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

 

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

(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)

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 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