[basic.life] - C++14 → C++17

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@@ -1,40 +1,51 @@
 ## Object lifetime <a id="basic.life">[[basic.life]]</a>
-The *lifetime* of an object is a runtime property of the object. An
-object is said to have non-trivial initialization if it is of a class or
-aggregate type and it or one of its members is initialized by a
-constructor other than a trivial default constructor. initialization by
-a trivial copy/move constructor is non-trivial initialization. The
-lifetime of an object of type `T` begins when:
 - storage with the proper alignment and size for type `T` is obtained,
   and
-- if the object has non-trivial initialization, its initialization is
-  complete.
-The lifetime of an object of type `T` ends when:
 - if `T` is a class type with a non-trivial destructor (
   [[class.dtor]]), the destructor call starts, or
-- the storage which the object occupies is reused or released.
-The lifetime of an array object starts as soon as storage with proper
-size and alignment is obtained, and its lifetime ends when the storage
-which the array occupies is reused or released. [[class.base.init]]
-describes the lifetime of base and member subobjects.
-The properties ascribed to objects throughout this International
-Standard apply for a given object only during its lifetime. In
-particular, before the lifetime of an object starts and after its
-lifetime ends there are significant restrictions on the use of the
-object, as described below, in  [[class.base.init]] and in
 [[class.cdtor]]. Also, the behavior of an object under construction and
 destruction might not be the same as the behavior of an object whose
 lifetime has started and not ended. [[class.base.init]] and
 [[class.cdtor]] describe the behavior of objects during the construction
-and destruction phases.
 A program may end the lifetime of any object by reusing the storage
 which the object occupies or by explicitly calling the destructor for an
 object of a class type with a non-trivial destructor. For an object of a
 class type with a non-trivial destructor, the program is not required to
@@ -46,32 +57,37 @@ called and any program that depends on the side effects produced by the
 destructor has undefined behavior.
 Before the lifetime of an object has started but after the storage which
 the object will occupy has been allocated[^16] or, after the lifetime of
 an object has ended and before the storage which the object occupied is
-reused or released, any pointer that refers to the storage location
-where the object will be or was located may be used but only in limited
-ways. For an object under construction or destruction, see
-[[class.cdtor]]. Otherwise, such a pointer refers to allocated storage (
-[[basic.stc.dynamic.deallocation]]), and using the pointer as if the
-pointer were of type `void*`, is well-defined. Indirection through such
-a pointer is permitted but the resulting lvalue may only be used in
-limited ways, as described below. The program has undefined behavior if:
 - the object will be or was of a class type with a non-trivial
   destructor and the pointer is used as the operand of a
   *delete-expression*,
 - the pointer is used to access a non-static data member or call a
   non-static member function of the object, or
 - the pointer is implicitly converted ([[conv.ptr]]) to a pointer to a
   virtual base class, or
 - the pointer is used as the operand of a `static_cast` (
   [[expr.static.cast]]), except when the conversion is to pointer to
- *cv* `void`, or to pointer to *cv* `void` and subsequently to pointer
- to either *cv* `char` or *cv* `unsigned char`, or
 - the pointer is used as the operand of a `dynamic_cast` (
   [[expr.dynamic.cast]]).
 ``` cpp
 #include <cstdlib>
 struct B {
   virtual void f();
@@ -90,16 +106,18 @@ limited ways, as described below. The program has undefined behavior if:
 void g() {
   void* p = std::malloc(sizeof(D1) + sizeof(D2));
   B* pb = new (p) D1;
   pb->mutate();
-    &pb;              // OK: pb points to valid memory
   void* q = pb;     // OK: pb points to valid memory
   pb->f();          // undefined behavior, lifetime of *pb has ended
 }
 ```
 Similarly, before the lifetime of an object has started but after the
 storage which the object will occupy has been allocated or, after the
 lifetime of an object has ended and before the storage which the object
 occupied is reused or released, any glvalue that refers to the original
 object may be used but only in limited ways. For an object under
@@ -107,14 +125,13 @@ construction or destruction, see  [[class.cdtor]]. Otherwise, such a
 glvalue refers to allocated storage (
 [[basic.stc.dynamic.deallocation]]), and using the properties of the
 glvalue that do not depend on its value is well-defined. The program has
 undefined behavior if:
-- an lvalue-to-rvalue conversion ([[conv.lval]]) is applied to such a
- glvalue,
-- the glvalue is used to access a non-static data member or call a
-  non-static member function of the object, or
 - the glvalue is bound to a reference to a virtual base class (
   [[dcl.init.ref]]), or
 - the glvalue is used as the operand of a `dynamic_cast` (
   [[expr.dynamic.cast]]) or as the operand of `typeid`.
