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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.

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

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.

Notes

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

Example

#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"

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