Question

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Best way to detect integer overflow in C/C++

i have tried to implement simple program which tests if overflow occurs during integer addition:

#include <climits>
#include <iostream>
#include <string>

using namespace std;

string overflow(long a,long b){
    return ((a+b)>UINT_MAX)?"true":"false";
}

int main(){
    long a, b;
    cout << "enter a and b: ";
    cin >> a >> b;
    string m = overflow(a,b);
    cout << m;

    return 0;
}

UINT_MAX=65535 so i have entered 65535 and 20 but it wrote false why?

Answer 1

Given unsigned int a, b, this expression can never evaluate to true:

(a+b) > UINT_MAX

The reason why it can’t ever be true is because UINT_MAX is the max, so whatever result (a+b) is can never be greater than UINT_MAX, because that would be a contradiction.

You tried to use a bigger data type long to get around this, but this is actually not guaranteed to be bigger than int in C++. You may want to try long long instead (although this type does not exist in standard C++).

Unlike, say, Java, C++ does not actually specify how large each data type must be, so the claim that UINT_MAX=65535 is not universally true. It’s only true for processors where an unsigned integer is held in 16 bits. For 32 bits, UINT_MAX is 4294967295.

Related questions

• Best way to detect integer overflow in C/C++

See also

• Wikipedia/Limits.h
• cplusplus.com/reference/library/climits