# std::filesystem::hash_value

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< cpp‎ | filesystem‎ | path

Defined in header `<filesystem>` |  |
std::size_t hash_value( const std::filesystem::path & p ) noexcept; |  | (since C++17)

### Parameters

p | - | a std::filesystem::path object

### 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`.

### 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`.

`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).

### 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"

### See also

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

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