Strange behaviour of macros C/C++

Question

I\'m using some macros, and observing some strange behaviour.

I\'ve defined PI as a constant, and then used it in macros to convert degrees to radians and radians to

Answer 1

First, cmath defines M_PI, use that.

Second, cpp macros do textual substitution. Which mean that this:

#define PI atan(1) * 4
a = 1 / PI;

will be turned into this:

a = 1 / atan(1) * 4;

before the c/c++ compiler gets a chance to see your code, and it will treat it equivalent to this:

a = (1 / atan(1)) * 4;

which is not what you want.

Your define should look like this:

#define PI (atan(1) * 4)

and everything should be fine.

This is not really strange behaviour, but well documented behaviour of the c-preprocessor.

You should search the web for other pitfalls with macros. (hint: parameter passing)