Question

I’m studying casting in C++ and the code after is magic to me.

#include <iostream>
using namespace std;

class Base {
public:
virtual void f() { }
};

#define SOME_VALUE 8

int main() {
cout << SOME_VALUE <<endl;
getchar();
}

the output is: 8

The code is very simple, but What type of SOME_VALUE? int, or double or char?

After is more complex:

#include <iostream>
using namespace std;

class Base {
public:
virtual void f() { }
};

#define SOME_VALUE 8
int main() {
cout << (Base*)SOME_VALUE-SOME_VALUE <<endl;
getchar();
}

The output is: FFFFFFE8

Following this code, I can understand that SOME_VALUE is numeric type. I also test sizeof(SOME_VALUE) and the out put is 4. But if SOME_WHAT is numeric, how can it change to object Pointer? And how Object Pointer can minus to integer?

Answer 1

SOME_VALUE is a macro–it doesn’t have a type. 8, however, is an integer.

Use #define SOME_VALUE ((Base*)8), if you want SOME_VALUE to always act like a Base*.