C++: variable 'std::ifstream ifs' has initializer but incomplete type

Sorry if this is pretty noobish, but I'm pretty new to C++. I'm trying to open a file and read it using ifstream:

vector<string> load_f(string file) {
  vector<string> text;

  ifstream ifs(file);
  string buffer, str_line;

  int brackets = 0;
  str_line = "";

  while ( getline(ifs, buffer) ) {
    buffer = Trim( buffer );
    size_t s = buffer.find_first_of("()");

    if (s == string::npos) str_line += "" + buffer;
    else {
      while ( s != string::npos ) {
        str_line += "" + buffer.substr(0, s + 1);
        brackets += (buffer[s] == '(' ? 1 : -1);

        if ( brackets == 0 ) {
          text.push_back( str_line );
          str_line = "";
        }

        buffer = buffer.substr(s + 1);
        s = buffer.find_first_of("()");
      }
    }
  }

  return text;
}

However, I'm getting the following error I'm not quite sure how to fix:

variable 'std::ifstream ifs' has initializer but incomplete type

Answers very appreciated. Note that I never forgot to #include <fstream>, since many have gotten the error due to just forgetting to include the header.

EDIT:

Turns out that I actually did forget to include fstream, but I had forgotten due to moving the function to another file.