Question

The following program gives a signed/unsigned mismatch warning:

#include <iostream>

int main()
{
unsigned int a = 2;
int b = -2;

if(a < b)
    std::cout << "a is less than b!";

return 0;
}

I’m trying to understand the problem when it comes to mixing signed and unsigned ints. From what I have been told, an int is typically stored in memory using two’s complement.

So, let’s say I have the number 2. Based on what I understand it will be represented in memory like this:

00000000 00000000 00000000 00000010

And -2 will be represented as the one’s compliment plus 1, or:

11111111 11111111 11111111 11111110

With two’s compliment there is no bit reserved for the sign like the “Sign-and-magnitude method”. If there is no sign bit, why are unsigned ints capable of storing larger positive numbers? What is an example of a problem which could occur when mixing signed/unsigned ints?

Answer 1

I’m trying to understand the problem when it comes to mixing signed and unsigned ints.

a < b

By the usual arithmetic conversions b is converted to an unsigned int, which is a huge number > a.

Here the expression a < b is the same as:

2U < (unsigned int) -2 which the same as:

2U < UINT_MAX - 1 (in most two’s complement systems) which is 1 (true).

With two’s compliment there is no bit reserved for the sign like the “Sign-and-magnitude method”.

In two’s complement representation if the most significant bit of a signed quantity is 1, the number is negative.