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


(until C++17)


(since C++17)

Member functions

creates a new weak_ptr
(public member function)
destroys a weak_ptr
(public member function)
assigns the weak_ptr
(public member function)
releases the ownership of the managed object
(public member function)
swaps the managed objects
(public member function)
returns the number of shared_ptr objects that manage the object
(public member function)
checks whether the referenced object was already deleted
(public member function)
creates a shared_ptr that manages the referenced object
(public member function)
provides owner-based ordering of weak pointers
(public member function)

Non-member functions

specializes the std::swap algorithm
(function template)

Helper classes

atomic weak pointer
(class template specialization)

Deduction guides (since C++17)


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.


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

#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;


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

smart pointer with unique object ownership semantics
(class template)
smart pointer with shared object ownership semantics
(class template)

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