std::weak_ptr

 

C++

Language

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)

 

Utilities library

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)

Swap, forward and move

swap

forward (C++11)

move (C++11)

move_if_noexcept (C++11)

declval (C++11)

 

Dynamic memory management

Low level memory management
Allocators
allocator
allocator_traits (C++11)
allocator_arg_t (C++11)
allocator_arg (C++11)
uses_allocator (C++11)
scoped_allocator_adaptor (C++11)
Uninitialized storage
uninitialized_copy
uninitialized_copy_n (C++11)
uninitialized_fill
uninitialized_fill_n
raw_storage_iterator
get_temporary_buffer
return_temporary_buffer
Smart pointers
unique_ptr (C++11)
shared_ptr (C++11)
weak_ptr (C++11)
auto_ptr (deprecated)
owner_less (C++11)
enable_shared_from_this (C++11)
bad_weak_ptr (C++11)
default_delete (C++11)
Garbage collection support
declare_reachable (C++11)
undeclare_reachable (C++11)
declare_no_pointers (C++11)
undeclare_no_pointers (C++11)
pointer_safety (C++11)
get_pointer_safety (C++11)
Miscellaneous
pointer_traits (C++11)
addressof (C++11)
align (C++11)
C Library
malloc calloc	realloc free

 

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

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.

#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