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# std::hash <std::unique_ptr>

From cppreference.com
< cpp‎ | memory‎ | unique ptr

template<class T, class Deleter> struct hash<unique_ptr<T, Deleter>>; |  | (since C++11)

The template specialization of std::hash for std::unique_ptr<T, Deleter> allows users to obtain hashes of objects of type std::unique_ptr<T, Deleter>.

The specialization std::hash<std::unique_ptr<T,D>> is enabled (see std::hash) if std::hash<typename std::unique_ptr<T,D>::pointer> is enabled, and is disabled otherwise. | (since C++17)

When enabled, (since C++17) for a given std::unique_ptr<T, D> p, this specialization ensures that std::hash<std::unique_ptr<T, D>>()(p) == std::hash<typename std::unique_ptr<T, D>::pointer>()(p.get()).

The member functions of this specialization are not guaranteed to be noexcept because the pointer may be a fancy pointer and its hash might throw.

### [edit] Example

Run this code

    #include <iostream>
    #include <memory>
    #include <functional>

    struct Foo {
        Foo() { std::cout << "Foo...\n"; }
        ~Foo() { std::cout << "~Foo...\n\n"; }
    };

    int main()
    {
        Foo* foo = new Foo();
        std::unique_ptr<Foo> up(foo);

        std::cout << "hash(up):  " << std::hash<std::unique_ptr<Foo>>()(up) << '\n';
        std::cout << "hash(foo): " << std::hash<Foo*>()(foo) << '\n';
    }

Output:

    Foo...
    hash(up):  3686401041
    hash(foo): 3686401041
    ~Foo...

### [edit] See also

hash (C++11) | hash function object (class template) [edit]

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