std::filesystem::hash_value
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< cpp | filesystem | path
C++
Feature test macros (C++20)
Concepts library (C++20)
Metaprogramming library (C++11)
Ranges library (C++20)
Filesystem library (C++17)
Concurrency support library (C++11)
Execution control library (C++26)
std::filesystem::path
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)
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
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"
[edit] See also
(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)
(function) [edit]