Defined in header <filesystem> | ||
|---|---|---|
std::size_t hash_value( const std::filesystem::path& p ) noexcept; | (since C++17) |
| p | - | a std::filesystem::path object |
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.
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).
#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"| compares the lexical representations of two paths lexicographically (public member function) |
|
|
(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) |
|
(C++17) | checks whether two paths refer to the same file system object (function) |
|
(C++11) | hash function object (class template) |
|
(C++17) | hash support for std::filesystem::path (class template specialization) |
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