation function has no explicit
-*noexcept-specifier*, it has a non-throwing exception specification (
-[[except.spec]]). — *end note*]
-## Initialization <a id="class.init">[[class.init]]</a>
-When no initializer is specified for an object of (possibly
-cv-qualified) class type (or array thereof), or the initializer has the
-form `()`, the object is initialized as specified in  [[dcl.init]].
-An object of class type (or array thereof) can be explicitly
-initialized; see  [[class.expl.init]] and  [[class.base.init]].
-When an array of class objects is initialized (either explicitly or
-implicitly) and the elements are initialized by constructor, the
-constructor shall be called for each element of the array, following the
-subscript order; see  [[dcl.array]].
-[*Note 1*: Destructors for the array elements are called in reverse
-order of their construction. — *end note*]
-### Explicit initialization <a id="class.expl.init">[[class.expl.init]]</a>
-An object of class type can be initialized with a parenthesized
-*expression-list*, where the *expression-list* is construed as an
-argument list for a constructor that is called to initialize the object.
-Alternatively, a single *assignment-expression* can be specified as an
-*initializer* using the `=` form of initialization. Either
-direct-initialization semantics or copy-initialization semantics apply;
-see  [[dcl.init]].
-[*Example 1*:
-``` cpp
-struct complex {
-  complex();
-  complex(double);
-  complex(double,double);
-};
-complex sqrt(complex,complex);
-complex a(1);                   // initialize by a call of complex(double)
-complex b = a;                  // initialize by a copy of a
-complex c = complex(1,2);       // construct complex(1,2) using complex(double,double),
-                                // copy/move it into c
-complex d = sqrt(b,c);          // call sqrt(complex,complex) and copy/move the result into d
-complex e;                      // initialize by a call of complex()
-complex f = 3;                  // construct complex(3) using complex(double), copy/move it into f
-complex g = { 1, 2 };           // initialize by a call of complex(double, double)
-```
-— *end example*]
-[*Note 1*:  Overloading of the assignment operator ([[over.ass]]) has
-no effect on initialization. — *end note*]
-An object of class type can also be initialized by a *braced-init-list*.
-List-initialization semantics apply; see  [[dcl.init]] and
-[[dcl.init.list]].
-[*Example 2*:
-``` cpp
-complex v[6] = { 1, complex(1,2), complex(), 2 };
-```
-Here, `complex::complex(double)` is called for the initialization of
-`v[0]` and `v[3]`, `complex::complex({}double, double)` is called for
-the initialization of `v[1]`, `complex::complex()` is called for the
-initialization `v[2]`, `v[4]`, and `v[5]`. For another example,
-``` cpp
-struct X {
-  int i;
-  float f;
-  complex c;
-} x = { 99, 88.8, 77.7 };
-```
-Here, `x.i` is initialized with 99, `x.f` is initialized with 88.8, and
-`complex::complex(double)` is called for the initialization of `x.c`.
-— *end example*]
-[*Note 2*: Braces can be elided in the *initializer-list* for any
-aggregate, even if the aggregate has members of a class type with
-user-defined type conversions; see  [[dcl.init.aggr]]. — *end note*]
-[*Note 3*: If `T` is a class type with no default constructor, any
-declaration of an object of type `T` (or array thereof) is ill-formed if
-no *initializer* is explicitly specified (see  [[class.init]] and
-[[dcl.init]]). — *end note*]
-[*Note 4*:  The order in which objects with static or thread storage
-duration are initialized is described in  [[basic.start.dynamic]] and
-[[stmt.dcl]]. — *end note*]
-### Initializing bases and members <a id="class.base.init">[[class.base.init]]</a>
-In the definition of a constructor for a class, initializers for direct
-and virtual base class subobjects and non-static data members can be
-specified by a *ctor-initializer*, which has the form
-``` bnf
-ctor-initializer:
-    ':' mem-initializer-list
-```
-``` bnf
-mem-initializer-list:
-    mem-initializer '...'ₒₚₜ
-    mem-initializer-list ',' mem-initializer '...'ₒₚₜ
-```
-``` bnf
-mem-initializer:
-    mem-initializer-id '(' expression-listₒₚₜ ')'
-    mem-initializer-id braced-init-list
-```
-``` bnf
-mem-initializer-id:
-    class-or-decltype
-    identifier
-```
-In a *mem-initializer-id* an initial unqualified *identifier* is looked
-up in the scope of the constructor’s class and, if not found in that
-scope, it is looked up in the scope containing the constructor’s
-definition.
-[*Note 1*: If the constructor’s class contains a member with the same
-name as a direct or virtual base class of the class, a
-*mem-initializer-id* naming the member or base class and composed of a
-single identifier refers to the class member. A *mem-initializer-id* for
-the hidden base class may be specified using a qualified
-name. — *end note*]
-Unless the *mem-initializer-id* names the constructor’s class, a
-non-static data member of the constructor’s class, or a direct or
-virtual base of that class, the *mem-initializer* is ill-formed.
-A *mem-initializer-list* can initialize a base class using any
-*class-or-decltype* that denotes that base class type.
-[*Example 1*:
-``` cpp
-struct A { A(); };
-typedef A global_A;
-struct B { };
-struct C: public A, public B { C(); };
-C::C(): global_A() { }          // mem-initializer for base A
-```
-— *end example*]
-If a *mem-initializer-id* is ambiguous because it designates both a
-direct non-virtual base class and an inherited virtual base class, the
-*mem-initializer* is ill-formed.
-[*Example 2*:
-``` cpp
-struct A { A(); };
-struct B: public virtual A { };
-struct C: public A, public B { C(); };
-C::C(): A() { }                 // ill-formed: which A?
-```
-— *end example*]
-A *ctor-initializer* may initialize a variant member of the
-constructor’s class. If a *ctor-initializer* specifies more than one
-*mem-initializer* for the same member or for the same base class, the
-*ctor-initializer* is ill-formed.
-A *mem-initializer-list* can delegate to another constructor of the
-constructor’s class using any *class-or-decltype* that denotes the
-constructor’s class itself. If a *mem-initializer-id* designates the
-constructor’s class, it shall be the only *mem-initializer*; the
-constructor is a *delegating constructor*, and the constructor selected
-by the *mem-initializer* is the *target constructor*. The target
-constructor is selected by overload resolution. Once the target
-constructor returns, the body of the delegating constructor is executed.
-If a constructor delegates to itself directly or indirectly, the program
-is ill-formed, no diagnostic required.
-[*Example 3*:
-``` cpp
-struct C {
-  C( int ) { }                  // #1: non-delegating constructor
-  C(): C(42) { }                // #2: delegates to #1
-  C( char c ) : C(42.0) { }     // #3: ill-formed due to recursion with #4
-  C( double d ) : C('a') { }    // #4: ill-formed due to recursion with #3
-};
-```
-— *end example*]
-The *expression-list* or *braced-init-list* in a *mem-initializer* is
-used to initialize the designated subobject (or, in the case of a
-delegating constructor, the complete class object) according to the
-initialization rules of  [[dcl.init]] for direct-initialization.
