[basic.life]
6 Basics [basic]
6.8 Memory and objects [basic.memobj]
6.8.4 Lifetime [basic.life]
In this subclause, “before” and “after” refer to the “happens before” relation ([intro.multithread]).
The lifetime of an object or reference is a runtime property of the object or reference.
A variable is said to have vacuous initialization if it is default-initialized, no other initialization is performed, and, if it is of class type or a (possibly multidimensional) array thereof, a trivial constructor of that class type is selected for the default-initialization.
The lifetime of an object o of type T ends when:
- if T is a non-class type, the object is destroyed, or
- if T is a class type, 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]).
When evaluating a new-expression, storage is considered reused after it is returned from the allocation function, but before the evaluation of the new-initializer ([expr.new]).
[Example 1: struct S { int m; }; void f() { S x{1}; new(&x) S(x.m); } — end example]
The lifetime of a reference begins when its initialization is complete.
The lifetime of a reference ends as if it were a scalar object requiring storage.
The properties ascribed to objects and references throughout this document apply for a given object or reference only during its lifetime.
[Note 2:
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 can differ from the behavior of an object whose lifetime has started and not ended.
[class.base.init] and [class.cdtor] describe the behavior of an object during its periods of construction and destruction.
— end note]
A program may end the lifetime of an object of class type without invoking the destructor, by reusing or releasing the storage as described above.
In this case, the destructor is not implicitly invoked.
Before the lifetime of an object has started but after the storage which the object will occupy has been allocated16 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.allocation]), 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 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 converted ([conv.ptr], [expr.static.cast]) to a pointer to a virtual base class or to a base class thereof, or
- the pointer is used as the operand of a dynamic_cast ([expr.dynamic.cast]).
[Example 2: #include <cstdlib> struct B { virtual void f(); void mutate(); virtual ~B(); }; struct D1 : B { void f(); }; struct D2 : B { void f(); }; void B::mutate() { new (this) D2; f(); ... = this; } void g() { void* p = std::malloc(sizeof(D1) + sizeof(D2)); B* pb = new (p) D1; pb->mutate(); *pb; void* q = pb; pb->f(); } — 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 construction or destruction, see [class.cdtor].
Otherwise, such a glvalue refers to allocated storage ([basic.stc.dynamic.allocation]), 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.
An object is transparently replaceable by an object if either
- and are complete objects for which:
- is not const,
- the storage that occupies exactly overlays the storage that occupied, and
- and are of the same type (ignoring the top-level cv-qualifiers), or
- and are corresponding direct subobjects ([intro.object]) for which:
- the complete object of is not const or
- is a mutable member subobject or a subobject thereof.
After the lifetime of an object has ended and before the storage which the object occupied is reused or released, if a new object is created at the storage location which the original object occupied and the original object was transparently replaceable by the new object, a pointer that pointed to the original object, a reference that referred to the original object, or the name of the original object will automatically refer to the new object and, once the lifetime of the new object has started, can be used to manipulate the new object.
[Example 3: struct C { int i; void f(); const C& operator=( const C& ); }; const C& C::operator=( const C& other) { if ( this != &other ) { this->~C(); new (this) C(other); f(); } return *this; } C c1; C c2; c1 = c2; c1.f(); — end example]
[Note 6:
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 ([ptr.launder]).
— 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 and another object of the original type does not occupy that same storage location when the implicit destructor call takes place, the behavior of the program is undefined.
This is true even if the block is exited with an exception.
[Example 4: class T { }; struct B { ~B(); }; void h() { B b; new (&b) T; } — 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 5: struct B { B(); ~B(); }; const B b; void h() { b.~B(); new (const_cast<B*>(&b)) const B; } — end example]