NAME

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

Synopsis

Defined in header <filesystem>
path canonical( const std::filesystem::path& p ); (1) (since C++17)
path canonical( const std::filesystem::path& p, (2) (since C++17)
std::error_code& ec );
path weakly_canonical(const std::filesystem::path& p); (3) (since C++17)
path weakly_canonical(const std::filesystem::path& p, (4) (since C++17)
std::error_code& ec);

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

The overload that does not take a std::error_code& parameter throws
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. 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().

Example

// Run this code

#include <iostream>
#include <filesystem>

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
{
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