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

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 ) return rv;

I'm sure I've heard people say that putting the constant first is a good idea, but why is that? Is it just so that if you have a large statement you can quickly see what you're comparing against or is there more to it?

So that you don't mix comparison (==) with assignment (=).

As you know, you can't assign to a constant. If you try, the compiler will give you an error.

Basically, it's one of defensive programming techniques. To protect yourself from yourself.

To stop you from writing:

 if ( pMsg = NULL ) return rv;

by mistake. A good compiler will warn you about this however, so most people don't use the "constant first" way, as they find it difficult to read.

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