Objective-C预处理器指令'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
}

Answer 1

you could try:

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

Answer 2

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

#ifndef SOME_VARIABLE

or

#if !defined(SOME_VARIABLE)

Answer 3

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.