-[*Example 4*:
-``` cpp
-struct B1 { B1(int); ... };
-struct B2 { B2(int); ... };
-struct D : B1, B2 {
-  D(int);
-  B1 b;
-  const int c;
-};
-D::D(int a) : B2(a+1), B1(a+2), c(a+3), b(a+4) { ... }
-D d(10);
-```
-— *end example*]
-[*Note 2*: The initialization performed by each *mem-initializer*
-constitutes a full-expression ([[intro.execution]]). Any expression in
-a *mem-initializer* is evaluated as part of the full-expression that
-performs the initialization. — *end note*]
-A *mem-initializer* where the *mem-initializer-id* denotes a virtual
-base class is ignored during execution of a constructor of any class
-that is not the most derived class.
-A temporary expression bound to a reference member in a
-*mem-initializer* is ill-formed.
-[*Example 5*:
-``` cpp
-struct A {
-  A() : v(42) { }   // error
-  const int& v;
-};
-```
-— *end example*]
-In a non-delegating constructor, if a given potentially constructed
-subobject is not designated by a *mem-initializer-id* (including the
-case where there is no *mem-initializer-list* because the constructor
-has no *ctor-initializer*), then
-- if the entity is a non-static data member that has a default member
-  initializer ([[class.mem]]) and either
-  - the constructor’s class is a union ([[class.union]]), and no other
-    variant member of that union is designated by a *mem-initializer-id*
-    or
-  - the constructor’s class is not a union, and, if the entity is a
-    member of an anonymous union, no other member of that union is
-    designated by a *mem-initializer-id*,
-  the entity is initialized from its default member initializer as
-  specified in  [[dcl.init]];
-- otherwise, if the entity is an anonymous union or a variant member (
-  [[class.union.anon]]), no initialization is performed;
-- otherwise, the entity is default-initialized ([[dcl.init]]).
-[*Note 3*: An abstract class ([[class.abstract]]) is never a most
-derived class, thus its constructors never initialize virtual base
-classes, therefore the corresponding *mem-initializer*s may be
-omitted. — *end note*]
-An attempt to initialize more than one non-static data member of a union
-renders the program ill-formed.
-[*Note 4*: After the call to a constructor for class `X` for an object
-with automatic or dynamic storage duration has completed, if the
-constructor was not invoked as part of value-initialization and a member
-of `X` is neither initialized nor given a value during execution of the
-*compound-statement* of the body of the constructor, the member has an
-indeterminate value. — *end note*]
-[*Example 6*:
-``` cpp
-struct A {
-  A();
-};
-struct B {
-  B(int);
-};
-struct C {
-  C() { }               // initializes members as follows:
-  A a;                  // OK: calls A::A()
-  const B b;            // error: B has no default constructor
-  int i;                // OK: i has indeterminate value
-  int j = 5;            // OK: j has the value 5
-};
-```
-— *end example*]
-If a given non-static data member has both a default member initializer
-and a *mem-initializer*, the initialization specified by the
-*mem-initializer* is performed, and the non-static data member’s default
-member initializer is ignored.
-[*Example 7*:
-Given
-``` cpp
-struct A {
-  int i = /* some integer expression with side effects */ ;
-  A(int arg) : i(arg) { }
-  // ...
-};
-```
-the `A(int)` constructor will simply initialize `i` to the value of
-`arg`, and the side effects in `i`’s default member initializer will not
-take place.
-— *end example*]
-A temporary expression bound to a reference member from a default member
-initializer is ill-formed.
-[*Example 8*:
-``` cpp
-struct A {
-  A() = default;        // OK
-  A(int v) : v(v) { }   // OK
-  const int& v = 42;    // OK
-};
-A a1;                   // error: ill-formed binding of temporary to reference
-A a2(1);                // OK, unfortunately
-```
-— *end example*]
-In a non-delegating constructor, the destructor for each potentially
-constructed subobject of class type is potentially invoked (
-[[class.dtor]]).
-[*Note 5*: This provision ensures that destructors can be called for
-fully-constructed subobjects in case an exception is thrown (
-[[except.ctor]]). — *end note*]
-In a non-delegating constructor, initialization proceeds in the
-following order:
-- First, and only for the constructor of the most derived class (
-  [[intro.object]]), virtual base classes are initialized in the order
-  they appear on a depth-first left-to-right traversal of the directed
-  acyclic graph of base classes, where “left-to-right” is the order of
-  appearance of the base classes in the derived class
-  *base-specifier-list*.
-- Then, direct base classes are initialized in declaration order as they
-  appear in the *base-specifier-list* (regardless of the order of the
-  *mem-initializer*s).
-- Then, non-static data members are initialized in the order they were
-  declared in the class definition (again regardless of the order of the
-  *mem-initializer*s).
-- Finally, the *compound-statement* of the constructor body is executed.
-[*Note 6*: The declaration order is mandated to ensure that base and
-member subobjects are destroyed in the reverse order of
-initialization. — *end note*]
-[*Example 9*:
-``` cpp
-struct V {
-  V();
-  V(int);
-};
-struct A : virtual V {
-  A();
-  A(int);
-};
-struct B : virtual V {
-  B();
-  B(int);
-};
-struct C : A, B, virtual V {
-  C();
-  C(int);
-};
-A::A(int i) : V(i) { ... }
-B::B(int i) { ... }
-C::C(int i) { ... }
-V v(1);             // use V(int)
-A a(2);             // use V(int)
-B b(3);             // use V()
-C c(4);             // use V()
-```
-— *end example*]
-Names in the *expression-list* or *braced-init-list* of a
-*mem-initializer* are evaluated in the scope of the constructor for
-which the *mem-initializer* is specified.
-[*Example 10*:
-``` cpp
-class X {
-  int a;
-  int b;
-  int i;
-  int j;
-public:
-  const int& r;
-  X(int i): r(a), b(i), i(i), j(this->i) { }
-};
-```
-initializes `X::r` to refer to `X::a`, initializes `X::b` with the value
-of the constructor parameter `i`, initializes `X::i` with the value of
-the constructor parameter `i`, and initializes `X::j` with the value of
-`X::i`; this takes place each time an object of class `X` is created.
-— *end example*]
-[*Note 7*: Because the *mem-initializer* are evaluated in the scope of
-the constructor, the `this` pointer can be used in the *expression-list*
-of a *mem-initializer* to refer to the object being
-initialized. — *end note*]
-Member functions (including virtual member functions, [[class.virtual]])
-can be called for an object under construction. Similarly, an object
-under construction can be the operand of the `typeid` operator (
-[[expr.typeid]]) or of a `dynamic_cast` ([[expr.dynamic.cast]]).
