deduction guides for std::weak_ptr

Defined in header <memory> 
template<class T>
weak_ptr(std::shared_ptr<T>) -> weak_ptr<T>;
 (since C++17)

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
}

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