std::weak_ptr<T>::weak_ptr

Since C++11

// 1)
constexpr weak_ptr() noexcept;
// 2)
weak_ptr( const weak_ptr& r ) noexcept;

template< class Y >
weak_ptr( const weak_ptr<Y>& r ) noexcept;

template< class Y >
weak_ptr( const std::shared_ptr<Y>& r ) noexcept;
// 3)
weak_ptr( weak_ptr&& r ) noexcept;

template< class Y >
weak_ptr( weak_ptr<Y>&& r ) noexcept;

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';
}

Result

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