-However, if these operations are performed in a *ctor-initializer* (or
-in a function called directly or indirectly from a *ctor-initializer*)
-before all the *mem-initializer*s for base classes have completed, the
-program has undefined behavior.
-[*Example 11*:
-``` cpp
-class A {
-public:
-  A(int);
-};
-class B : public A {
-  int j;
-public:
-  int f();
-  B() : A(f()),     // undefined: calls member function but base A not yet initialized
-  j(f()) { }        // well-defined: bases are all initialized
-};
-class C {
-public:
-  C(int);
-};
-class D : public B, C {
-  int i;
-public:
-  D() : C(f()),     // undefined: calls member function but base C not yet initialized
-  i(f()) { }        // well-defined: bases are all initialized
-};
-```
-— *end example*]
-[*Note 8*:  [[class.cdtor]] describes the result of virtual function
-calls, `typeid` and `dynamic_cast`s during construction for the
-well-defined cases; that is, describes the *polymorphic behavior* of an
-object under construction. — *end note*]
-A *mem-initializer* followed by an ellipsis is a pack expansion (
-[[temp.variadic]]) that initializes the base classes specified by a pack
-expansion in the *base-specifier-list* for the class.
-[*Example 12*:
-``` cpp
-template<class... Mixins>
-class X : public Mixins... {
-public:
-  X(const Mixins&... mixins) : Mixins(mixins)... { }
-};
-```
-— *end example*]
-### Initialization by inherited constructor <a id="class.inhctor.init">[[class.inhctor.init]]</a>
-When a constructor for type `B` is invoked to initialize an object of a
-different type `D` (that is, when the constructor was inherited (
-[[namespace.udecl]])), initialization proceeds as if a defaulted default
-constructor were used to initialize the `D` object and each base class
-subobject from which the constructor was inherited, except that the `B`
-subobject is initialized by the invocation of the inherited constructor.
-The complete initialization is considered to be a single function call;
-in particular, the initialization of the inherited constructor’s
-parameters is sequenced before the initialization of any part of the `D`
-object.
-[*Example 1*:
-``` cpp
-struct B1 {
-  B1(int, ...) { }
-};
-struct B2 {
-  B2(double) { }
-};
-int get();
-struct D1 : B1 {
-  using B1::B1;     // inherits B1(int, ...)
-  int x;
-  int y = get();
-};
-void test() {
-  D1 d(2, 3, 4);    // OK: B1 is initialized by calling B1(2, 3, 4),
-                    // then d.x is default-initialized (no initialization is performed),
-                    // then d.y is initialized by calling get()
-  D1 e;             // error: D1 has a deleted default constructor
-}
-struct D2 : B2 {
-  using B2::B2;
-  B1 b;
-};
-D2 f(1.0);          // error: B1 has a deleted default constructor
-struct W { W(int); };
-struct X : virtual W { using W::W; X() = delete; };
-struct Y : X { using X::X; };
-struct Z : Y, virtual W { using Y::Y; };
-Z z(0);             // OK: initialization of Y does not invoke default constructor of X
-template<class T> struct Log : T {
-  using T::T;       // inherits all constructors from class T
-  ~Log() { std::clog << "Destroying wrapper" << std::endl; }
-};
-```
-Class template `Log` wraps any class and forwards all of its
-constructors, while writing a message to the standard log whenever an
-object of class `Log` is destroyed.
-— *end example*]
-If the constructor was inherited from multiple base class subobjects of
-type `B`, the program is ill-formed.
-[*Example 2*:
-``` cpp
-struct A { A(int); };
-struct B : A { using A::A; };
-struct C1 : B { using B::B; };
-struct C2 : B { using B::B; };
-struct D1 : C1, C2 {
-  using C1::C1;
-  using C2::C2;
-};
-struct V1 : virtual B { using B::B; };
-struct V2 : virtual B { using B::B; };
-struct D2 : V1, V2 {
-  using V1::V1;
-  using V2::V2;
-};
-D1 d1(0);           // ill-formed: ambiguous
-D2 d2(0);           // OK: initializes virtual B base class, which initializes the A base class
-                    // then initializes the V1 and V2 base classes as if by a defaulted default constructor
-struct M { M(); M(int); };
-struct N : M { using M::M; };
-struct O : M {};
-struct P : N, O { using N::N; using O::O; };
-P p(0);             // OK: use M(0) to initialize N's base class,
-                    // use M() to initialize O's base class
-```
-— *end example*]
-When an object is initialized by an inherited constructor,
-initialization of the object is complete when the initialization of all
-subobjects is complete.
-## Construction and destruction <a id="class.cdtor">[[class.cdtor]]</a>
-For an object with a non-trivial constructor, referring to any
-non-static member or base class of the object before the constructor
-begins execution results in undefined behavior. For an object with a
-non-trivial destructor, referring to any non-static member or base class
-of the object after the destructor finishes execution results in
-undefined behavior.
-[*Example 1*:
-``` cpp
-struct X { int i; };
-struct Y : X { Y(); };                  // non-trivial
-struct A { int a; };
-struct B : public A { int j; Y y; };    // non-trivial
-extern B bobj;
-B* pb = &bobj;                          // OK
-int* p1 = &bobj.a;                      // undefined, refers to base class member
-int* p2 = &bobj.y.i;                    // undefined, refers to member's member
-A* pa = &bobj;                          // undefined, upcast to a base class type
-B bobj;                                 // definition of bobj
-extern X xobj;
-int* p3 = &xobj.i;                      // OK, X is a trivial class
-X xobj;
-```
-For another example,
-``` cpp
-struct W { int j; };
-struct X : public virtual W { };
-struct Y {
-  int* p;
-  X x;
-  Y() : p(&x.j) {   // undefined, x is not yet constructed
-    }
-};
-```
-— *end example*]
-To explicitly or implicitly convert a pointer (a glvalue) referring to
-an object of class `X` to a pointer (reference) to a direct or indirect
-base class `B` of `X`, the construction of `X` and the construction of
-all of its direct or indirect bases that directly or indirectly derive
-from `B` shall have started and the destruction of these classes shall
-not have completed, otherwise the conversion results in undefined
-behavior. To form a pointer to (or access the value of) a direct
-non-static member of an object `obj`, the construction of `obj` shall
-have started and its destruction shall not have completed, otherwise the
-computation of the pointer value (or accessing the member value) results
-in undefined behavior.
