std::filesystem:: filesystem_error

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

Member functions

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.

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';
}

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