std::weak_ptr<T>::weak_ptr

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std::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

Example

Run this code
#include <iostream>
#include <memory>
 
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)
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