We used the ReadClasses function in this example to load the map of objects. Here, the key was an image class index and the value was a textual class description. This function is trivial and reads the synset file line by line. In such a file, each line contains a number and a class description string, separated with the space character. The following code shows its definition:

using Classes = std::map<size_t, std::string>;
Classes ReadClasses(const std::string& file_name) {
  Classes classes;
  std::ifstream file(file_name);
  if (file) {
    std::string line;
    std::string id;
    std::string label;
    std::string token;
    size_t idx = 1;
    while (std::getline(file, line)) {
       std::stringstream line_stream(line);
       size_t i = 0;
       while (std::getline(line_stream, token, ' ')) {
          switch (i) {
            case 0:
              id = token;
              break;
            case 1:
              label = token;
              break;
          }
          token.clear();
          ++i;
       }
       classes.insert({idx, label});
       ++idx;
    }
  }
  return classes;
}

Notice that we used the std::getline function in the internal while loop to tokenize a single line string. We did this by specifying the third parameter that defines the delimiter character value.