转载链接：http://www.jianshu.com/p/dc0404c09060
简书作者：MajorLMJ
在我们的工程中，很多时候只希望在满足一定条件的时候才进行编译，这是就需要用到条件编译。
#if #elif #else #endif
#if 条件1
代码块1
#elif 条件2
代码块2
...
#else
代码块n
#endif 用法举例
在开发中通常会在测试、线上等服务器间切换，我们可以写这样一个API的条件编译，根据情况使用条件控制不同的服务器API。
#if 0
#define API_SEVER @"http://101.8.8.251:9000" //本地test
#elif 1
#define API_SEVER @"http://124.182.199.114:9000" //线上test
#elif 0
#define API_SEVER @"http://service1.com" //正式1
#elif 0
#define API_SEVER @"http://service2.com" //正式2
#elif 0
#define API_SEVER @"" //备用
#endif这只是一个简单的应用，还比如有时候我们在真机和模拟器上的编码并不是一样，利用条件编译就可以节省我们切换代码的工作。
#ifdef和#ifndef
#ifdef MACRO_Define // 如果已定义MACRO_Define这个宏
代码块1
#else
代码块2
#endif或
#ifndef MACRO_Define // 如果未定义MACRO_Define这个宏
代码块1
#else
代码块2
#endif defined()函数
我们可以使用defined()和#if的组合，来代替上边的用法。使用defined()和#if的组合能判断更复杂的条件。
#if ( defined(MACRO_Define1) || (!defined(MACRO_Define2) && defined(MACRO_Define3) )
代码块1
#else
代码块2
#endif补充
在系统库的的TargetConditionals.h文件中（我当前使用的iOS版本为9.3）
苹果已经定义了一些在不同编译器下的宏，这些宏我们在系统库的头文件或者其它地方我们会经常看到，在条件编译的时候我们也可以使用这些宏，大家可以了解一下。
- 运行于Mac OS X的gcc编译器下
/* * gcc based compiler used on Mac OS X */ #if defined(__GNUC__) && ( defined(__APPLE_CPP__) || defined(__APPLE_CC__) || defined(__MACOS_CLASSIC__) ) #define TARGET_OS_MAC 1 #define TARGET_OS_WIN32 0 #define TARGET_OS_UNIX 0 #define TARGET_OS_IPHONE 1 #define TARGET_OS_IOS 1 #define TARGET_OS_WATCH 0 #define TARGET_OS_TV 0 #define TARGET_OS_SIMULATOR 1 #define TARGET_OS_EMBEDDED 0 #define TARGET_IPHONE_SIMULATOR TARGET_OS_SIMULATOR /* deprecated */ #define TARGET_OS_NANO TARGET_OS_WATCH /* deprecated */ #if defined(__ppc__) #define TARGET_CPU_PPC 1 #define TARGET_CPU_PPC64 0 #define TARGET_CPU_68K 0 #define TARGET_CPU_X86 0 #define TARGET_CPU_X86_64 0 #define TARGET_CPU_ARM 0 #define TARGET_CPU_ARM64 0 #define TARGET_CPU_MIPS 0 #define TARGET_CPU_SPARC 0 #define TARGET_CPU_ALPHA 0 #define TARGET_RT_LITTLE_ENDIAN 0 #define TARGET_RT_BIG_ENDIAN 1 #define TARGET_RT_64_BIT 0 #ifdef __MACOS_CLASSIC__ #define TARGET_RT_MAC_CFM 1 #define TARGET_RT_MAC_MACHO 0 #else #define TARGET_RT_MAC_CFM 0 #define TARGET_RT_MAC_MACHO 1 #endif #elif defined(__ppc64__) #define TARGET_CPU_PPC 0 #define TARGET_CPU_PPC64 1 #define TARGET_CPU_68K 0 #define TARGET_CPU_X86 0 #define TARGET_CPU_X86_64 0 #define TARGET_CPU_ARM 0 #define TARGET_CPU_ARM64 0 #define TARGET_CPU_MIPS 0 #define TARGET_CPU_SPARC 0 #define TARGET_CPU_ALPHA 0 #define TARGET_RT_LITTLE_ENDIAN 0 #define TARGET_RT_BIG_ENDIAN 1 #define TARGET_RT_64_BIT 1 #define TARGET_RT_MAC_CFM 0 #define TARGET_RT_MAC_MACHO 1 #elif defined(__i386__) #define TARGET_CPU_PPC 0 #define TARGET_CPU_PPC64 0 #define TARGET_CPU_68K 0 #define TARGET_CPU_X86 1 #define TARGET_CPU_X86_64 0 #define TARGET_CPU_ARM 0 #define TARGET_CPU_ARM64 0 #define TARGET_CPU_MIPS 0 #define TARGET_CPU_SPARC 0 #define TARGET_CPU_ALPHA 0 #define TARGET_RT_MAC_CFM 0 #define