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.

(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)
Technical Specifications

Type support (basic types, RTTI, type traits)
Dynamic memory management
Error handling
Program utilities
Variadic functions
Date and time
Function objects
initializer_list (C++11)
bitset
hash (C++11)
Relational operators
rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=
Pairs and tuples
pair
tuple (C++11)
piecewise_construct_t (C++11)
piecewise_construct (C++11)
integer_sequence (C++14)
Swap, forward and move
swap
exchange (C++14)
forward (C++11)
move (C++11)
move_if_noexcept (C++11)
Type operations
declval (C++11)
as_const (C++17)

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

std::weak_ptr

Contents

[edit] Member types

[edit] Member functions

Modifiers

Observers

[edit] Non-member functions

[edit] Example

Member type	Definition
element_type	T