# std::weak_ptr::weak_ptr

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< cpp‎ | memory‎ | 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

operator= | assigns the `weak_ptr` (public member function)

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