std::filesystem::canonical, std::filesystem::weakly_canonical

Defined in header <filesystem>
path canonical( const std::filesystem::path& p );
(1) (since C++17)
path canonical( const std::filesystem::path& p,
                std::error_code& ec );
(2) (since C++17)
path weakly_canonical( const std::filesystem::path& p );
(3) (since C++17)
path weakly_canonical( const std::filesystem::path& p,
                       std::error_code& ec );
(4) (since C++17)
1,2) Converts path p to a canonical absolute path, i.e. an absolute path that has no dot, dot-dot elements or symbolic links in its generic format representation. If p is not an absolute path, the function behaves as if it is first made absolute by std::filesystem::absolute(p). The path p must exist.
3,4) Returns a path composed by operator/= from the result of calling canonical() with a path argument composed of the leading elements of p that exist (as determined by status(p) or status(p, ec)), if any, followed by the elements of p that do not exist. The resulting path is in normal form.


[edit] Parameters

p - a path which may be absolute or relative; for canonical it must be an existing path
ec - error code to store error status to

[edit] Return value

1,2) An absolute path that resolves to the same file as std::filesystem::absolute(p).
3,4) A normal path of the form canonical(x)/y, where x is a path composed of the longest leading sequence of elements in p that exist, and y is a path composed of the remaining trailing non-existent elements of p.

[edit] Exceptions

Any overload not marked noexcept may throw std::bad_alloc if memory allocation fails.

1,3) Throws std::filesystem::filesystem_error on underlying OS API errors, constructed with p as the first path argument and the OS error code as the error code argument.
2,4) Sets a std::error_code& parameter to the OS API error code if an OS API call fails, and executes ec.clear() if no errors occur.

[edit] Notes

The function canonical() is modeled after the POSIX realpath.

The function weakly_canonical() was introduced to simplify operational semantics of relative().

[edit] Example

#include <filesystem>
#include <iostream>
int main()
    /* set up sandbox directories:
     └── b
         ├── c1
         │   └── d <== current path
         └── c2
             └── e
    auto old = std::filesystem::current_path();
    auto tmp = std::filesystem::temp_directory_path();
    auto d1 = tmp / "a/b/c1/d";
    auto d2 = tmp / "a/b/c2/e";
    auto p1 = std::filesystem::path("../../c2/./e");
    auto p2 = std::filesystem::path("../no-such-file");
    std::cout << "Current path is "
              << std::filesystem::current_path() << '\n'
              << "Canonical path for " << p1 << " is "
              << std::filesystem::canonical(p1) << '\n'
              << "Weakly canonical path for " << p2 << " is "
              << std::filesystem::weakly_canonical(p2) << '\n';
        [[maybe_unused]] auto x_x = std::filesystem::canonical(p2);
        // NOT REACHED
    catch (const std::exception& ex)
        std::cout << "Canonical path for " << p2 << " threw exception:\n"
                  << ex.what() << '\n';
    // cleanup
    const auto count = std::filesystem::remove_all(tmp / "a");
    std::cout << "Deleted " << count << " files or directories.\n";

Possible output:

Current path is "/tmp/a/b/c1/d"
Canonical path for "../../c2/./e" is "/tmp/a/b/c2/e"
Weakly canonical path for "../no-such-file" is "/tmp/a/b/c1/no-such-file"
Canonical path for "../no-such-file" threw exception:
filesystem error: in canonical: No such file or directory [../no-such-file] [/tmp/a/b/c1/d]
Deleted 6 files or directories.

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2956 C++17 canonical has a spurious base parameter removed

[edit] See also

represents a path
(class) [edit]
composes an absolute path
(function) [edit]
composes a relative path
(function) [edit]