deduction guides for std::weak_ptr
Since C++17
template<class T>weak_ptr(std::shared_ptr<T>) -> weak_ptr<T>;
One deduction guide is provided for std::weak_ptr to account for the edge case missed by the implicit deduction guides.
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}