W3cubDocs

/C++

std::weak_ptr<T>::weak_ptr

constexpr weak_ptr() noexcept;
 (1) (since C++11) 
weak_ptr( const weak_ptr& r ) noexcept;
 (2) (since C++11) 
template< class Y > 
weak_ptr( const weak_ptr<Y>& r ) noexcept;
 (2) (since C++11) 
template< class Y > 
weak_ptr( const std::shared_ptr<Y>& r ) noexcept;
 (2) (since C++11) 
weak_ptr( weak_ptr&& r ) noexcept;
 (3) (since C++11) 
template< class Y > 
weak_ptr( weak_ptr<Y>&& r ) noexcept;
 (3) (since C++11)

Constructs new weak_ptr that potentially shares an object with r.

1) Default constructor. Constructs empty weak_ptr.
2) Constructs new weak_ptr which shares an object managed by r. If r manages no object, *this manages no object too. The templated overloads don't participate in the overload resolution unless Y* is implicitly convertible to T*, or Y is the type "array of N U" for some type U and some number N, and T is the type "array of unknown bound of (possibly cv-qualified) U" (since C++17).
3) Move constructors. Moves a weak_ptr instance from r into *this. After this, r is empty and r.use_count()==0. The templated overload doesn't participate in the overload resolution unless Y* is implicitly convertible to T*.

Parameters

r - a std::shared_ptr or std::weak_ptr that will be viewed by this std::weak_ptr.

Notes

Because the default constructor is constexpr, static std::weak_ptrs are initialized as part of static non-local initialization, before any dynamic non-local initialization begins. This makes it safe to use a std::weak_ptr in a constructor of any static object.

Example

#include <memory>
#include <iostream>
 
struct Foo {};
 
int main()
{
   std::weak_ptr<Foo> w_ptr;
 
   {
      auto ptr = std::make_shared<Foo>();
      w_ptr = ptr;
      std::cout << "w_ptr.use_count() inside scope: " << w_ptr.use_count() << '\n';
   }
 
   std::cout << "w_ptr.use_count() out of scope: " << w_ptr.use_count() << '\n';
   std::cout << "w_ptr.expired() out of scope: " << std::boolalpha << w_ptr.expired() << '\n';
}

Output:

w_ptr.use_count() inside scope: 1
w_ptr.use_count() out of scope: 0
w_ptr.expired() out of scope: true

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 2315 C++11 move semantic was not enabled for weak_ptr enabled

See also

assigns the weak_ptr
(public member function)

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/memory/weak_ptr/weak_ptr