NAME¶

std::hash - std::hash

Synopsis¶

Defined in header <filesystem>
template<> (since C++17)
struct hash<std::filesystem::path>;

The template specialization of std::hash for std::filesystem::path allows users to
obtain hash values of std::filesystem::path.

The operator() of this specialization is noexcept. For every std::filesystem::path
value p, std::hash<std::filesystem::path>{}(p) is equal to
std::filesystem::hash_value(p).

This specialization was absent from the C++17 standard publication, see LWG issue
3657.

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)
<< std::hash<fs::path>{}(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);

std::unordered_set<fs::path, std::hash<fs::path>> 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¶

hash hash function object
(C++11) (class template)
hash_value calculates a hash value for a path object
(C++17) (function)