What's the reasoning behind putting constants in if statements first?

Question

I was looking at some example C++ code for a hardware interface I\'m working with and noticed a lot of statements along the following lines:

if ( NULL == pMsg )          


        

Answer 1

It stops the single = assignment bug.

Eg,

if ( NULL = pMsg ) return rv;

won't compile, where as

if ( pMsg =  NULL) return rv;

will compile and give you headaches