ios #if 和 #ifdef Objective-C 预处理器宏之间有什么区别？
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What's the difference between #if and #ifdef Objective-C preprocessor macro?
提问by Geri Borbás
How to define preprocessor macros in build settings, like IPAD_BUILD, and IPHONE_BUILD (and how to use them in my factory methods)?
如何在构建设置中定义预处理器宏，例如 IPAD_BUILD 和 IPHONE_BUILD（以及如何在我的工厂方法中使用它们）？
I'm using these by heart now, would be cool to know what is going behind.
我现在正在用心使用这些，知道后面发生了什么会很酷。
回答by Stas
/#if works as usual if:
/#if 正常工作，如果：
#if __IPHONE_OS_VERSION_MAX_ALLOWED >= 30200
if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad) {
return YES;
}
#endif
return NO;
}
/#ifdef means "if defined - some value or macros":
/#ifdef 表示“如果已定义 - 某些值或宏”：
#ifdef RKL_APPEND_TO_ICU_FUNCTIONS
#define RKL_ICU_FUNCTION_APPEND(x) _RKL_CONCAT(x, RKL_APPEND_TO_ICU_FUNCTIONS)
#else // RKL_APPEND_TO_ICU_FUNCTIONS
#define RKL_ICU_FUNCTION_APPEND(x) x
#endif // RKL_APPEND_TO_ICU_FUNCTIONS
or:
或者：
#ifdef __OBJC__
#import <Foundation/Foundation.h>
#endif
Use this link for more information http://www.techotopia.com/index.php/Using_Objective-C_Preprocessor_Directives
使用此链接获取更多信息 http://www.techotopia.com/index.php/Using_Objective-C_Preprocessor_Directives
To test whether you running iPad or not you should have smth like this:
要测试您是否运行 iPad，您应该像这样：
#define USING_IPAD UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad
if (USING_IPAD) {
NSLog(@"running iPad");
}
Here's another useful preprocessor functions:
这是另一个有用的预处理器函数：
#ifdef DEBUG
//here we run application through xcode (either simulator or device). You usually place some test code here (e.g. hardcoded login-passwords)
#else
//this is a real application downloaded from appStore
#endif
回答by gnasher729
A macro can be undefined, it can be defined with no value, or it can be defined with some value, possibly a number. Examples:
宏可以是未定义的，也可以不带值定义，或者可以用某个值（可能是数字）来定义。例子：
#undef MACRO
#define MACRO
#define MACRO ??????
#define MACRO 0
#define MACRO 1
#ifdef MACRO or #if defined (MACRO) checks whether the macro is defined, with or without value.
#ifdef MACRO 或 #if defined (MACRO) 检查宏是否已定义，有或没有值。
#if MACRO substitutes the macro definition; if the macro is not defined then it substitutes 0. It then evaluates the expression that it find. If we take the five examples above, #if MACRO will be turned into
#if MACRO 替换宏定义；如果未定义宏，则将其替换为 0。然后计算它找到的表达式。如果我们拿上面的五个例子，#if MACRO 就会变成
#if 0
#if
#if ??????
#if 0
#if 1
Number 2 and 3 give a compile time error. Number 1 and 4 evaluate to false, so the following code is skipped. Number 5 evaluates to true.
数字 2 和 3 给出了编译时错误。数字 1 和 4 的计算结果为 false，因此跳过以下代码。数字 5 评估为真。
#if is more flexible: You could write
#if 更灵活：你可以写
#if MACRO == 2
which will only compile the following code if the macro was defined for example as
如果宏被定义为例如，它只会编译以下代码
#define MACRO 2
