Why put the constant before the variable in a comparison?

I noticed for a while now the following syntax in some of our code:

if( NULL == var){
   //...
}

or

if( 0 == var){
  //...
}

and similar things.

Can someone please explain why did the person who wrote this choose this notation instead of the common var == 0 way)?

Is it a matter of style, or does it somehow affect performance?

It's a mechanism to avoid mistakes like this:

if ( var = NULL ) {
  // ...
}

If you write it with the variable name on the right hand side the compiler will be able catch certain mistakes:

if ( NULL = var ) {  // not legal, won't compile
  // ...
}

Of course this won't work if variable names appear on both sides of the equal sign and some people find this style unappealing.

Edit:

As Evan mentioned in the comments, any decent compiler will warn you about this if you enable warnings, for example, gcc -Wall will give you the following:

warning: suggest parentheses around assignment used as truth value

You should always enable warnings on your compiler, it is the cheapest way to find errors.

Lastly, as Mike B points out, this is a matter of style and doesn't affect the performance of the program.

If you mistakenly put

if ( var = NULL )

instead of

if ( var == NULL )

then there will only be a compiler warning. If you reverse the order:

if ( NULL == var )

then there will be a compiler error if you put

if ( NULL = var )

Personally, I hate to read code written that way, and I only made that mistake once in my first year of coding. =)

To avoid the

if (var = NULL)

bug

Corollary: try to use const as much as you can.

const int val = 42;

if (val = 43) {
    ...
}

will not compile.

Quoting Joel On Software, The Guerrilla Guide to Interviewing:

Occasionally, you will see a C programmer write something like if (0==strlen(x)), putting the constant on the left hand side of the == . This is a really good sign. It means that they were stung once too many times by confusing = and == and have forced themselves to learn a new habit to avoid that trap.

(I'm not really a fan of this "best practice".)