std::weak_ptr<T>::weak_ptr
// 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
.
Default constructor. Constructs empty weak_ptr.
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)
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';
}
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 |