std::filesy...::hash_value(3)

💖 std::filesystem::hash_value 💖

okay bestie, welcome to std::filesystem::hash_value from the C++ Standard Libary manual — section (3), yep, that's the header's spelling, not me 😭 — and this page is literally all about hashing a std::filesystem::path. 💅

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🌸 NAME 🌸

std::filesystem::hash_value — that's the name, bestie. It's the std::filesystem::hash_value function, and she's the main character of this page. 👑

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✨ Synopsis ✨

Defined in header <filesystem>:

std::size_t hash_value( const std::filesystem::path& p ) noexcept;

(since C++17) — lowkey a classic. 🫶

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🎀 Parameters 🎀

• p — a std::filesystem::path object. One parameter, zero drama, iconic behavior. ✨

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💖 Return value 💖

A hash value such that if for two paths, p1 == p2 then hash_value(p1) == hash_value(p2).

The return value is consistent with std::hash. No cap. 🫶

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🦋 Notes 🦋

Equality of two paths is determined by comparing each component separately, so, for example, "a//b" equals "a/b" and has the same hash_value. The way that works is honestly so satisfying. 💜

hash_value originates from the Boost.filesystem library where it was used for interoperability with boost.hash (which calls hash_value found by argument-dependent lookup or boost::hash_value where available). History! 🌈

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🔥 Example 🔥

       // Run this code

        #include <cassert>
        #include <cstddef>
        #include <filesystem>
        #include <iomanip>
        #include <iostream>
        #include <unordered_set>
        namespace fs = std::filesystem;

        void show_hash(fs::path const& p)
        {
            std::cout << std::hex << std::uppercase << std::setw(16)
                      << fs::hash_value(p) << " : " << p << '\n';
        }

        int main()
        {
            auto tmp1 = fs::path{"/tmp"};
            auto tmp2 = fs::path{"/tmp/../tmp"};
            assert(!(tmp1 == tmp2));
            assert(fs::equivalent(tmp1, tmp2));
            show_hash(tmp1);
            show_hash(tmp2);

            for (auto s : {"/a///b", "/a//b", "/a/c", "...", "..", ".", ""})
                show_hash(s);

            // A hash function object to work with unordered_* containers:
            struct PathHash
            {
                std::size_t operator()(fs::path const& p) const noexcept
                {
                    return fs::hash_value(p);
                }
            };
            std::unordered_set<fs::path, PathHash> dirs{
                "/bin", "/bin", "/lib", "/lib", "/opt", "/opt", "/tmp",
       "/tmp/../tmp"};
            for (fs::path const& p : dirs)
                std::cout << p << ' ';
            std::cout << '\n';
        }

🌈 Possible output: 🌈

        6050C47ADB62DFE5 : "/tmp"
        62795A58B69AD90A : "/tmp/../tmp"
        FF302110C9991974 : "/a///b"
        FF302110C9991974 : "/a//b"
        FD6167277915D464 : "/a/c"
        C42040F82CD8B542 : "..."
        D2D30154E0B78BBC : ".."
        D18C722215ED0530 : "."
                       0 : ""
        "/tmp/../tmp" "/opt" "/lib" "/tmp" "/bin"

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👑 SEE ALSO 👑

• compare — compares the lexical representations of two paths lexicographically (public member function).
• operator== — lexicographically compares two paths (function) — (C++17).
• operator!= — lexicographically compares two paths (function) — (C++17)(until C++20).
• operator< — lexicographically compares two paths (function) — (C++17)(until C++20).
• operator<= — lexicographically compares two paths (function) — (C++17)(until C++20).
• operator> — lexicographically compares two paths (function) — (C++17)(until C++20).
• operator>= — lexicographically compares two paths (function) — (C++17)(until C++20).
• operator<=> — lexicographically compares two paths (function) — (C++20).
• equivalent — checks whether two paths refer to the same file system object (function) — (C++17).
• hash — hash function object (class template) — (C++11).
• std::hash<std::filesystem::path> — hash support for std::filesystem::path (class template specialization) — (C++17).

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byeee bestie 🫶💖 — this page is from http://cppreference.com, version/date 2024.06.10, and the manual page is std::filesy...::hash_value(3). mwah 💋