AND Operation for -1

Question

While implementing logical operations in the code, I discovered a phenomenon where it should not be entered in the if statement. It turns out that this is the AND (&&) operation of -1 and natural numbers.

I don't know why the value 1 is printed in the same code below. I ran direct calculations such as 1's complement and 2's complement, but no 1 came out.

#include <iostream>

using namespace std;

int main()
{
    int a = 10;
    int b = -1;
    
    int c = a && b;
    
    printf("test = %d",c);

    return 0;
}

Answer 1

The expression a && b is a bool type and is either true or false: it is true if and only if both a and b are non-zero, and false otherwise. As your question mentions complementing schemes (note that from C++20, an int is always 2's complement), -0 and +0 are both zero for the purpose of &&.

When assigned to the int type c, that bool type is converted implicitly to either 0 (if false) or 1 (if true).

(In C the analysis is similar except that a && b is an int type, and the implicit conversion therefore does not take place.)