@@ -134,10 +151,13 @@ started, can be used to manipulate the new object, if:
   class type, does not contain any non-static data member whose type is
   const-qualified or a reference type, and
 - the original object was a most derived object ([[intro.object]]) of
   type `T` and the new object is a most derived object of type `T` (that
   is, they are not base class subobjects).
 ``` cpp
 struct C {
   int i;
   void f();
   const C& operator=( const C& );
@@ -156,18 +176,26 @@ started, can be used to manipulate the new object, if:
 C c2;
 c1 = c2;                        // well-defined
 c1.f();                         // well-defined; c1 refers to a new object of type C
 ```
 If a program ends the lifetime of an object of type `T` with static (
 [[basic.stc.static]]), thread ([[basic.stc.thread]]), or automatic (
 [[basic.stc.auto]]) storage duration and if `T` has a non-trivial
 destructor,[^17] the program must ensure that an object of the original
 type occupies that same storage location when the implicit destructor
 call takes place; otherwise the behavior of the program is undefined.
 This is true even if the block is exited with an exception.
 ``` cpp
 class T { };
 struct B {
    ~B();
 };
@@ -176,14 +204,18 @@ void h() {
    B b;
    new (&b) T;
 }                               // undefined behavior at block exit
 ```
-Creating a new object at the storage location that a `const` object with
-static, thread, or automatic storage duration occupies or, at the
-storage location that such a `const` object used to occupy before its
-lifetime ended results in undefined behavior.
 ``` cpp
 struct B {
   B();
   ~B();
@@ -195,10 +227,14 @@ void h() {
   b.~B();
   new (const_cast<B*>(&b)) const B;     // undefined behavior
 }
 ```
 In this section, “before” and “after” refer to the “happens before”
-relation ([[intro.multithread]]). Therefore, undefined behavior results
-if an object that is being constructed in one thread is referenced from
-another thread without adequate synchronization.

 ## Object lifetime <a id="basic.life">[[basic.life]]</a>
+The *lifetime* of an object or reference is a runtime property of the
+object or reference. An object is said to have *non-vacuous
+initialization* if it is of a class or aggregate type and it or one of
+its subobjects is initialized by a constructor other than a trivial
+default constructor.
+[*Note 1*: Initialization by a trivial copy/move constructor is
+non-vacuous initialization. — *end note*]
+The lifetime of an object of type `T` begins when:
 - storage with the proper alignment and size for type `T` is obtained,
   and
+- if the object has non-vacuous initialization, its initialization is
+  complete,
+except that if the object is a union member or subobject thereof, its
+lifetime only begins if that union member is the initialized member in
+the union ([[dcl.init.aggr]], [[class.base.init]]), or as described in
+[[class.union]]. The lifetime of an object *o* of type `T` ends when:
 - if `T` is a class type with a non-trivial destructor (
   [[class.dtor]]), the destructor call starts, or
+- the storage which the object occupies is released, or is reused by an
+  object that is not nested within *o* ([[intro.object]]).
+The lifetime of a reference begins when its initialization is complete.
+The lifetime of a reference ends as if it were a scalar object.
+[*Note 2*:  [[class.base.init]] describes the lifetime of base and
+member subobjects. — *end note*]
+The properties ascribed to objects and references throughout this
+International Standard apply for a given object or reference only during
+its lifetime.
+[*Note 3*: In particular, before the lifetime of an object starts and
+after its lifetime ends there are significant restrictions on the use of
+the object, as described below, in  [[class.base.init]] and in
 [[class.cdtor]]. Also, the behavior of an object under construction and
 destruction might not be the same as the behavior of an object whose
 lifetime has started and not ended. [[class.base.init]] and
 [[class.cdtor]] describe the behavior of objects during the construction
+and destruction phases. — *end note*]
 A program may end the lifetime of any object by reusing the storage
 which the object occupies or by explicitly calling the destructor for an
 object of a class type with a non-trivial destructor. For an object of a
 class type with a non-trivial destructor, the program is not required to
 destructor has undefined behavior.
