Is there a cross platform way to open files in a subdirectory in c++?

Question

Currently I am appending the folder to the filename to open a file in a sub-directory, and was wondering if there is a cleaner way to do it that makes it work on Windows and Unix based systems.

filein.open("./folder/" + filename);

I am thinking to make it work on windows I can do something like this:

filein.open("./folder/" + filename);
if(!filein.is_open()){
     filein.open(".\\folder\\" + filename);
     if(!filein.is_open()){
          cout << "Unable to open file" << endl;
     }
}

I am interested in knowing if there is something like a library function that can handle this.