TARGET_RT_MAC_MACHO 1 #define TARGET_RT_LITTLE_ENDIAN 1 #define TARGET_RT_BIG_ENDIAN 0 #define TARGET_RT_64_BIT 0 #elif defined(__x86_64__) #define TARGET_CPU_PPC 0 #define TARGET_CPU_PPC64 0 #define TARGET_CPU_68K 0 #define TARGET_CPU_X86 0 #define TARGET_CPU_X86_64 1 #define TARGET_CPU_ARM 0 #define TARGET_CPU_ARM64 0 #define TARGET_CPU_MIPS 0 #define TARGET_CPU_SPARC 0 #define TARGET_CPU_ALPHA 0 #define TARGET_RT_MAC_CFM 0 #define TARGET_RT_MAC_MACHO 1 #define TARGET_RT_LITTLE_ENDIAN 1 #define TARGET_RT_BIG_ENDIAN 0 #define TARGET_RT_64_BIT 1 #elif defined(__arm__) #define TARGET_CPU_PPC 0 #define TARGET_CPU_PPC64 0 #define TARGET_CPU_68K 0 #define TARGET_CPU_X86 0 #define TARGET_CPU_X86_64 0 #define TARGET_CPU_ARM 1 #define TARGET_CPU_ARM64 0 #define TARGET_CPU_MIPS 0 #define TARGET_CPU_SPARC 0 #define TARGET_CPU_ALPHA 0 #define TARGET_RT_MAC_CFM 0 #define TARGET_RT_MAC_MACHO 1 #define TARGET_RT_LITTLE_ENDIAN 1 #define TARGET_RT_BIG_ENDIAN 0 #define TARGET_RT_64_BIT 0 #elif defined(__arm64__) #define TARGET_CPU_PPC 0 #define TARGET_CPU_PPC64 0 #define TARGET_CPU_68K 0 #define TARGET_CPU_X86 0 #define TARGET_CPU_X86_64 0 #define TARGET_CPU_ARM 0 #define TARGET_CPU_ARM64 1 #define TARGET_CPU_MIPS 0 #define TARGET_CPU_SPARC 0 #define TARGET_CPU_ALPHA 0 #define TARGET_RT_MAC_CFM 0 #define TARGET_RT_MAC_MACHO 1 #define TARGET_RT_LITTLE_ENDIAN 1 #define TARGET_RT_BIG_ENDIAN 0 #define TARGET_RT_64_BIT 1 #else #error unrecognized GNU C compiler #endif - 来自于Metrowerks/Motorola的CodeWarrior编译器
/* * CodeWarrior compiler from Metrowerks/Motorola */ #elif defined(__MWERKS__) #define TARGET_OS_MAC 1 #define TARGET_OS_WIN32 0 #define TARGET_OS_UNIX 0 #define TARGET_OS_EMBEDDED 0 #if defined(__POWERPC__) #define TARGET_CPU_PPC 1 #define TARGET_CPU_PPC64 0 #define TARGET_CPU_68K 0 #define TARGET_CPU_X86 0 #define TARGET_CPU_MIPS 0 #define TARGET_CPU_SPARC 0 #define TARGET_CPU_ALPHA 0 #define TARGET_RT_LITTLE_ENDIAN 0 #define TARGET_RT_BIG_ENDIAN 1 #elif defined(__INTEL__) #define TARGET_CPU_PPC 0 #define TARGET_CPU_PPC64 0 #define TARGET_CPU_68K 0 #define TARGET_CPU_X86 1 #define TARGET_CPU_MIPS 0 #define TARGET_CPU_SPARC 0 #define TARGET_CPU_ALPHA 0 #define TARGET_RT_LITTLE_ENDIAN 1 #define TARGET_RT_BIG_ENDIAN 0 #else #error unknown Metrowerks CPU type #endif #define TARGET_RT_64_BIT 0 #ifdef __MACH__ #define TARGET_RT_MAC_CFM 0 #define TARGET_RT_MAC_MACHO 1 #else #define TARGET_RT_MAC_CFM 1 #define TARGET_RT_MAC_MACHO 0 #endif -
未知编译器
