std::filesystem::hash_value

 

C++

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Feature test macros (C++20)

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Concepts library (C++20)

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Filesystem library (C++17)

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Technical specifications

Symbols index

External libraries

 

Filesystem library

Classes
filesystem::path filesystem::filesystem_error filesystem::directory_entry filesystem::directory_iterator filesystem::recursive_directory_iterator      filesystem::file_status	filesystem::space_info filesystem::file_type filesystem::perms filesystem::perm_options filesystem::copy_options filesystem::directory_options filesystem::file_time_type
Functions
filesystem::absolute filesystem::canonicalfilesystem::weakly_canonical filesystem::relativefilesystem::proximate filesystem::copy filesystem::copy_file filesystem::copy_symlink filesystem::create_directory filesystem::create_directories filesystem::create_hard_link filesystem::create_symlink filesystem::create_directory_symlink filesystem::current_path	filesystem::exists filesystem::equivalent filesystem::file_size filesystem::hard_link_count filesystem::last_write_time filesystem::permissions filesystem::read_symlink filesystem::remove filesystem::remove_all filesystem::rename filesystem::resize_file filesystem::space filesystem::status filesystem::symlink_status filesystem::temp_directory_path
File types
filesystem::is_block_file filesystem::is_character_file filesystem::is_directory filesystem::is_empty filesystem::status_known	filesystem::is_fifo filesystem::is_other filesystem::is_regular_file filesystem::is_socket filesystem::is_symlink

 

std::filesystem::path

Constants
path::native_formatpath::generic_formatpath::auto_format
Member functions
path::path path::~path path::operator= path::assign path::appendpath::operator/= path::concatpath::operator+= path::clear path::make_preferred path::remove_filename path::replace_filename path::replace_extension      path::swap path::compare path::beginpath::end	path::c_strpath::nativepath::operator string_type path::stringpath::u8stringpath::u16stringpath::u32stringpath::wstring path::generic_stringpath::generic_u8stringpath::generic_u16stringpath::generic_u32stringpath::generic_wstring path::lexically_normalpath::lexically_relativepath::lexically_proximate
Path decomposition
path::root_name path::root_directory path::root_path path::relative_path path::parent_path path::filename path::stem path::extension path::empty	path::has_root_pathpath::has_root_namepath::has_root_directorypath::has_relative_pathpath::has_parent_pathpath::has_filenamepath::has_stempath::has_extension path::is_absolutepath::is_relative
Non-member functions
operator==operator!=operator<operator<=operator>operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)	operator/ operator<<operator>> swap(path) hash_value u8path
Helper Classes
hash

 

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

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