std::weak_ptr

 

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std::weak_ptr

Member functions

weak_ptr::weak_ptr

weak_ptr::~weak_ptr

weak_ptr::operator=

Modifiers

weak_ptr::reset

weak_ptr::swap

Observers

weak_ptr::use_count

weak_ptr::expired

weak_ptr::lock

weak_ptr::owner_before

Non-member functions

std::swap

Helper classes

std::atomic<std::weak_ptr> (C++20)

Deduction guides(C++17)

 

Defined in header <memory>
template< class T > class weak_ptr;		(since C++11)

std::weak_ptr is a smart pointer that holds a non-owning ("weak") reference to an object that is managed by std::shared_ptr. It must be converted to std::shared_ptr in order to access the referenced object.

std::weak_ptr models temporary ownership: when an object needs to be accessed only if it exists, and it may be deleted at any time by someone else, std::weak_ptr is used to track the object, and it is converted to std::shared_ptr to assume temporary ownership. If the original std::shared_ptr is destroyed at this time, the object's lifetime is extended until the temporary std::shared_ptr is destroyed as well.

Another use for std::weak_ptr is to break reference cycles formed by objects managed by std::shared_ptr. If such cycle is orphaned (i.e., there are no outside shared pointers into the cycle), the shared_ptr reference counts cannot reach zero and the memory is leaked. To prevent this, one of the pointers in the cycle can be made weak.

Member types

Member type	Definition
element_type	T (until C++17) std::remove_extent_t<T> (since C++17)

Member functions

(constructor)	creates a new weak_ptr (public member function)
(destructor)	destroys a weak_ptr (public member function)
operator=	assigns the weak_ptr (public member function)
Modifiers
reset	releases the ownership of the managed object (public member function)
swap	swaps the managed objects (public member function)
Observers
use_count	returns the number of shared_ptr objects that manage the object (public member function)
expired	checks whether the referenced object was already deleted (public member function)
lock	creates a shared_ptr that manages the referenced object (public member function)
owner_before	provides owner-based ordering of weak pointers (public member function)

Non-member functions

std::swap(std::weak_ptr) (C++11)

specializes the std::swap algorithm (function template)

Helper classes

std::atomic<std::weak_ptr> (C++20)

atomic weak pointer (class template specialization)

Notes

Like std::shared_ptr, a typical implementation of weak_ptr stores two pointers:

• a pointer to the control block; and
• the stored pointer of the shared_ptr it was constructed from.

A separate stored pointer is necessary to ensure that converting a shared_ptr to weak_ptr and then back works correctly, even for aliased shared_ptrs. It is not possible to access the stored pointer in a weak_ptr without locking it into a shared_ptr.

Example

Demonstrates how lock is used to ensure validity of the pointer.

Run this code

#include <iostream>
#include <memory>
 
std::weak_ptr<int> gw;
 
void observe()
{
    std::cout << "gw.use_count() == " << gw.use_count() << "; ";
    // we have to make a copy of shared pointer before usage:
    if (std::shared_ptr<int> spt = gw.lock())
        std::cout << "*spt == " << *spt << '\n';
    else
        std::cout << "gw is expired\n";
}
 
int main()
{
    {
        auto sp = std::make_shared<int>(42);
        gw = sp;
 
        observe();
    }
 
    observe();
}

Output:

gw.use_count() == 1; *spt == 42
gw.use_count() == 0; gw is expired

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
LWG 3001	C++17	element_type was not updated for array support	updated

See also

unique_ptr (C++11)	smart pointer with unique object ownership semantics (class template)
shared_ptr (C++11)	smart pointer with shared object ownership semantics (class template)