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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