< cpp‎ | memory
Dynamic memory management
Uninitialized storage
Garbage collection support
C Library
Low level memory management
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
T (until C++17)
std::remove_extent_t<T> (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.

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


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

#include <iostream>
#include <memory>
std::weak_ptr<int> gw;
void observe()
    std::cout << "use_count == " << gw.use_count() << ": ";
    if (auto spt = gw.lock()) { // Has to be copied into a shared_ptr before usage
	std::cout << *spt << "\n";
    else {
        std::cout << "gw is expired\n";
int main()
        auto sp = std::make_shared<int>(42);
	gw = sp;


use_count == 1: 42
use_count == 0: gw is expired