NAME

std::weak_ptr::weak_ptr - std::weak_ptr::weak_ptr

Synopsis

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

// 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

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.