-[*Example 2*:
-``` cpp
-struct A { };
-struct B : virtual A { };
-struct C : B { };
-struct D : virtual A { D(A*); };
-struct X { X(A*); };
-struct E : C, D, X {
-  E() : D(this),    // undefined: upcast from E* to A* might use path E* → D* → A*
-                    // but D is not constructed
-                    // ``D((C*)this)'' would be defined: E* → C* is defined because E() has started,
-                    // and C* → A* is defined because C is fully constructed
-  X(this) {}        // defined: upon construction of X, C/B/D/A sublattice is fully constructed
-};
-```
-— *end example*]
-Member functions, including virtual functions ([[class.virtual]]), can
-be called during construction or destruction ([[class.base.init]]).
-When a virtual function is called directly or indirectly from a
-constructor or from a destructor, including during the construction or
-destruction of the class’s non-static data members, and the object to
-which the call applies is the object (call it `x`) under construction or
-destruction, the function called is the final overrider in the
-constructor’s or destructor’s class and not one overriding it in a
-more-derived class. If the virtual function call uses an explicit class
-member access ([[expr.ref]]) and the object expression refers to the
-complete object of `x` or one of that object’s base class subobjects but
-not `x` or one of its base class subobjects, the behavior is undefined.
-[*Example 3*:
-``` cpp
-struct V {
-  virtual void f();
-  virtual void g();
-};
-struct A : virtual V {
-  virtual void f();
-};
-struct B : virtual V {
-  virtual void g();
-  B(V*, A*);
-};
-struct D : A, B {
-  virtual void f();
-  virtual void g();
-  D() : B((A*)this, this) { }
-};
-B::B(V* v, A* a) {
-  f();              // calls V::f, not A::f
-  g();              // calls B::g, not D::g
-  v->g();           // v is base of B, the call is well-defined, calls B::g
-  a->f();           // undefined behavior, a's type not a base of B
-}
-```
-— *end example*]
-The `typeid` operator ([[expr.typeid]]) can be used during construction
-or destruction ([[class.base.init]]). When `typeid` is used in a
-constructor (including the *mem-initializer* or default member
-initializer ([[class.mem]]) for a non-static data member) or in a
-destructor, or used in a function called (directly or indirectly) from a
-constructor or destructor, if the operand of `typeid` refers to the
-object under construction or destruction, `typeid` yields the
-`std::type_info` object representing the constructor or destructor’s
-class. If the operand of `typeid` refers to the object under
-construction or destruction and the static type of the operand is
-neither the constructor or destructor’s class nor one of its bases, the
-behavior is undefined.
-`dynamic_cast`s ([[expr.dynamic.cast]]) can be used during construction
-or destruction ([[class.base.init]]). When a `dynamic_cast` is used in
-a constructor (including the *mem-initializer* or default member
-initializer for a non-static data member) or in a destructor, or used in
-a function called (directly or indirectly) from a constructor or
-destructor, if the operand of the `dynamic_cast` refers to the object
-under construction or destruction, this object is considered to be a
-most derived object that has the type of the constructor or destructor’s
-class. If the operand of the `dynamic_cast` refers to the object under
-construction or destruction and the static type of the operand is not a
-pointer to or object of the constructor or destructor’s own class or one
-of its bases, the `dynamic_cast` results in undefined behavior.
-[*Example 4*:
-``` cpp
-struct V {
-  virtual void f();
-};
-struct A : virtual V { };
-struct B : virtual V {
-  B(V*, A*);
-};
-struct D : A, B {
-  D() : B((A*)this, this) { }
-};
-B::B(V* v, A* a) {
-  typeid(*this);                // type_info for B
-  typeid(*v);                   // well-defined: *v has type V, a base of B yields type_info for B
-  typeid(*a);                   // undefined behavior: type A not a base of B
-  dynamic_cast<B*>(v);          // well-defined: v of type V*, V base of B results in B*
-  dynamic_cast<B*>(a);          // undefined behavior, a has type A*, A not a base of B
-}
-```
-— *end example*]
-## Copying and moving class objects <a id="class.copy">[[class.copy]]</a>
-A class object can be copied or moved in two ways: by initialization (
-[[class.ctor]], [[dcl.init]]), including for function argument passing (
-[[expr.call]]) and for function value return ([[stmt.return]]); and by
-assignment ([[expr.ass]]). Conceptually, these two operations are
-implemented by a copy/move constructor ([[class.ctor]]) and copy/move
-assignment operator ([[over.ass]]).
-A program is ill-formed if the copy/move constructor or the copy/move
-assignment operator for an object is implicitly odr-used and the special
-member function is not accessible (Clause  [[class.access]]).
-[*Note 1*: Copying/moving one object into another using the copy/move
-constructor or the copy/move assignment operator does not change the
-layout or size of either object. — *end note*]
-### Copy/move constructors <a id="class.copy.ctor">[[class.copy.ctor]]</a>
-A non-template constructor for class `X` is a copy constructor if its
-first parameter is of type `X&`, `const X&`, `volatile X&` or
-`const volatile X&`, and either there are no other parameters or else
-all other parameters have default arguments ([[dcl.fct.default]]).
-[*Example 1*:
-`X::X(const X&)`
-and `X::X(X&,int=1)` are copy constructors.
-``` cpp
-struct X {
-  X(int);
-  X(const X&, int = 1);
-};
-X a(1);             // calls X(int);
-X b(a, 0);          // calls X(const X&, int);
-X c = b;            // calls X(const X&, int);
-```
-— *end example*]
-A non-template constructor for class `X` is a move constructor if its
-first parameter is of type `X&&`, `const X&&`, `volatile X&&`, or
-`const volatile X&&`, and either there are no other parameters or else
-all other parameters have default arguments ([[dcl.fct.default]]).
-[*Example 2*:
-`Y::Y(Y&&)` is a move constructor.
-``` cpp
-struct Y {
-  Y(const Y&);
-  Y(Y&&);
-};
-extern Y f(int);
-Y d(f(1));          // calls Y(Y&&)
-Y e = d;            // calls Y(const Y&)
-```
-— *end example*]
-[*Note 1*:
-All forms of copy/move constructor may be declared for a class.
-[*Example 3*:
-``` cpp
-struct X {
-  X(const X&);
-  X(X&);            // OK
-  X(X&&);
-  X(const X&&);     // OK, but possibly not sensible
-};
-```
-— *end example*]
-— *end note*]
-[*Note 2*:
-If a class `X` only has a copy constructor with a parameter of type
-`X&`, an initializer of type `const` `X` or `volatile` `X` cannot
-initialize an object of type (possibly cv-qualified) `X`.
-[*Example 4*:
-``` cpp
-struct X {
-  X();              // default constructor
-  X(X&);            // copy constructor with a non-const parameter
-};
-const X cx;
-X x = cx;           // error: X::X(X&) cannot copy cx into x
-```
-— *end example*]
-— *end note*]
-A declaration of a constructor for a class `X` is ill-formed if its
-first parameter is of type (optionally cv-qualified) `X` and either
-there are no other parameters or else all other parameters have default
-arguments. A member function template is never instantiated to produce
-such a constructor signature.
