Understanding side effect in #define usage

Question

I am referring to this question

#define max(a,b) ((a<b)?b:a)

this will have a some side effect as stated in the answer;

The side effects appear if you use max(a++,b++) for example (a or b will be incremented twice)

I am not able to understand this side effect; why a or b will be incremented twice when we use max(a++,b++) ?

Remember that macros are actually text replacement. Expand out the macro and you should see the problem. — Fred Larson, Dec 12 '13 at 06:01
Also read [Sequence point](http://en.wikipedia.org/wiki/Sequence_point) — Grijesh Chauhan, Dec 12 '13 at 06:25

score 7 · Accepted Answer · answered Dec 12 '13 at 06:03

7

If you use max(a++,b++) in your code like this,

x = max(a++,b++);

a text replacement happens as

x = ((a++<b++)? b++ : a++);
      ^   ^     ^---------Increment if condition is true
      |---|---------Increment

So you will be incrementing either a or b twice...

answered Dec 12 '13 at 06:03

1

There is no parenthesis enclosing `a++` and `b++`. – timrau Dec 12 '13 at 06:04
The text replacement will actually be `x = ((a++ – Nikos C. Dec 12 '13 at 06:06
@timrau Ya was trying to edit the text, but the browser was stuck. Sorry for the mistake... – Dec 12 '13 at 06:08
@NikosC. Ya was trying to edit the text, but the browser was stuck. Sorry for the mistake... – Dec 12 '13 at 06:09

score 1 · Answer 2 · answered Dec 12 '13 at 06:05

max(a++, b++) will be expanded as ((a++ < b++) ? b++ : a++). While evaluating from the left the expression (a++ < b++) get precedence and will increment both a and b. This is the first increment. Then depending the output of < operator, either a or b will get incremented again (this is the second increment).

Understanding side effect in #define usage

2 Answers2