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

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< cpp‎ | filesystem

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

Run this code

    #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

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

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