-[*Example 5*:
-``` cpp
-struct S {
-  template<typename T> S(T);
-  S();
-};
-S g;
-void h() {
-  S a(g);           // does not instantiate the member template to produce S::S<S>(S);
-                    // uses the implicitly declared copy constructor
-}
-```
-— *end example*]
-If the class definition does not explicitly declare a copy constructor,
-a non-explicit one is declared *implicitly*. If the class definition
-declares a move constructor or move assignment operator, the implicitly
-declared copy constructor is defined as deleted; otherwise, it is
-defined as defaulted ([[dcl.fct.def]]). The latter case is deprecated
-if the class has a user-declared copy assignment operator or a
-user-declared destructor.
-The implicitly-declared copy constructor for a class `X` will have the
-form
-``` cpp
-X::X(const X&)
-```
-if each potentially constructed subobject of a class type `M` (or array
-thereof) has a copy constructor whose first parameter is of type `const`
-`M&` or `const` `volatile` `M&`.[^4] Otherwise, the implicitly-declared
-copy constructor will have the form
-``` cpp
-X::X(X&)
-```
-If the definition of a class `X` does not explicitly declare a move
-constructor, a non-explicit one will be implicitly declared as defaulted
-if and only if
-- `X` does not have a user-declared copy constructor,
-- `X` does not have a user-declared copy assignment operator,
-- `X` does not have a user-declared move assignment operator, and
-- `X` does not have a user-declared destructor.
-[*Note 3*: When the move constructor is not implicitly declared or
-explicitly supplied, expressions that otherwise would have invoked the
-move constructor may instead invoke a copy constructor. — *end note*]
-The implicitly-declared move constructor for class `X` will have the
-form
-``` cpp
-X::X(X&&)
-```
-An implicitly-declared copy/move constructor is an `inline` `public`
-member of its class. A defaulted copy/move constructor for a class `X`
-is defined as deleted ([[dcl.fct.def.delete]]) if `X` has:
-- a variant member with a non-trivial corresponding constructor and `X`
-  is a union-like class,
-- a potentially constructed subobject type `M` (or array thereof) that
-  cannot be copied/moved because overload resolution ([[over.match]]),
-  as applied to find `M`’s corresponding constructor, results in an
-  ambiguity or a function that is deleted or inaccessible from the
-  defaulted constructor,
-- any potentially constructed subobject of a type with a destructor that
-  is deleted or inaccessible from the defaulted constructor, or,
-- for the copy constructor, a non-static data member of rvalue reference
-  type.
-A defaulted move constructor that is defined as deleted is ignored by
-overload resolution ([[over.match]], [[over.over]]).
-[*Note 4*: A deleted move constructor would otherwise interfere with
-initialization from an rvalue which can use the copy constructor
-instead. — *end note*]
-A copy/move constructor for class `X` is trivial if it is not
-user-provided and if:
-- class `X` has no virtual functions ([[class.virtual]]) and no virtual
-  base classes ([[class.mi]]), and
-- the constructor selected to copy/move each direct base class subobject
-  is trivial, and
-- for each non-static data member of `X` that is of class type (or array
-  thereof), the constructor selected to copy/move that member is
-  trivial;
-otherwise the copy/move constructor is *non-trivial*.
-A copy/move constructor that is defaulted and not defined as deleted is
-*implicitly defined* if it is odr-used ([[basic.def.odr]]) or when it
-is explicitly defaulted after its first declaration.
-[*Note 5*: The copy/move constructor is implicitly defined even if the
-implementation elided its odr-use ([[basic.def.odr]],
-[[class.temporary]]). — *end note*]
-If the implicitly-defined constructor would satisfy the requirements of
-a constexpr constructor ([[dcl.constexpr]]), the implicitly-defined
-constructor is `constexpr`.
-Before the defaulted copy/move constructor for a class is implicitly
-defined, all non-user-provided copy/move constructors for its
-potentially constructed subobjects shall have been implicitly defined.
-[*Note 6*: An implicitly-declared copy/move constructor has an implied
-exception specification ([[except.spec]]). — *end note*]
-The implicitly-defined copy/move constructor for a non-union class `X`
-performs a memberwise copy/move of its bases and members.
-[*Note 7*: Default member initializers of non-static data members are
-ignored. See also the example in  [[class.base.init]]. — *end note*]
-The order of initialization is the same as the order of initialization
-of bases and members in a user-defined constructor (see
-[[class.base.init]]). Let `x` be either the parameter of the constructor
-or, for the move constructor, an xvalue referring to the parameter. Each
-base or non-static data member is copied/moved in the manner appropriate
-to its type:
-- if the member is an array, each element is direct-initialized with the
-  corresponding subobject of `x`;
-- if a member `m` has rvalue reference type `T&&`, it is
-  direct-initialized with `static_cast<T&&>(x.m)`;
-- otherwise, the base or member is direct-initialized with the
-  corresponding base or member of `x`.
-Virtual base class subobjects shall be initialized only once by the
-implicitly-defined copy/move constructor (see  [[class.base.init]]).
-The implicitly-defined copy/move constructor for a union `X` copies the
-object representation ([[basic.types]]) of `X`.
-### Copy/move assignment operator <a id="class.copy.assign">[[class.copy.assign]]</a>
-A user-declared *copy* assignment operator `X::operator=` is a
-non-static non-template member function of class `X` with exactly one
-parameter of type `X`, `X&`, `const` `X&`, `volatile` `X&` or `const`
-`volatile` `X&`.[^5]
-[*Note 1*: An overloaded assignment operator must be declared to have
-only one parameter; see  [[over.ass]]. — *end note*]
-[*Note 2*: More than one form of copy assignment operator may be
-declared for a class. — *end note*]
-[*Note 3*:
-If a class `X` only has a copy assignment operator with a parameter of
-type `X&`, an expression of type const `X` cannot be assigned to an
-object of type `X`.
-[*Example 1*:
-``` cpp
-struct X {
-  X();
-  X& operator=(X&);
-};
-const X cx;
-X x;
-void f() {
-  x = cx;           // error: X::operator=(X&) cannot assign cx into x
-}
-```
-— *end example*]
-— *end note*]
-If the class definition does not explicitly declare a copy assignment
-operator, one is declared *implicitly*. If the class definition declares
-a move constructor or move assignment operator, the implicitly declared
-copy assignment operator is defined as deleted; otherwise, it is defined
-as defaulted ([[dcl.fct.def]]). The latter case is deprecated if the
-class has a user-declared copy constructor or a user-declared
-destructor. The implicitly-declared copy assignment operator for a class
-`X` will have the form
-``` cpp
-X& X::operator=(const X&)
-```
-if
-- each direct base class `B` of `X` has a copy assignment operator whose
-  parameter is of type `const` `B&`, `const` `volatile` `B&` or `B`, and
-- for all the non-static data members of `X` that are of a class type
-  `M` (or array thereof), each such class type has a copy assignment
-  operator whose parameter is of type `const` `M&`, `const` `volatile`
-  `M&` or `M`.[^6]
-Otherwise, the implicitly-declared copy assignment operator will have
-the form
-``` cpp
-X& X::operator=(X&)
-```
-A user-declared move assignment operator `X::operator=` is a non-static
-non-template member function of class `X` with exactly one parameter of
-type `X&&`, `const X&&`, `volatile X&&`, or `const volatile X&&`.
