Question

I’m having a annoying problem with a C++ function that I wrote and whose purpose is to validate de user input. The function reads the user input, verifies if it’s a number and, if so, if it is in the range [min, max].

The problem occurs when I invoke the template function with a unsigned type, like size_t, and the input is a negative number. The string stream converts the string to something like 4294967291. I can see that the program is converting the data to a value near de maximum value of the unsigned data type (defined in the numeric_limits header) but my question is why, since the if statement should stop at sstream >> value?

My code:

template <class T>
T getNumberInput(std::string prompt, T min, T max) {
    std::string input;
    T value;

    while (true) {
        try {
            std::cout << prompt;
            std::cin.clear();
            std::getline(std::cin, input);
            std::stringstream sstream(input);

            if (input.empty()) {
                throw EmptyInput<std::string>(input);
            } else if (sstream >> value && value >= min && value <= max) {
                std::cout << std::endl;
                return value;
            } else {
                throw InvalidInput<std::string>(input);
            }
        } catch (EmptyInput<std::string> & emptyInput) {
            std::cout << "O campo não pode ser vazio!\n" << std::endl;
        } catch (InvalidInput<std::string> & invalidInput){
            std::cout << "Tipo de dados inválido!\n" << std::endl;
        }
    }
}

Thank you for your time!

Answer 1

In C++ arithmetic involving an unsigned type with n value bits, is guaranteed to be modulo 2^n. That means any result is wrapped back into the range 0 through 2^n-1, by adding or subtracting a suitable multiple of 2^n. This is so also in C.

So you need to check the input for minus sign, or add some other check.

By the way, your if with >> and && produced some effect on my bad-code-meter. I can never remember the operator precedences for >> versus &&. I guess if it compiled it must be OK, though, since >> can’t take a value right-hand-side. Checking… OK, but I’d use parentheses to clarify that.

Also, on the code structure, it would be a good idea to separate the interactive input thing from the checking of the input. E.g., can you use any of that code in a GUI program, with input from an edit field? No, not as it is…

Cheers & hth.,