/* * unknown compiler */ #else #if defined(TARGET_CPU_PPC) && TARGET_CPU_PPC #define TARGET_CPU_PPC64 0 #define TARGET_CPU_68K 0 #define TARGET_CPU_X86 0 #define TARGET_CPU_X86_64 0 #define TARGET_CPU_ARM 0 #define TARGET_CPU_ARM64 0 #define TARGET_CPU_MIPS 0 #define TARGET_CPU_SPARC 0 #define TARGET_CPU_ALPHA 0 #elif defined(TARGET_CPU_PPC64) && TARGET_CPU_PPC64 #define TARGET_CPU_PPC 0 #define TARGET_CPU_68K 0 #define TARGET_CPU_X86 0 #define TARGET_CPU_X86_64 0 #define TARGET_CPU_ARM 0 #define TARGET_CPU_ARM64 0 #define TARGET_CPU_MIPS 0 #define TARGET_CPU_SPARC 0 #define TARGET_CPU_ALPHA 0 #elif defined(TARGET_CPU_X86) && TARGET_CPU_X86 #define TARGET_CPU_PPC 0 #define TARGET_CPU_PPC64 0 #define TARGET_CPU_X86_64 0 #define TARGET_CPU_68K 0 #define TARGET_CPU_ARM 0 #define TARGET_CPU_ARM64 0 #define TARGET_CPU_MIPS 0 #define TARGET_CPU_SPARC 0 #define TARGET_CPU_ALPHA 0 #elif defined(TARGET_CPU_X86_64) && TARGET_CPU_X86_64 #define TARGET_CPU_PPC 0 #define TARGET_CPU_PPC64 0 #define TARGET_CPU_X86 0 #define TARGET_CPU_68K 0 #define TARGET_CPU_ARM 0 #define TARGET_CPU_ARM64 0 #define TARGET_CPU_MIPS 0 #define TARGET_CPU_SPARC 0 #define TARGET_CPU_ALPHA 0 #elif defined(TARGET_CPU_ARM) && TARGET_CPU_ARM #define TARGET_CPU_PPC 0 #define TARGET_CPU_PPC64 0 #define TARGET_CPU_X86 0 #define TARGET_CPU_X86_64 0 #define TARGET_CPU_68K 0 #define TARGET_CPU_ARM64 0 #define TARGET_CPU_MIPS 0 #define TARGET_CPU_SPARC 0 #define TARGET_CPU_ALPHA 0 #elif defined(TARGET_CPU_ARM64) && TARGET_CPU_ARM64 #define TARGET_CPU_PPC 0 #define TARGET_CPU_PPC64 0 #define TARGET_CPU_X86 0 #define TARGET_CPU_X86_64 0 #define TARGET_CPU_68K 0 #define TARGET_CPU_ARM 0 #define TARGET_CPU_MIPS 0 #define TARGET_CPU_SPARC 0 #define TARGET_CPU_ALPHA 0 #else /* NOTE: If your compiler errors out here then support for your compiler has not yet been added to TargetConditionals.h. TargetConditionals.h is designed to be plug-and-play. It auto detects which compiler is being run and configures the TARGET_ conditionals appropriately. The short term work around is to set the TARGET_CPU_ and TARGET_OS_ on the command line to the compiler (e.g. -DTARGET_CPU_MIPS=1 -DTARGET_OS_UNIX=1) The long term solution is to add a new case to this file which auto detects your compiler and sets up the TARGET_ conditionals. Then submit the changes to Apple Computer. */ #error TargetConditionals.h: unknown compiler (see comment above) #define TARGET_CPU_PPC 0 #define TARGET_CPU_68K 0 #define TARGET_CPU_X86 0 #define TARGET_CPU_ARM 0 #define TARGET_CPU_ARM64 0 #define TARGET_CPU_MIPS 0 #define TARGET_CPU_SPARC 0 #define TARGET_CPU_ALPHA 0 #endif #define TARGET_OS_MAC 1 #define TARGET_OS_WIN32 0 #define TARGET_OS_UNIX 0 #define TARGET_OS_EMBEDDED 0 #if TARGET_CPU_PPC || TARGET_CPU_PPC64 #define TARGET_RT_BIG_ENDIAN 1 #define TARGET_RT_LITTLE_ENDIAN 0 #else #define TARGET_RT_BIG_ENDIAN 0 #define TARGET_RT_LITTLE_ENDIAN 1 #endif #if TARGET_CPU_PPC64 || TARGET_CPU_X86_64 #define TARGET_RT_64_BIT 1 #else #define TARGET_RT_64_BIT 0 #endif #ifdef __MACH__ #define TARGET_RT_MAC_MACHO 1 #define TARGET_RT_MAC_CFM 0 #else #define TARGET_RT_MAC_MACHO 0 #define TARGET_RT_MAC_CFM 1 #endif #endif
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