# deduction guides for `std::weak_ptr`

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< cpp‎ | memory‎ | 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

Run this code

    #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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