Advantages of conditional-preprocessor over conditional statements

Question

I have never worked with #if, #ifdef, #ifndef, #else, #elif and #endif

Answer 1

Conditional preprocessor doesn't work like in your first example.

It's working with constants, you see? At compile time, it looks at the various conditions and puts in/omits source code according to it.

For example:

#define HAS_COMPARISON

int main() {
    #ifdef HAS_COMPARISON
        int i = 0;
        if(i == 0) std::cout << "This";
        else
    #else
        std::cout << "That";
    #endif
}

With the define set, it will set the variable i and perform the comparison...in short, it will output This. If you comment that define, the entire block will not be in your program which means that it will always output That, without ever setting the variable or doing the comparison.

That's the most common use of preprocessor defines. You can also define values and compare those to have variable behaviour with the same define, but that's another issue.

Once more: Conditional preprocessor is evaluated at compile time, variable conditions are evaluated at runtime.