deduction guides for std::shared_ptr

Since C++17

Header: <memory>

These deduction guides are provided for std::shared_ptr to account for the edge cases missed by the implicit deduction guides.

# Declarations

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

(since C++17)

template< class T, class D >
shared_ptr( std::unique_ptr<T, D> ) -> shared_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
    std::shared_ptr p2{w}; // explicit deduction guide is used in this case
}