std::filesystem::hash_value

Since C++17

Header: <filesystem>

1 Parameters

# Declarations

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

(since C++17)

# Parameters

# Return value

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

# 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

#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';
}

# See also