How to check for read and write permission in C++ using std::filesystem

I have a unix codebase that is using access(path, W_OK) and access(path, R_OK), available in unistd.h, in order to see if the file is writeable/readable.
Examples at the end of the page here.

I have seen that Windows offer the same function, under the name of _access, but I wanted an implementation leveraging only std::filesystem available in C++17.

I thought about using std::filesystem::perms and perform a manual check for both read and write, but how can I pefectly mimick the checks made by access?

See the reference cppreference
If you want to check you can use this:

std::filesystem::status("filename").permissions()

That will give you list of permissions. And you can use some approach like this:

#include <filesystem>
#define F_OK 0
#define R_OK 1
#define W_OK 2
#define X_OK 3
namespace fs = std::filesystem;

int access(const fs::path& path, int amode){
    auto a=fs::status(path).permissions();
    switch(amode){
    case F_OK:
        return fs::perms::unknown!=a? 0 : -1;
    case R_OK:
        return (fs::perms::owner_read & a) == fs::perms::owner_read ? 0 : -1;
    case W_OK:
        return (fs::perms::owner_read & a) == fs::perms::owner_read  && (fs::perms::owner_write & a) == fs::perms::owner_write ? 0 : -1;
    case X_OK:
        return (fs::perms::owner_read & a) == fs::perms::owner_read  && (fs::perms::owner_exec & a) == fs::perms::owner_exec ? 0 : -1;
    default:
        return -1;
    }
}

This might not work as you expected because I don't even controlled file owner is me or Am I in same group with group owner. You need to check each case, for example," read can be operate, whether file have owner_read and you are the file owner or file have group_read and you are same group with file owner or file have others_read"
It makes something more complicated.

You need to check something like that and it is so complex I guess.

Or you can easily use fstat for Unix and GetFileAttributes for Windows and in your code just check whether your system is Unix or Windows using directives.