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std::filesystem::filesystem_error(3) C++ Standard Libary std::filesystem::filesystem_error(3)


std::filesystem::filesystem_error - std::filesystem::filesystem_error


Defined in header <filesystem>
class filesystem_error; (since C++17)

The class std::filesystem::filesystem_error defines an exception object that is
thrown on failure by the throwing overloads of the functions in the filesystem

std-filesystem-filesystem error-inheritance.svg

Inheritance diagram

Member functions

constructor constructs the exception object
(public member function)
operator= replaces the exception object
(public member function)
path1 returns the paths that were involved in the operation that caused the
path2 error
(public member function)
what returns the explanatory string
(public member function)

Inherited from std::system_error

Member functions

code returns error code
(public member function of std::system_error)
what returns an explanatory string
[virtual] (virtual public member function of std::system_error)

Inherited from std::runtime_error

Inherited from std::exception

Member functions

destructor destroys the exception object
[virtual] (virtual public member function of std::exception)
what returns an explanatory string
[virtual] (virtual public member function of std::exception)


In order to ensure that copy functions of filesystem_error are noexcept, typical
implementations store an object holding the return value of what() and two
std::filesystem::path objects referenced by path1() and path2() respectively in a
separately-allocated reference-counted storage.

Currently the MS STL implementation is non-conforming: objects mentioned above are
stored directly in the filesystem object, which makes the copy functions not


// Run this code

#include <system_error>
#include <filesystem>
#include <iostream>

int main()
const std::filesystem::path from{"/nonexistent1/a"}, to{"/nonexistent2/b"};
try {
std::filesystem::copy_file(from, to); // throws: files do not exist
catch(std::filesystem::filesystem_error const& ex) {
<< "what(): " << ex.what() << '\n'
<< "path1(): " << ex.path1() << '\n'
<< "path2(): " << ex.path2() << '\n'
<< "code().value(): " << ex.code().value() << '\n'
<< "code().message(): " << ex.code().message() << '\n'
<< "code().category(): " << ex.code().category().name() << '\n';

// All functions have non-throwing equivalents
std::error_code ec;
std::filesystem::copy_file(from, to, ec); // does not throw
std::cout << "\nnon-throwing form sets error_code: " << ec.message() << '\n';

Possible output:

what(): filesystem error: cannot copy file: No such file or directory [/nonexistent1/a] [/nonexistent2/b]
path1(): "/nonexistent1/a"
path2(): "/nonexistent2/b"
code().value(): 2
code().message(): No such file or directory
code().category(): generic

non-throwing form sets error_code: No such file or directory