 Before the lifetime of an object has started but after the storage which
 the object will occupy has been allocated[^16] or, after the lifetime of
 an object has ended and before the storage which the object occupied is
+reused or released, any pointer that represents the address of the
+storage location where the object will be or was located may be used but
+only in limited ways. For an object under construction or destruction,
+see  [[class.cdtor]]. Otherwise, such a pointer refers to allocated
+storage ([[basic.stc.dynamic.deallocation]]), and using the pointer as
+if the pointer were of type `void*`, is well-defined. Indirection
+through such a pointer is permitted but the resulting lvalue may only be
+used in limited ways, as described below. The program has undefined
+behavior if:
 - the object will be or was of a class type with a non-trivial
   destructor and the pointer is used as the operand of a
   *delete-expression*,
 - the pointer is used to access a non-static data member or call a
   non-static member function of the object, or
 - the pointer is implicitly converted ([[conv.ptr]]) to a pointer to a
   virtual base class, or
 - the pointer is used as the operand of a `static_cast` (
   [[expr.static.cast]]), except when the conversion is to pointer to
+  cv `void`, or to pointer to cv `void` and subsequently to pointer to
+  cv `char`, cv `unsigned char`, or cv `std::byte` ([[cstddef.syn]]),
+  or
 - the pointer is used as the operand of a `dynamic_cast` (
   [[expr.dynamic.cast]]).
+[*Example 1*:
 ``` cpp
 #include <cstdlib>
 struct B {
   virtual void f();
 void g() {
   void* p = std::malloc(sizeof(D1) + sizeof(D2));
   B* pb = new (p) D1;
   pb->mutate();
+  *pb;              // OK: pb points to valid memory
   void* q = pb;     // OK: pb points to valid memory
   pb->f();          // undefined behavior, lifetime of *pb has ended
 }
 ```
+— *end example*]
 Similarly, before the lifetime of an object has started but after the
 storage which the object will occupy has been allocated or, after the
 lifetime of an object has ended and before the storage which the object
 occupied is reused or released, any glvalue that refers to the original
 object may be used but only in limited ways. For an object under
 glvalue refers to allocated storage (
 [[basic.stc.dynamic.deallocation]]), and using the properties of the
 glvalue that do not depend on its value is well-defined. The program has
 undefined behavior if:
+- the glvalue is used to access the object, or
+- the glvalue is used to call a non-static member function of the
+  object, or
 - the glvalue is bound to a reference to a virtual base class (
   [[dcl.init.ref]]), or
 - the glvalue is used as the operand of a `dynamic_cast` (
   [[expr.dynamic.cast]]) or as the operand of `typeid`.
   class type, does not contain any non-static data member whose type is
   const-qualified or a reference type, and
 - the original object was a most derived object ([[intro.object]]) of
   type `T` and the new object is a most derived object of type `T` (that
   is, they are not base class subobjects).
+[*Example 2*:
 ``` cpp
 struct C {
   int i;
   void f();
   const C& operator=( const C& );
 C c2;
 c1 = c2;                        // well-defined
 c1.f();                         // well-defined; c1 refers to a new object of type C
 ```
+— *end example*]
+[*Note 4*: If these conditions are not met, a pointer to the new object
+can be obtained from a pointer that represents the address of its
+storage by calling `std::launder` ([[support.dynamic]]). — *end note*]
 If a program ends the lifetime of an object of type `T` with static (
 [[basic.stc.static]]), thread ([[basic.stc.thread]]), or automatic (
 [[basic.stc.auto]]) storage duration and if `T` has a non-trivial
 destructor,[^17] the program must ensure that an object of the original
 type occupies that same storage location when the implicit destructor
 call takes place; otherwise the behavior of the program is undefined.
 This is true even if the block is exited with an exception.
+[*Example 3*:
 ``` cpp
 class T { };
 struct B {
    ~B();
 };
    B b;
    new (&b) T;
 }                               // undefined behavior at block exit
 ```
+— *end example*]
+Creating a new object within the storage that a `const` complete object
+with static, thread, or automatic storage duration occupies, or within
+the storage that such a `const` object used to occupy before its
+lifetime ended, results in undefined behavior.
+[*Example 4*:
 ``` cpp
 struct B {
   B();
   ~B();
   b.~B();
   new (const_cast<B*>(&b)) const B;     // undefined behavior
 }
 ```
+— *end example*]
 In this section, “before” and “after” refer to the “happens before”
+relation ([[intro.multithread]]).
+[*Note 5*: Therefore, undefined behavior results if an object that is
+being constructed in one thread is referenced from another thread
+without adequate synchronization. — *end note*]

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