Thread: Preventing overflow

It depends on the input of the variable. Can it be represented as a hexadecimal value? Can it be negative? Assuming length of input string can be formatted (0x, etc) and inifite, no pure and simple solution comes to my mind.

You can load this into a larger data type and then check, but I think that the best solution would be a custom string parser.

Because size of long = int in this case.

Think, if x is unsigned, how can it be less than 0..

You can first read this into a string.

Then perform a conversion using a stringstream. Then you can perform all kinds of checks on the input.

Code:

#include <string>
#include <iostream>
#include <sstream>
#include <cctype>

bool read_ulong(unsigned long& out)
{
    std::string input;
    std::cin >> input;
    if (input[0] == '-') { //streams will happily read negative values into unsigned types
        return false;
    }
    std::stringstream ss(input);
    if (! (ss >> out) ) {  //input is not a number at all
        return false;
    }
    if (!ss.eof()) {
        char ch;
        ss >> ch;
        return !isdigit(ch);  //at least one unread digit - overflow
    }
    return true;
}

int main()
{
    unsigned long n;
    while (true) {
        std::cout << "Enter unsigned integer: ";
        if (read_ulong(n)) {
            std::cout << n << '\n';
            break;
        }
        else {
            std::cout << "Bad input\n";
        }
    }
}

Example run:

Code:

Enter unsigned integer: abc
Bad input
Enter unsigned integer: -123
Bad input
Enter unsigned integer: 123456678901234567890
Bad input
Enter unsigned integer: 453.232
453

As you can see, this truncates rational numbers, which you can also report as an error or accept.