Objective C "#if" syntax

Question

I'm a little confused by the "pound if" or #if syntax I see when I look at some classes.

For example:

#if someConstant == someNumber
  do something
#elif
  etc

versus:

if (someConstant == someNumber) 
   do something
else if {
   do more stuff
}

what's the difference, and why use #if ?

Answer 1

#if etc are preprocessor directives. This means that they are dealt with before compiling and not at runtime. This can be useful, for example, in defining debugging behaviour that only compiles when you build for debug and not release:

#if DEBUG
    #define ISRelease(x) [x release]
#else
    #define ISRelease(x) [x release], x = nil
#endif

(Code courtesy of Jeff LaMarche's blog.)

This way you don't need to go through your entire application's code just before you submit your app and remove a load of debugging code. This is just one small example of the use of these directives.

Answer 2

#if is a preprocessor directive.

if is a language construct.

The difference is in the way that the final program is compiled into. When you use #if, the result of that directive is what the final program will contain on those lines. When you use the language construct, the expression that is passed to the construct will be evaluated at runtime, and not compile-time.