-[*Note 4*: An overloaded assignment operator must be declared to have
-only one parameter; see  [[over.ass]]. — *end note*]
-[*Note 5*: More than one form of move assignment operator may be
-declared for a class. — *end note*]
-If the definition of a class `X` does not explicitly declare a move
-assignment operator, one will be implicitly declared as defaulted if and
-only if
-- `X` does not have a user-declared copy constructor,
-- `X` does not have a user-declared move constructor,
-- `X` does not have a user-declared copy assignment operator, and
-- `X` does not have a user-declared destructor.
-[*Example 2*:
-The class definition
-``` cpp
-struct S {
-  int a;
-  S& operator=(const S&) = default;
-};
-```
-will not have a default move assignment operator implicitly declared
-because the copy assignment operator has been user-declared. The move
-assignment operator may be explicitly defaulted.
-``` cpp
-struct S {
-  int a;
-  S& operator=(const S&) = default;
-  S& operator=(S&&) = default;
-};
-```
-— *end example*]
-The implicitly-declared move assignment operator for a class `X` will
-have the form
-``` cpp
-X& X::operator=(X&&);
-```
-The implicitly-declared copy/move assignment operator for class `X` has
-the return type `X&`; it returns the object for which the assignment
-operator is invoked, that is, the object assigned to. An
-implicitly-declared copy/move assignment operator is an `inline`
-`public` member of its class.
-A defaulted copy/move assignment operator for class `X` is defined as
-deleted if `X` has:
-- a variant member with a non-trivial corresponding assignment operator
-  and `X` is a union-like class, or
-- a non-static data member of `const` non-class type (or array thereof),
-  or
-- a non-static data member of reference type, or
-- a direct non-static data member of class type `M` (or array thereof)
-  or a direct base class `M` that cannot be copied/moved because
-  overload resolution ([[over.match]]), as applied to find `M`’s
-  corresponding assignment operator, results in an ambiguity or a
-  function that is deleted or inaccessible from the defaulted assignment
-  operator.
-A defaulted move assignment operator that is defined as deleted is
-ignored by overload resolution ([[over.match]], [[over.over]]).
-Because a copy/move assignment operator is implicitly declared for a
-class if not declared by the user, a base class copy/move assignment
-operator is always hidden by the corresponding assignment operator of a
-derived class ([[over.ass]]). A *using-declaration* (
-[[namespace.udecl]]) that brings in from a base class an assignment
-operator with a parameter type that could be that of a copy/move
-assignment operator for the derived class is not considered an explicit
-declaration of such an operator and does not suppress the implicit
-declaration of the derived class operator; the operator introduced by
-the *using-declaration* is hidden by the implicitly-declared operator in
-the derived class.
-A copy/move assignment operator for class `X` is trivial if it is not
-user-provided and if:
-- class `X` has no virtual functions ([[class.virtual]]) and no virtual
-  base classes ([[class.mi]]), and
-- the assignment operator selected to copy/move each direct base class
-  subobject is trivial, and
-- for each non-static data member of `X` that is of class type (or array
-  thereof), the assignment operator selected to copy/move that member is
-  trivial;
-otherwise the copy/move assignment operator is *non-trivial*.
-A copy/move assignment operator for a class `X` that is defaulted and
-not defined as deleted is *implicitly defined* when it is odr-used (
-[[basic.def.odr]]) (e.g., when it is selected by overload resolution to
-assign to an object of its class type) or when it is explicitly
-defaulted after its first declaration. The implicitly-defined copy/move
-assignment operator is `constexpr` if
-- `X` is a literal type, and
-- the assignment operator selected to copy/move each direct base class
-  subobject is a constexpr function, and
-- for each non-static data member of `X` that is of class type (or array
-  thereof), the assignment operator selected to copy/move that member is
-  a constexpr function.
-Before the defaulted copy/move assignment operator for a class is
-implicitly defined, all non-user-provided copy/move assignment operators
-for its direct base classes and its non-static data members shall have
-been implicitly defined.
-[*Note 6*: An implicitly-declared copy/move assignment operator has an
-implied exception specification ([[except.spec]]). — *end note*]
-The implicitly-defined copy/move assignment operator for a non-union
-class `X` performs memberwise copy/move assignment of its subobjects.
-The direct base classes of `X` are assigned first, in the order of their
-declaration in the *base-specifier-list*, and then the immediate
-non-static data members of `X` are assigned, in the order in which they
-were declared in the class definition. Let `x` be either the parameter
-of the function or, for the move operator, an xvalue referring to the
-parameter. Each subobject is assigned in the manner appropriate to its
-type:
-- if the subobject is of class type, as if by a call to `operator=` with
-  the subobject as the object expression and the corresponding subobject
-  of `x` as a single function argument (as if by explicit qualification;
-  that is, ignoring any possible virtual overriding functions in more
-  derived classes);
-- if the subobject is an array, each element is assigned, in the manner
-  appropriate to the element type;
-- if the subobject is of scalar type, the built-in assignment operator
-  is used.
-It is unspecified whether subobjects representing virtual base classes
-are assigned more than once by the implicitly-defined copy/move
-assignment operator.
-[*Example 3*:
-``` cpp
-struct V { };
-struct A : virtual V { };
-struct B : virtual V { };
-struct C : B, A { };
-```
-It is unspecified whether the virtual base class subobject `V` is
-assigned twice by the implicitly-defined copy/move assignment operator
-for `C`.
-— *end example*]
-The implicitly-defined copy assignment operator for a union `X` copies
-the object representation ([[basic.types]]) of `X`.
