std::weak_ptr

From cppreference.com

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.


(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)

[edit] Non-member functions


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

[edit] 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()
{
    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:

42
gw is expired

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)

std::weak_ptr

Contents

[edit] Member types

[edit] Member functions

Modifiers

Observers

[edit] Non-member functions

[edit] Example