ios 'if not' 的 Objective-C 预处理器指令
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Objective-C preprocessor directive for 'if not'
提问by Undistraction
I understand how to use a preprocessor directive like this:
我了解如何使用这样的预处理器指令：
#if SOME_VARIABLE
// Do something
#else
// Do something else
#endif
But what if I only want to do something IF NOT SOME_VARIABLE.
但是如果我只想做一些 IF NOT SOME_VARIABLE 怎么办。
Obviously I still could do this:
显然我仍然可以这样做：
#if SOME_VARIABLE
#else
// Do something else
#endif
. . . leaving the if empty, But is there a way to do:
. . . 将 if 留空，但有没有办法做到：
#if not SOME_VARIABLE
// Do something
#endif
Apple documentation heresuggests not, but this seems like a very basic need.
此处的Apple 文档不建议这样做，但这似乎是一个非常基本的需求。
Basically I want to do the preprocessor equivalent of:
基本上我想做的预处理器相当于：
if(!SOME_VARIABLE)(
{
// Do Something
}
回答by CarlJ
you could try:
你可以试试：
#if !(SOME_VARIABLE)
// Do something
#endif
回答by xuzhe
Are you trying to check if something is defined or not? If yes, you can try:
您是否要检查是否已定义某些内容？如果是，您可以尝试：
#ifndef SOME_VARIABLE
#ifndef SOME_VARIABLE
or
或者
#if !defined(SOME_VARIABLE)
#if !defined(SOME_VARIABLE)
回答by Peter M
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.
Apple 文档（If - The C Preprocessor）是正确的，这就是 C 预处理器语句从一开始就采用的方式。根据相同的文档，您所能做的就是制作一个计算结果为零或非零值的表达式并使用它。
Meccan's answers is correct as TARGET_IPHONE_SIMULATORis defined as TRUEor FALSEdepending on the platform, so the expression will evaluate to either zero or a non-zero amount.
Meccan 的答案是正确的，因为平台TARGET_IPHONE_SIMULATOR定义TRUE或FALSE取决于平台，因此表达式的计算结果为零或非零。
In general these macros (#ifetc) are used for including or excluding things based on whether a symbol is defined or not. For that use case the pre-processor has #ifdefand #ifndefwhich covers what has historically been accepted as the most important cases.
通常，这些宏（#if等）用于根据是否定义符号来包含或排除事物。对于该用例，预处理器具有#ifdef并且#ifndef涵盖了历史上被认为是最重要的情况。
Also given that the subject of these statements can only be other pre-processor defined symbols (via #define) then this limitation is reasonable.
还考虑到这些语句的主题只能是其他预处理器定义的符号（via #define），那么这种限制是合理的。
