C++ How to make a non void function return nothing for if else statements?

So my problem goes like this:

string something (string something){
    string letter;
    cout << "press a";
    cin >> letter;
    if (letter==a){
        return a;
    }
    else
    cout << "wrong letter";
    //what should I put here if I don't want it to return any value?
}

Any alternatives would also be welcome

>Solution :

Consider using the exception mechanism for this: this carries the advantage that you can keep a reasonable return type, and you can also delegate handling of the exception to the caller of the function.

First, write

struct WrongLetterException : public std::exception{};

Then change the function to

std::string something (std::string something/*ToDo - is this correct?*/){
    std::string letter;
    std::cout << "press a";
    std::cin >> letter;
    if (letter == 'a'/*note the change here*/){
        return a;
    }
    throw WrongLetterException();
}

In the caller to the function, you then catch (const WrongLetterException& ex) and deal with it as you see fit.