std:: weak_ptr

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 acquire 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 functions

Non-member functions

Helper classes

Deduction guides (since C++17)

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_ptr s. It is not possible to access the stored pointer in a weak_ptr without locking it into a shared_ptr .

Example

#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();
}

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.

See also