std::weak_ptr::weak_ptr

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Low level memory management
 
std::weak_ptr
 
constexpr weak_ptr();
 (1) (since C++11)
weak_ptr( const weak_ptr& r );
 (2) (since C++11)
template< class Y >
weak_ptr( const weak_ptr<Y>& r );
 (2) (since C++11)
template< class Y >
weak_ptr( const std::shared_ptr<Y>& r );
 (2) (since C++11)
weak_ptr( weak_ptr&& r );
 (3) (since C++14)
template< class Y >
weak_ptr( weak_ptr<Y>&& r );
 (3) (since C++14)

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

Exceptions

noexcept specification:  
noexcept
  

Notes

Because the default constructor is constexpr, static 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 weak_ptr in a constructor of any static object.

Example

Run this code
#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

See also

 assigns the weak_ptr
(public member function)
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