-### Copy/move elision <a id="class.copy.elision">[[class.copy.elision]]</a>
-When certain criteria are met, an implementation is allowed to omit the
-copy/move construction of a class object, even if the constructor
-selected for the copy/move operation and/or the destructor for the
-object have side effects. In such cases, the implementation treats the
-source and target of the omitted copy/move operation as simply two
-different ways of referring to the same object. If the first parameter
-of the selected constructor is an rvalue reference to the object’s type,
-the destruction of that object occurs when the target would have been
-destroyed; otherwise, the destruction occurs at the later of the times
-when the two objects would have been destroyed without the
-optimization.[^7] This elision of copy/move operations, called *copy
-elision*, is permitted in the following circumstances (which may be
-combined to eliminate multiple copies):
-- in a `return` statement in a function with a class return type, when
-  the *expression* is the name of a non-volatile automatic object (other
-  than a function parameter or a variable introduced by the
-  *exception-declaration* of a *handler* ([[except.handle]])) with the
-  same type (ignoring cv-qualification) as the function return type, the
-  copy/move operation can be omitted by constructing the automatic
-  object directly into the function call’s return object
-- in a *throw-expression* ([[expr.throw]]), when the operand is the
-  name of a non-volatile automatic object (other than a function or
-  catch-clause parameter) whose scope does not extend beyond the end of
-  the innermost enclosing *try-block* (if there is one), the copy/move
-  operation from the operand to the exception object ([[except.throw]])
-  can be omitted by constructing the automatic object directly into the
-  exception object
-- when the *exception-declaration* of an exception handler (Clause
-  [[except]]) declares an object of the same type (except for
-  cv-qualification) as the exception object ([[except.throw]]), the
-  copy operation can be omitted by treating the *exception-declaration*
-  as an alias for the exception object if the meaning of the program
-  will be unchanged except for the execution of constructors and
-  destructors for the object declared by the *exception-declaration*.
-  \[*Note 1*: There cannot be a move from the exception object because
-  it is always an lvalue. — *end note*]
-Copy elision is required where an expression is evaluated in a context
-requiring a constant expression ([[expr.const]]) and in constant
-initialization ([[basic.start.static]]).
-[*Note 2*: Copy elision might not be performed if the same expression
-is evaluated in another context. — *end note*]
-[*Example 1*:
-``` cpp
-class Thing {
-public:
-  Thing();
-  ~Thing();
-  Thing(const Thing&);
-};
-Thing f() {
-  Thing t;
-  return t;
-}
-Thing t2 = f();
-struct A {
-  void *p;
-  constexpr A(): p(this) {}
-};
-constexpr A g() {
-  A a;
-  return a;
-}
-constexpr A a;          // well-formed, a.p points to a
-constexpr A b = g();    // well-formed, b.p points to b
-void g() {
-  A c = g();            // well-formed, c.p may point to c or to an ephemeral temporary
-}
-```
-Here the criteria for elision can eliminate the copying of the local
-automatic object `t` into the result object for the function call `f()`,
-which is the global object `t2`. Effectively, the construction of the
-local object `t` can be viewed as directly initializing the global
-object `t2`, and that object’s destruction will occur at program exit.
-Adding a move constructor to `Thing` has the same effect, but it is the
-move construction from the local automatic object to `t2` that is
-elided.
-— *end example*]
-In the following copy-initialization contexts, a move operation might be
-used instead of a copy operation:
-- If the *expression* in a `return` statement ([[stmt.return]]) is a
-  (possibly parenthesized) *id-expression* that names an object with
-  automatic storage duration declared in the body or
-  *parameter-declaration-clause* of the innermost enclosing function or
-  *lambda-expression*, or
-- if the operand of a *throw-expression* ([[expr.throw]]) is the name
-  of a non-volatile automatic object (other than a function or
-  catch-clause parameter) whose scope does not extend beyond the end of
-  the innermost enclosing *try-block* (if there is one),
-overload resolution to select the constructor for the copy is first
-performed as if the object were designated by an rvalue. If the first
-overload resolution fails or was not performed, or if the type of the
-first parameter of the selected constructor is not an rvalue reference
-to the object’s type (possibly cv-qualified), overload resolution is
-performed again, considering the object as an lvalue.
-[*Note 3*: This two-stage overload resolution must be performed
-regardless of whether copy elision will occur. It determines the
-constructor to be called if elision is not performed, and the selected
-constructor must be accessible even if the call is
-elided. — *end note*]
-[*Example 2*:
-``` cpp
-class Thing {
-public:
-  Thing();
-  ~Thing();
-  Thing(Thing&&);
-private:
-  Thing(const Thing&);
-};
-Thing f(bool b) {
-  Thing t;
-  if (b)
-    throw t;            // OK: Thing(Thing&&) used (or elided) to throw t
-  return t;             // OK: Thing(Thing&&) used (or elided) to return t
-}
-Thing t2 = f(false);    // OK: no extra copy/move performed, t2 constructed by call to f
-struct Weird {
-  Weird();
-  Weird(Weird&);
-};
-Weird g() {
-  Weird w;
-  return w;             // OK: first overload resolution fails, second overload resolution selects Weird(Weird&)
-}
-```
-— *end example*]
-<!-- Link reference definitions -->
-[basic.def.odr]: basic.md#basic.def.odr
-[basic.life]: basic.md#basic.life
-[basic.lookup]: basic.md#basic.lookup
-[basic.lval]: basic.md#basic.lval
-[basic.start.dynamic]: basic.md#basic.start.dynamic
-[basic.start.static]: basic.md#basic.start.static
-[basic.start.term]: basic.md#basic.start.term
-[basic.stc.auto]: basic.md#basic.stc.auto
-[basic.stc.dynamic]: basic.md#basic.stc.dynamic
-[basic.stc.dynamic.deallocation]: basic.md#basic.stc.dynamic.deallocation
-[basic.stc.static]: basic.md#basic.stc.static
-[basic.stc.thread]: basic.md#basic.stc.thread
-[basic.types]: basic.md#basic.types
-[class]: class.md#class
-[class.abstract]: class.md#class.abstract
-[class.access]: class.md#class.access
-[class.base.init]: #class.base.init
-[class.cdtor]: #class.cdtor
-[class.conv]: #class.conv
-[class.conv.ctor]: #class.conv.ctor
-[class.conv.fct]: #class.conv.fct
-[class.copy]: #class.copy
-[class.copy.assign]: #class.copy.assign
-[class.copy.ctor]: #class.copy.ctor
-[class.copy.elision]: #class.copy.elision
-[class.ctor]: #class.ctor
-[class.dtor]: #class.dtor
-[class.expl.init]: #class.expl.init
-[class.free]: #class.free
-[class.friend]: class.md#class.friend
-[class.inhctor.init]: #class.inhctor.init
-[class.init]: #class.init
-[class.mem]: class.md#class.mem
