NAME¶

std::filesystem::hash_value - std::filesystem::hash_value

Synopsis¶

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 <iomanip>
#include <iostream>
#include <filesystem>
#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 << ' '; }
}

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¶

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