deduction guides for std::weak_ptr

Since C++17

Header: <memory>

One deduction guide is provided for std::weak_ptr to account for the edge case missed by the implicit deduction guides.

# Declarations

template< class T >
weak_ptr( std::shared_ptr<T> ) -> weak_ptr<T>;

(since C++17)

# Example

#include <memory>
 
int main()
{
    auto p = std::make_shared<int>(42);
    std::weak_ptr w{p}; // explicit deduction guide is used in this case
}