-[class.member.lookup]: class.md#class.member.lookup
-[class.mfct]: class.md#class.mfct
-[class.mi]: class.md#class.mi
-[class.qual]: basic.md#class.qual
-[class.temporary]: #class.temporary
-[class.union]: class.md#class.union
-[class.union.anon]: class.md#class.union.anon
-[class.virtual]: class.md#class.virtual
-[conv]: conv.md#conv
-[conv.array]: conv.md#conv.array
-[conv.rval]: conv.md#conv.rval
-[dcl.array]: dcl.md#dcl.array
-[dcl.constexpr]: dcl.md#dcl.constexpr
-[dcl.fct]: dcl.md#dcl.fct
-[dcl.fct.def]: dcl.md#dcl.fct.def
-[dcl.fct.def.delete]: dcl.md#dcl.fct.def.delete
-[dcl.fct.default]: dcl.md#dcl.fct.default
-[dcl.fct.spec]: dcl.md#dcl.fct.spec
-[dcl.init]: dcl.md#dcl.init
-[dcl.init.aggr]: dcl.md#dcl.init.aggr
-[dcl.init.list]: dcl.md#dcl.init.list
-[dcl.init.ref]: dcl.md#dcl.init.ref
-[dcl.spec.auto]: dcl.md#dcl.spec.auto
-[dcl.type.cv]: dcl.md#dcl.type.cv
-[diff.special]: compatibility.md#diff.special
-[except]: except.md#except
-[except.ctor]: except.md#except.ctor
-[except.handle]: except.md#except.handle
-[except.spec]: except.md#except.spec
-[except.throw]: except.md#except.throw
-[expr]: expr.md#expr
-[expr.ass]: expr.md#expr.ass
-[expr.call]: expr.md#expr.call
-[expr.cast]: expr.md#expr.cast
-[expr.const]: expr.md#expr.const
-[expr.const.cast]: expr.md#expr.const.cast
-[expr.delete]: expr.md#expr.delete
-[expr.dynamic.cast]: expr.md#expr.dynamic.cast
-[expr.mptr.oper]: expr.md#expr.mptr.oper
-[expr.new]: expr.md#expr.new
-[expr.prim]: expr.md#expr.prim
-[expr.prim.lambda.capture]: expr.md#expr.prim.lambda.capture
-[expr.pseudo]: expr.md#expr.pseudo
-[expr.ref]: expr.md#expr.ref
-[expr.sizeof]: expr.md#expr.sizeof
-[expr.static.cast]: expr.md#expr.static.cast
-[expr.sub]: expr.md#expr.sub
-[expr.throw]: expr.md#expr.throw
-[expr.type.conv]: expr.md#expr.type.conv
-[expr.typeid]: expr.md#expr.typeid
-[expr.unary.op]: expr.md#expr.unary.op
-[intro.execution]: intro.md#intro.execution
-[intro.object]: intro.md#intro.object
-[namespace.udecl]: dcl.md#namespace.udecl
-[over.ass]: over.md#over.ass
-[over.best.ics]: over.md#over.best.ics
-[over.ics.ref]: over.md#over.ics.ref
-[over.match]: over.md#over.match
-[over.match.best]: over.md#over.match.best
-[over.match.copy]: over.md#over.match.copy
-[over.over]: over.md#over.over
-[special]: #special
-[stmt.dcl]: stmt.md#stmt.dcl
-[stmt.return]: stmt.md#stmt.return
-[temp.dep.type]: temp.md#temp.dep.type
-[temp.variadic]: temp.md#temp.variadic
-[^1]: The same rules apply to initialization of an `initializer_list`
-    object ([[dcl.init.list]]) with its underlying temporary array.
-[^2]: These conversions are considered as standard conversions for the
-    purposes of overload resolution ([[over.best.ics]],
-    [[over.ics.ref]]) and therefore initialization ([[dcl.init]]) and
-    explicit casts ([[expr.static.cast]]). A conversion to `void` does
-    not invoke any conversion function ([[expr.static.cast]]). Even
-    though never directly called to perform a conversion, such
-    conversion functions can be declared and can potentially be reached
-    through a call to a virtual conversion function in a base class.
-[^3]: A similar provision is not needed for the array version of
-    `operator` `delete` because  [[expr.delete]] requires that in this
-    situation, the static type of the object to be deleted be the same
-    as its dynamic type.
-[^4]: This implies that the reference parameter of the
-    implicitly-declared copy constructor cannot bind to a `volatile`
-    lvalue; see  [[diff.special]].
-[^5]: Because a template assignment operator or an assignment operator
-    taking an rvalue reference parameter is never a copy assignment
-    operator, the presence of such an assignment operator does not
-    suppress the implicit declaration of a copy assignment operator.
-    Such assignment operators participate in overload resolution with
-    other assignment operators, including copy assignment operators,
-    and, if selected, will be used to assign an object.
-[^6]: This implies that the reference parameter of the
-    implicitly-declared copy assignment operator cannot bind to a
-    `volatile` lvalue; see  [[diff.special]].
-[^7]: Because only one object is destroyed instead of two, and one
-    copy/move constructor is not executed, there is still one object
-    destroyed for each one constructed.

+### Special member functions <a id="special">[[special]]</a>
+Default constructors [[class.default.ctor]], copy constructors, move
+constructors [[class.copy.ctor]], copy assignment operators, move
+assignment operators [[class.copy.assign]], and prospective destructors
+[[class.dtor]] are *special member functions*.
 [*Note 1*: The implementation will implicitly declare these member
 functions for some class types when the program does not explicitly
 declare them. The implementation will implicitly define them if they are
+odr-used [[basic.def.odr]] or needed for constant evaluation
+[[expr.const]]. — *end note*]
 An implicitly-declared special member function is declared at the
 closing `}` of the *class-specifier*. Programs shall not define
 implicitly-declared special member functions.
 Programs may explicitly refer to implicitly-declared special member
 functions.
 [*Example 1*:
+A program may explicitly call or form a pointer to member to an
+implicitly-declared special member function.
 ``` cpp
 struct A { };                   // implicitly declared A::operator=
 struct B : A {
   B& operator=(const B &);
 };
 B& B::operator=(const B& s) {
+  this->A::operator=(s);        // well-formed
   return *this;
 }
 ```
 — *end example*]
 [*Note 2*: The special member functions affect the way objects of class
 type are created, copied, moved, and destroyed, and how values can be
 converted to values of other types. Often such special member functions
 are called implicitly. — *end note*]
+Special member functions obey the usual access rules [[class.access]].
+[*Example 2*: Declaring a constructor protected ensures that only
 derived classes and friends can create objects using
 it. — *end example*]
+Two special member functions are of the same kind if:
+- they are both default constructors,
+- they are both copy or move constructors with the same first parameter
+  type, or
+- they are both copy or move assignment operators with the same first
+  parameter type and the same *cv-qualifier*s and *ref-qualifier*, if
+  any.
+An *eligible special member function* is a special member function for
+which:
+- the function is not deleted,
+- the associated constraints [[temp.constr]], if any, are satisfied, and
+- no special member function of the same kind is more constrained
+  [[temp.constr.order]].
 For a class, its non-static data members, its non-virtual direct base
+classes, and, if the class is not abstract [[class.abstract]], its
 virtual base classes are called its *potentially constructed
 subobjects*.
+A defaulted special member function is *constexpr-compatible* if the
+corresponding implicitly-declared special member function would be a
+constexpr function.

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