C++ Read int from istream, detect overflow

Question

If I read an integer from a istream using the >> operator, and the represented integer larger than INT_MAX then the operation just returns INT_MAX.

I am currently doing a comparison against INT_MAX to detect the overflow, but if the operation is inputted "2147483647" then it will return an error when in reality there was none and the result is valid.

Example: http://ideone.com/4bXyGd

#include <iostream>
#include <sstream>
#include <climits>

int main() {
    std::istringstream st("1234567890123"); // Try with 2147483647
    int result;

    st >> result;

    if (result == INT_MAX)
        std::cout << "Overflow!" << std::endl;
    else
        std::cout << result << std::endl;

    return 0;
}

What is the ideologically correct way to do this?

Answer 1

The best way to do this is to read the value as a string and then convert it into an integer. During the conversion, you can catch whether the value fits within the range of the type you're converting to.

boost::lexical_cast is a good library for this. It will throw an exception if the value cannot fit in the target type.

Answer 2

For general parsing failures (including the number being too large or too small) you can simply check the fail bit of the stringstream has been set. The easiest way to do this is:

if (!st) {
    std::cout << "Could not parse number" << std::endl;
}

Before C++11 there was no way to check specifically for overflow or underflow using this method. However in C++11 if the parsed value would be too large or too small small for the type, the result will be set to the largest value the type can hold (std::numeric_limits<Type>::max() or std::numeric_limits<Type>::min()), in addition to the fail bit being set.

So in C++11 to check if the value was too large or small you can do:

if (!st) {
    if (result == std::numeric_limits<int>::max()) {
        std::cout << "Overflow!" << std::endl;
    } else if (result == std::numeric_limits<int>::min()) {
        std::cout << "Underflow!" << std::endl;
    } else {
        std::cout << "Some other parse error" << std::endl;
    }
}