Question

Running this code:

#include <string>
#include <iostream>
using namespace std;
int main() {
    string a = "Hello, world!";
    cout << (-1)*a.size() << endl;
}

I get:

18446744073709551603

which is clearly not what I want. This annoying bug happens every time I try to multiply a string::size() by a negative number (but not for positive!). I can’t figure out what is the problem here.

Answer 1

In (-1)*a.size(), (-1) has type int and a.size() has an unsigned type (probably size_t).

(-1)*a.size() is computed as an unsigned integer, because promotion and conversion rules favor unsigned types when binary operations are applied to a signed type and an unsigned type.