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

From cppreference.com

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)
relative proximate (C++17)	composes a relative path (function)