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Lifetime

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Every object and reference has a lifetime, which is a runtime property: for any object or reference, there is a point of execution of a program when its lifetime begins, and there is a moment when it ends.

The lifetime of an object begins when:

  • 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, or it is made active,
  • if the object is nested in a union object, its lifetime may begin if the containing union object is assigned or constructed by a trivial special member function,
  • an array object's lifetime may also begin if it is allocated by std::allocator::allocate.

Some operations implicitly create objects of implicit-lifetime types (see below) in given region of storage and start their lifetime.

The lifetime of an object ends when:

  • if it is of a non-class type, the object is destroyed (maybe via a pseudo-destructor call) (since C++20), or
  • if it is of 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 it.

Lifetime of an object is equal to or is nested within the lifetime of its storage, see storage duration.

The lifetime of a reference begins when its initialization is complete and ends as if it were a scalar object.

Note: the lifetime of the referred object may end before the end of the lifetime of the reference, which makes dangling references possible.

Lifetimes of non-static data members and base subobjects begin and end following class initialization order.

Contents

[edit] Implicit-lifetime types

An object type is called implicit-lifetime type if it is:

  • a scalar type, or
  • an array type, or
  • an aggregate class type, or
  • a class type that has
  • at least one trivial eligible constructor, and
  • a trivial, non-deleted destructor,
  • or a cv-qualified version of one of above types.

Note that if a subobject of an implicitly created object is not of an implicit-lifetime type, its lifetime does not begin implicitly.

[edit] Temporary object lifetime

Temporary objects are created when a prvalue is materialized so that it can be used as a glvalue, which occurs (since C++17) in the following situations:

(since C++11)
  • returning a prvalue from a function
  • conversion that creates a prvalue (including T(a,b,c) and T{})
(since C++11)
(until C++17)
(since C++17)

The materialization of a temporary object is generally delayed as long as possible in order to avoid creating unnecessary temporary object: see copy elision.

(since C++17)

All temporary objects are destroyed as the last step in evaluating the full-expression that (lexically) contains the point where they were created, and if multiple temporary objects were created, they are destroyed in the order opposite to the order of creation. This is true even if that evaluation ends in throwing an exception.

There are two exceptions from that:

  • The lifetime of a temporary object may be extended by binding to a const lvalue reference or to an rvalue reference (since C++11), see reference initialization for details.
  • The lifetime of a temporary object created when evaluating the default arguments of a default or copy constructor used to initialize an element of an array ends before the next element of the array begins initialization.

[edit] Storage reuse

A program is not required to call the destructor of an object to end its lifetime if the object is trivially-destructible or if the program does not rely on the side effects of the destructor. However, if a program ends the lifetime of an non-trivially destructible object that is a variable explicitly, it must ensure that a new object of the same type is constructed in-place (e.g. via placement new) before the destructor may be called implicitly, i.e. due to scope exit or exception for automatic objects, due to thread exit for thread-local objects, or due to program exit for static objects; otherwise the behavior is undefined.

class T {}; // trivial
struct B {
    ~B() {} // non-trivial
};
void x() {
    long long n; // automatic, trivial
    new (&n) double(3.14); // reuse with a different type okay
} // okay
void h() {
    B b; // automatic non-trivially destructible
    b.~B(); // end lifetime (not required, since no side-effects)
    new (&b) T; // wrong type: okay until the destructor is called
} // destructor is called: undefined behavior

It is undefined behavior to reuse storage that is or was occupied by a const complete object of static, thread-local, or automatic storage duration because such objects may be stored in read-only memory.

struct B {
    B(); // non-trivial
    ~B(); // non-trivial
};
const B b; // const static
void h() {
    b.~B(); // end the lifetime of b
    new (const_cast<B*>(&b)) const B; // undefined behavior: attempted reuse of a const
}

If a new object is created at the address that was occupied by another object, then all pointers, references, and the name of the original object will automatically refer to the new object and, once the lifetime of the new object begins, can be used to manipulate the new object, but only if the original object is transparently replaceable by the new object.

Object x is transparently replaceable by object y if:

  • the storage for y exactly overlays the storage location which x occupied
  • y is of the same type as x (ignoring the top-level cv-qualifiers)
  • x is not a complete const object
  • neither x nor y is a base class subobject, or a member subobject declared with [[no_unique_address]] (since C++20)
  • either
  • x and y are both complete objects, or
  • x and y are direct subobjects of objects ox and oy respectively, and ox is transparently replaceable by oy.
struct C {
  int i;
  void f();
  const C& operator=( const C& );
};
const C& C::operator=( const C& other) {
  if ( this != &other ) {
    this->~C();          // lifetime of *this ends
    new (this) C(other); // new object of type C created
    f();                 // well-defined
  }
  return *this;
}
C c1;
C c2;
c1 = c2; // well-defined
c1.f();  // well-defined; c1 refers to a new object of type C

If the conditions listed above are not met, a valid pointer to the new object may still be obtained by applying the pointer optimization barrier std::launder:

struct A { 
    virtual int transmogrify();
};
 
struct B : A {
    int transmogrify() override { ::new(this) A; return 2; }
};
 
inline int A::transmogrify() { ::new(this) B; return 1; }
 
void test()
{
    A i;
    int n = i.transmogrify();
    // int m = i.transmogrify(); // undefined behavior:
    // the new A object is a base subobject, while the old one is a complete object
    int m = std::launder(&i)->transmogrify(); // OK
    assert(m + n == 3);
}
(since C++17)

Similarly, if an object is created in the storage of a class member or array element, the created object is only a subobject (member or element) of the original object's containing object if:

  • the lifetime of the containing object has begun and not ended
  • the storage for the new object exactly overlays the storage of the original object
  • the new object is of the same type as the original object (ignoring cv-qualification).

