std::hash,s...,std::hash,(3)

🌟 std::hash for string_view besties 🌟

Okay so this is the C++ Standard Libary man page, and the page label is literally std::hash,s...,std::hash,(3) — yes, the commas are having a moment. We're talking about the std::hash specializations for the string view family. It's giving “hash me, keep me in an unordered container, and don't let the referenced string dangle.” 💅

🌸 NAME 🌸

std::hash,std::hash,std::hash, — std::hash,std::hash,std::hash, (that's the literal NAME, bestie; we are not fixing it, we are respecting the chaos).

✨ Synopsis ✨

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)

These are the template specializations of std::hash for the various view classes, made for hashing views. Periodt. 🫶

And the slay detail: these hashes equal the hashes of the 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))

No cap, that's the guarantee. 💖

🔥 Example 🔥

// Run this code — literally, go off:

#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 — hash function object (C++11) (class template)
• std::hash<std::basic_string> — hash support for strings (C++11) (class template specialization)

Source: http://cppreference.com · version/date: 2024.06.10 · page id: std::hash,s...,std::hash,(3). Bye bestie, stay hashed and hydrated 🫶✨