How does the unary minus operator work on booleans in C++?

Question

I am currently converting some OpenCV code from C++ to Java. I can\'t use JavaCV, as we need the conversion in native Java, not a JNA. At one point in the code, I get the fo

Answer 1

The expression (kHit >= kForeground) yields a boolean that has value true or false. When the unary - is applied, the bool gets promoted to an int, and the conversion yields 1 for true or 0 for false. After the promotion, the sign is changed into -1 or 0 and then it is converted to uchar by the outer cast.

Note that the important bit of information is that the unary operator- is not applied to a boolean, but the boolean is converted to int and it is then applied. That can be tested with a bit of template magic:

template 
struct same_type {
    static const bool value = false;
};
template 
struct same_type {
    static const bool value = true;
};
template 
void f( T value ) {
    std::cout << "Is int? " << std::boolalpha << same_type::value << "\n";
    std::cout << "Is bool? " << same_type::value << "\n";
}
int main() {
    f(-true);
}

The f template tests the type of the passed argument against int and bool by using the same_type templates above (trivial enough to understand). If we call the f template with -true as argument type deduction will set T to be the type of the expression -true. If you run the program, you will see that it prints Is int? true\nIs bool? false.