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std::weak_ptr

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.

In addition, std::weak_ptr is used to break circular references of std::shared_ptr.

Member types

Member type Definition
element_type
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)
Modifiers
releases the ownership of the managed object
(public member function)
swaps the managed objects
(public member function)
Observers
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

(C++11)
 specializes the std::swap algorithm
(function template)

Example

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

#include <iostream>
#include <memory>
 
std::weak_ptr<int> gw;
 
void f()
{
    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;
 
        f();
    }
 
    f();
}

Output:

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

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