Unexpected Result in Macro

Question

I have a simple program to calculate the volume of a cube. It runs fine, but the result I get is wrong. It comes out as \"Y is 392\". Can anyone help me understand why it is

Answer 1

This is because the macro expands to:

y = ++x * ++x * ++x;

This is a very badly written macro, for this very reason; it looks like a function call (which would evaluate the argument only once) but it really evaluates it three times.

This gives undefined behavior since there is a lack of sequence points.

Answer 2

The reason this happens is that the macro pre-processor substitutes the parameters as they are. So, CUBE(++x) is expanded to:

++x*++x*++x

You should avoid using expressions with side effects in macros because of this.