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

Helper classes

std::atomic<std::weak_ptr> (C++20)

Deduction guides(C++17)

 

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)

Helper classes

std::atomic<std::weak_ptr> (C++20)

atomic weak pointer (class template specialization)

Notes

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

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 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;
 
	observe();
    }
 
    observe();
}

Output:

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