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Showing content from https://en.cppreference.com/w/cpp/language/../header/../filesystem/filesystem_error.html below:

std::filesystem::filesystem_error - cppreference.com

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 library.

Inheritance diagram

[edit] Member functions constructs the exception object
(public member function) [edit] replaces the exception object
(public member function) [edit] returns the paths that were involved in the operation that caused the error
(public member function) [edit] returns the explanatory string
(public member function) [edit] Inherited from std::system_error Member functions returns error code
(public member function of std::system_error) [edit] returns an explanatory string
(virtual public member function of std::system_error) [edit] Inherited from std::exception Member functions destroys the exception object
(virtual public member function of std::exception) [edit] returns an explanatory string
(virtual public member function of std::exception) [edit] [edit] Notes

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 noexcept.

[edit] Example

#include <filesystem>
#include <iostream>
#include <system_error>
 
int main()
{
    const std::filesystem::path from{"/none1/a"}, to{"/none2/b"};
 
    try
    {
        std::filesystem::copy_file(from, to); // throws: files do not exist
    }
    catch (std::filesystem::filesystem_error const& ex)
    {
        std::cout << "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 [/none1/a] [/none2/b]
path1(): "/none1/a"
path2(): "/none2/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

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