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std::weak_ptr::weak_ptr(3) C++ Standard Libary std::weak_ptr::weak_ptr(3) 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 partici- pate 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 retroac- tively to previously published C++ standards. DR Applied to Behavior as published Cor- rect behavior LWG 2315 C++11 move semantic was not enabled for weak_ptr en- abled See also operator= assigns the weak_ptr (public member function) http://cppreference.com 2022.07.31 std::weak_ptr::weak_ptr(3)
NAME | Synopsis | Parameters | Notes | Example | Output: | See also
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