incrementating integers

Code:

#include <iostream>
using namespace std;

#define SQR(x) ((x) * (x))

int main()
{
int num=1;

cout << SQR(++num) << endl ;
return 0;
}

Can you explain how the output is: 9 ?
I could have thought it would be 1 or if num is incremented by 1 then 4 but how is it 9?

Very cute.

The preprocessor replaces
SQR(++num)
with
((++num) * (++num))

When the program runs, the outer expression
((++num) * (++num))
can't be evaluated until after both of the inner expressions are evaluated. I don't know which one goes first (maybe C++ leaves this up to the compiler to decide), but either way, first one of the (++num)'s increments num to 2, then the other one increments num to 3, then finally the multiplication is done.
num * num = 9

Yet another example of why #define macros shouldn't be used when functions will do.

ahh cheers, that makes sense,

I forgot about the preprocessing part!
thanks

yeh joshdick but this was in a revision quiz and was the only question to "make me think". I totally forgot about how preprocessing worked

I just wanted to point out that this behavior is undefined. It could produce 1, 2, 3, 4, 6, 9, or 572749.

could you expand on what you mean by undefined and your example?

The variable 'x' is modified twice between sequence points. The C++ standard places no requirements on what happens under such circumstances. There's a very good chance that running the code on different compilers will produce a variety of different results.

Originally posted by kernal32
yeh joshdick but this was in a revision quiz and was the only question to "make me think". I totally forgot about how preprocessing worked

My compiler outputs 6. I hope the revision quiz gave the answer: undefined.

And it really doesn't matter that it's a macro. This produces the same result:
cout << ++num * ++num << endl;

gcc on linux gives me 9, thats the compiler we are using and the quiz perhaps assumes this, althought we could infact compile it and and give the answer.

The only valid answer was 9....

I have a written exam on this tomorrow, I'll put a note that it depends on the compiler then give an answer on what I think gcc would give.

>>The only valid answer was 9....
No it ain't. The only valid answer is "undefined", but I'm sure you've realised this already. Exam questions expecting a specific number as the answer (like 9), are wrong, plain and simple. The term "undefined" means that anything could happen, which means the program isn't even guaranteed to attempt the calculation at all.

If you get stuck, or just fancy an argument with your teacher, point them to the C++ standard:

ISO/IEC 14882:1998(E)

5 Expressions

4 Except where noted, the order of evaluation of operands of individual operators and subexpressions of individual
expressions, and the order in which side effects take place, is unspecified. Between the previous
and next sequence point a scalar object shall have its stored value modified at most once by the evaluation
of an expression. Furthermore, the prior value shall be accessed only to determine the value to be stored.
The requirements of this paragraph shall be met for each allowable ordering of the subexpressions of a full
expression; otherwise the behavior is undefined. [Example:

Code:

i = v[i++]; // the behavior is unspecified
i = 7, i++, i++; // i becomes 9
i = ++i + 1; // the behavior is unspecified
i = i + 1; // the value of i is incremented

—end example]

You shouldn't be using #define's for that type of thing in C++ anyway. inline functions will do a much better job. #define's are more prominent in C, avoid them in C++ where you can.

Hammer, I understand.
I meant however that in the multichoice quiz the only "Accepted" answer was 9. There was no undefined option

Originally posted by kernal32
Hammer, I understand.
I meant however that in the multichoice quiz the only "Accepted" answer was 9. There was no undefined option

Then highlight this as a mistake to your instructor.