# std::hash (std::string_view, std::wstring_view, std::u16string_view, std::u32string_view)

From cppreference.com
< cpp‎ | string‎ | basic string view

Defined in header `<string_view>` |  |
template<> struct hash<std::string_view>; |  | (since C++17)
template<> struct hash<std::wstring_view>; |  | (since C++17)
template<> struct hash<std::u8string_view>; |  | (since C++20)
template<> struct hash<std::u16string_view>; |  | (since C++17)
template<> struct hash<std::u32string_view>; |  | (since C++17)

Template specializations of std::hash for the various view classes for hashing views.

These hashes equal the hashes of corresponding std::basic_string classes: If S is one of the standard basic_string types, SV is the corresponding string view type, and s is an object of type S, then std::hash<S>()(s) == std::hash<SV>()(SV(s)).

### Example

Run this code

    #include <iostream>
    #include <string_view>
    #include <unordered_set>
    using namespace std::literals;

    int main()
    {
        std::cout << "\"A\"   #: " << std::hash<std::string_view>{}("A"sv) << '\n';
        std::cout << "L\"B\"  #: " << std::hash<std::wstring_view>{}(L"B"sv) << '\n';
        std::cout << "u8\"C\" #: " << std::hash<std::u8string_view>{}(u8"C"sv) << '\n';
        std::cout << "u\"D\"  #: " << std::hash<std::u16string_view>{}(u"D"sv) << '\n';
        std::cout << "U\"E\"  #: " << std::hash<std::u32string_view>{}(U"E"sv) << '\n';

        // std::hash for string_view family makes it possible to keep these view-types
        // in unordered_* associative containers, such as unordered_set. But ensure
        // the lifespan of referenced strings is no less than lifespan of the container,
        // i.e. no dangling references occurred.

        std::unordered_set stars{"Rigel"sv, "Capella"sv, "Vega"sv, "Arcturus"sv};

        for (std::string_view const& s : stars)
            std::cout << s << ' ';
        std::cout << '\n';
    }

Possible output:

    "A"   #: 6919333181322027406
    L"B"  #: 11959850520494268278
    u8"C" #: 12432341034569643010
    u"D"  #: 312659256970442235
    U"E"  #: 18073225910249204957
    Arcturus Vega Capella Rigel

### See also

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

std::hash<std::string>std::hash<std::u8string>std::hash<std::u16string>std::hash<std::u32string>std::hash<std::wstring>std::hash<std::pmr::string>std::hash<std::pmr::u8string>std::hash<std::pmr::u16string>std::hash<std::pmr::u32string>std::hash<std::pmr::wstring> (C++11)(C++20)(C++11)(C++11)(C++11)(C++17)(C++20)(C++17)(C++17)(C++17) | hash support for strings (class template specialization)

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