filesystem::copy returns Invalid argument, but argument is existing fs::path variable

Question

I'm working with files in my project using std::filesystem (fs) library. All paths I store as fs::path variables. My code is:

int main(){

    std::filesystem::path root_dir = "/home/xyz/study/misis2023f-22-3-kovaleva-m-a/prj.cw/test_dir";

    std::filesystem::path from_path = "/home/xyz/study/misis2023f-22-3-kovaleva-m-a/prj.cw/test_dir/misis2023_kovaleva_m_a/make.cpp";
    std::cout << std::filesystem::exists(from_path) << '\n';

    std::filesystem::copy_file(from_path, root_dir, std::filesystem::copy_options::update_existing);
}

I face the problem when I call fs::copy function, Invalid argument. But as you see, other functions (ex. fs::exists) work fine with those variables:

terminate called after throwing an instance of 'std::filesystem::__cxx11::filesystem_error' what(): filesystem error: cannot copy file: Invalid argument [/home/xyz/study/misis2023f-22-3-kovaleva-m-a/prj.cw/test_dir/misis2023_kovaleva_m_a/make.cpp] [/home/xyz/study/misis2023f-22-3-kovaleva-m-a/prj.cw/test_dir]

function signature: (from cpp-reference)

void copy( const std::filesystem::path& from,
           const std::filesystem::path& to, 
           std::filesystem::copy_options options );

How can I fix this problem with argument?

I use cmake 3.22.1, ubuntu 22.04

I tried to pass argument as a string:

std::filesystem::copy_file("/home/xyz/study/misis2023f-22-3-kovaleva-m-a/prj.cw/test_dir/misis2023_kovaleva_m_a/make.cpp", root_dir, std::filesystem::copy_options::update_existing);

I tried to pass shorter argument:

std::filesystem::path from_path = "/home/xyz/study/test_copy.txt";
std::filesystem::copy_file(from_path, root_dir, std::filesystem::copy_options::update_existing);

I also tried to pass argument without extension:

std::filesystem::path from_path = "/home/xyz/study/what";
std::filesystem::copy_options::update_existing);

But results is the same: Invalid argument

I also tried to check file status:

std::filesystem::file_status from_path_status = std::filesystem::status(from_path);

Output is:

_M_type: std::filesystem:;file_type::regular
_M_perms: 436

Answer 1

You are trying to copy a file to a directory without specifying a target file name.

std::filesystem::copy_file() expects two file paths as it merely copies one file to another file. It cannot copy a file to a directory, like you are trying to do. Since you are passing it a directory path, it fails with the error.

You need to either

• specify a target file name for std::filesystem::copy_file().

• use std::filesystem::copy() instead.