deduction guides for std::shared_ptr

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

template <class T, class D>
shared_ptr(std::unique_ptr<T, D>) ->  shared_ptr<T>;

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

Note that there is no class template argument deduction from pointer types because it is impossible to distinguish pointers obtained from array and non-array forms of new.

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
}