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std::filesystem::hash_value

From cppreference.com
< cpp‎ | filesystem‎ | path
 
 
 
 
Defined in header <filesystem>
std::size_t hash_value( const std::filesystem::path& p ) noexcept;
(since C++17)

Contents

[edit] Parameters

p - a std::filesystem::path object

[edit] 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.

[edit] 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).

[edit] 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';
}

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"

[edit] See also

compares the lexical representations of two paths lexicographically
(public member function) [edit]
(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) [edit]
checks whether two paths refer to the same file system object
(function) [edit]
(C++11)
hash function object
(class template) [edit]
hash support for std::filesystem::path
(class template specialization) [edit]