Objective-C preprocessor directive for 'if not'

Question

I understand how to use a preprocessor directive like this:

#if SOME_VARIABLE
    // Do something
#else
    // Do something else
#endif

But what if I only want to do something IF NOT SOME_VARIABLE.

Obviously I still could do this:

#if SOME_VARIABLE

#else
    // Do something else
#endif

. . . leaving the if empty, But is there a way to do:

#if not SOME_VARIABLE
   // Do something
#endif

Apple documentation here suggests not, but this seems like a very basic need.

Basically I want to do the preprocessor equivalent of:

if(!SOME_VARIABLE)(
{
   // Do Something
}

CarlJ · Accepted Answer · 2012-06-04T12:08:36.917

92

you could try:

#if !(SOME_VARIABLE)
   // Do something
#endif

edited Jun 04 '12 at 12:08

answered Jun 04 '12 at 11:32

CarlJ

9,461
3
33
47

I've removed all references to TARGET_IPHONE_SIMULATOR. That was just an example. I just want to know how to say IF NOT in a preprocessor directive. – Undistraction Jun 04 '12 at 11:39
6

Does the parentheses around SOME_VARIABLE matter? – Thomas Watson Mar 05 '14 at 07:25

score 24 · Answer 2 · answered Jun 04 '12 at 11:47

24

Are you trying to check if something is defined or not? If yes, you can try:

#ifndef SOME_VARIABLE

or

#if !defined(SOME_VARIABLE)

answered Jun 04 '12 at 11:47

xuzhe

5,100
22
28

My question has nothing to do whether it is defined or not. Just wanted a way to do the equivalent if if(!some_var){//do something}; – Undistraction Jun 04 '12 at 11:58

Peter M · Answer 3 · 2012-06-04T11:52:45.800

The Apple documentation (If - The C Preprocessor) is correct and this is the way that C pre-processor statements have been since the dawn of time. As per that same documentation all you can do is craft an expression that evaluates to either zero or a non-zero value and use that.

Meccan's answers is correct as TARGET_IPHONE_SIMULATOR is defined as TRUE or FALSE depending on the platform, so the expression will evaluate to either zero or a non-zero amount.

In general these macros (#if etc) are used for including or excluding things based on whether a symbol is defined or not. For that use case the pre-processor has #ifdef and #ifndef which covers what has historically been accepted as the most important cases.

Also given that the subject of these statements can only be other pre-processor defined symbols (via #define) then this limitation is reasonable.

Objective-C preprocessor directive for 'if not'

3 Answers3

Linked

Related