Distinguishing operands from operations from user input

In the main loop of evaluate_rpn, we take the current string token from the iterator and then see whether it is an operand or not. If the string can be parsed into a double variable, then it is a number, and hence also an operand. We consider everything which is not easily parseable as a number (such as "+", for example) to be an operation.

The naked code skeleton for exactly this task is as follows:

stringstream ss {*it};
if (double val; ss >> val) {
    // It's a number!
} else {
    // It's something else than a number - an operation!
}

The stream operator >> tells us if it is a number. First, we wrapped the string into an std::stringstream. Then we use the stringstream object's capability to stream from an std::string into a double variable, which involves parsing. If the parsing fails, we know that it does so, because we asked it to parse something into a number, which is no number.