Otherwise, the name of the original subobject cannot be used to access the new object without std::launder:

(since C++17)

As a special case, objects can be created in arrays of unsigned char or std::byte (since C++17) (in which case it is said that the array provides storage for the object) if

  • the lifetime of the array has begun and not ended
  • the storage for the new object fits entirely within the array
  • there is no array object that satisfies these constraints nested within the array.

If that portion of the array previously provided storage for another object, the lifetime of that object ends because its storage was reused, however the lifetime of the array itself does not end (its storage is not considered to have been reused)

template<typename ...T>
struct AlignedUnion {
  alignas(T...) unsigned char data[max(sizeof(T)...)];
};
int f() {
  AlignedUnion<int, char> au;
  int *p = new (au.data) int;     // OK, au.data provides storage
  char *c = new (au.data) char(); // OK, ends lifetime of *p
  char *d = new (au.data + 1) char();
  return *c + *d; // OK
}

[edit] Access outside of lifetime

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, the following uses of the glvalue expression that identifies that object are undefined, unless the object is being constructed or destructed (separate set of rules applies):

  1. Lvalue to rvalue conversion (e.g. function call to a function that takes a value).
  2. Access to a non-static data member or a call to a non-static member function.
  3. Binding a reference to a virtual base class subobject.
  4. dynamic_cast or typeid expressions.

The above rules apply to pointers as well (binding a reference to virtual base is replaced by implicit conversion to a pointer to virtual base), with two additional rules:

  1. static_cast of a pointer to storage without an object is only allowed when casting to (possibly cv-qualified) void*.
  2. Pointers to storage without an object that were cast to possibly cv-qualified void* can only be static_cast to pointers to possibly cv-qualified char, or possibly cv-qualified unsigned char, or possibly cv-qualified std::byte (since C++17).

During construction and destruction it is generally allowed to call non-static member functions, access non-static data members, and use typeid and dynamic_cast. However, because the lifetime either has not begun yet (during construction) or has already ended (during destruction), only specific operations are allowed. For one restriction, see virtual function calls during construction and destruction.

[edit] Notes

Until the resolution of core issue 2256, the end of lifetime rules are different between non-class objects (end of storage duration) and class objects (reverse order of construction):

struct A {
  int* p;
  ~A() { std::cout << *p; } // undefined behavior since CWG2256: n does not outlive a
                            // well-defined until CWG2256: prints 123
};
void f() {
  A a;
  int n = 123; // if n did not outlive a, this could have been optimized out (dead store)
  a.p = &n;
}

Until the resolution of RU007, a non-static member of a const-qualified type or a reference type prevents its containing object from being transparently replaceable, which makes std::vector and std::deque hard to implement:

struct X { const int n; };
union U { X x; float f; };
void tong() {
  U u = { { 1 } };
  u.f = 5.f;                          // OK: creates new subobject of 'u'
  X *p = new (&u.x) X {2};            // OK: creates new subobject of 'u'
  assert(p->n == 2);                  // OK
  assert(u.x.n == 2);                 // undefined until RU007:
                                      // 'u.x' does not name the new subobject
  assert(*std::launder(&u.x.n) == 2); // OK even until RU007
}

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
CWG 201 C++98 lifetime of a temporary object in a default argument of a default constructor
was required to end when the initialization of the array completes
lifetime ends before the
initialization of the next element
CWG 2012 C++98 lifetime of references was specified to match storage duration, requiring that
extern references are alive before their initializers run
lifetime begins at initialization
CWG 2107 C++98 the resolution of CWG 124 was not applied to copy constructors applied
P0137R1 C++98 creating an object in an array of unsigned char reused its storage its storage is not reused
CWG 2256 C++98 lifetime of trivially destructible objects were inconsistent with other objects made consistent
P1971R0 C++98 a non-static data member of a const-qualified type or a reference type
prevented its containing object from being transparently replaceable
restriction removed
P2103R0 C++98 transparently replaceability did not require keeping the original structure requires
CWG 2470 C++98 there might be more than one arrays providing storage for the same object only one provides

[edit] References

  • C++20 standard (ISO/IEC 14882:2020):
  • 6.7.3 Object lifetime [basic.life]
  • 11.10.4 Construction and destruction [class.cdtor]
  • C++17 standard (ISO/IEC 14882:2017):
  • 6.8 Object lifetime [basic.life]
  • 15.7 Construction and destruction [class.cdtor]
  • C++14 standard (ISO/IEC 14882:2014):
  • 3 Object lifetime [basic.life]
  • 12.7 Construction and destruction [class.cdtor]
  • C++11 standard (ISO/IEC 14882:2011):
  • 3.8 Object lifetime [basic.life]
  • 12.7 Construction and destruction [class.cdtor]
  • C++03 standard (ISO/IEC 14882:2003):
  • 3.8 Object lifetime [basic.life]
  • 12.7 Construction and destruction [class.cdtor]
  • C++98 standard (ISO/IEC 14882:1998):
  • 3.8 Object lifetime [basic.life]
  • 12.7 Construction and destruction [class.cdtor]

[edit] See also

C documentation for Lifetime