# std::hash<std::unique_ptr>

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< cpp‎ | memory‎ | unique ptr

template< class T, class Deleter > struct hash< std::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.

When enabled, 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.

### Example

Run this code

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

    struct Foo
    {
        Foo(int num) : nr(num) { std::cout << "Foo(" << nr << ")\n"; }

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

        bool operator==(const Foo &other) const { return nr == other.nr; };

        int nr;
    };

    int main()
    {
        std::cout << std::boolalpha << std::hex;

        Foo* foo = new Foo(5);
        std::unique_ptr<Foo> up(foo);
        std::cout << "hash(up):    " << std::hash<std::unique_ptr<Foo>>()(up) << '\n'
                  << "hash(foo):   " << std::hash<Foo*>()(foo) << '\n'
                  << "*up==*foo:   " << (*up == *foo) << "\n\n";

        std::unique_ptr<Foo> other = std::make_unique<Foo>(5);
        std::cout << "hash(up):    " << std::hash<std::unique_ptr<Foo>>()(up) << '\n'
                  << "hash(other): " << std::hash<std::unique_ptr<Foo>>()(other) << '\n'
                  << "*up==*other: " <<(*up == *other) << "\n\n";
    }

Possible output:

    Foo(5)
    hash(up):    acac20
    hash(foo):   acac20
    *up==*foo:   true

    Foo(5)
    hash(up):    acac20
    hash(other): acbc50
    *up==*other: true

    ~Foo()
    ~Foo()

### See also

hash (C++11) | hash function object (class template)

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