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, if any. The resulting path is in normal form.


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


The overload that does not take a std::error_code& parameter throws 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. The overload taking a std::error_code& parameter sets it to the OS API error code if an OS API call fails, and executes ec.clear() if no errors occur. Any overload not marked noexcept may throw std::bad_alloc if memory allocation fails.


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

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

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


#include <iostream>
#include <filesystem>
namespace fs = std::filesystem;
int main()
    fs::path p = fs::path("..") / ".." / "AppData";
    std::cout << "Current path is " << fs::current_path() << '\n'
              << "Canonical path for " << p << " is " << fs::canonical(p) << '\n';

Possible output:

Current path is "C:\Users\abcdef\AppData\Local\Temp"
Canonical path for "..\..\AppData" is "C:\Users\abcdef\AppData"

See also

represents a path
composes an absolute path
composes a relative path