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.

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

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.

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.

Notes

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

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

Example

Run this code

#include <filesystem>
#include <iostream>
 
int main()
{
    /* set up sandbox directories:
     a
     └── b
         ├── c1
         │   └── d <== current path
         └── c2
             └── e
    */
    auto old = std::filesystem::current_path();
    auto tmp = std::filesystem::temp_directory_path();
    std::filesystem::current_path(tmp);
    auto d1 = tmp / "a/b/c1/d";
    auto d2 = tmp / "a/b/c2/e";
    std::filesystem::create_directories(d1);
    std::filesystem::create_directories(d2);
    std::filesystem::current_path(d1);
 
    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';
    try
    {
        [[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
    std::filesystem::current_path(old);
    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.

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

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path (C++17)	represents a path (class)
absolute (C++17)	composes an absolute path (function)
relativeproximate (C++17)	composes a relative path (function)