Multiplication macro gives the wrong answer

Question

#include <iostream>
using namespace std;

#define MULTIPLY(a, b) a*b

int main(){
    cout << MULTIPLY(2+3, 3+5);
    return 0;
}

I expected this to print 40 since five times eight is forty. Why does it print 16?

Try this instead `#define MULTIPLY(a, b) ((a)*(b))` and think about what the difference is. — john, May 17 '20 at 19:05
But even better than any macros `int MULTIPLY(int a, int b) { return a * b; }`. Macros are The Devil™. — Eljay, May 17 '20 at 19:06
A good illustration why Macros are bad: `MULTIPLY(4, "Hello");` If `MULTIPLY` was a function, the compiler would catch it. — Thomas Matthews, May 17 '20 at 19:12
@ThomasMatthews Why would the compiler not catch it in a macro? That should not compile. — STF_ZBR, May 17 '20 at 19:28

score 5 · Answer 1 · edited May 17 '20 at 19:11

5

Because C++ macros are not functions. They are text copies, so that means:

cout << 2+3*3+5;

Which is 2 + (3*3) + 5

edited May 17 '20 at 19:11

John Kugelman

answered May 17 '20 at 19:06

STF_ZBR

1 Answers1