clang -rewrite-objc 的作用是把oc代码转写成c/c++代码，常用它来查看OC的底层实现。

1.基本使用

//使用命令行进入main.m的目录，执行下面的命令clang -rewrite-objc main.m//会产生一个main.cpp文件复制代码

2.查看机器上装有的的sdk

xcodebuild -showsdks iOS SDKs:	iOS 11.1                      	-sdk iphoneos11.1iOS Simulator SDKs:	Simulator - iOS 11.1          	-sdk iphonesimulator11.1macOS SDKs:	macOS 10.13                   	-sdk macosx10.13tvOS SDKs:	tvOS 11.1                     	-sdk appletvos11.1tvOS Simulator SDKs:	Simulator - tvOS 11.1         	-sdk appletvsimulator11.1watchOS SDKs:	watchOS 4.1                   	-sdk watchos4.1watchOS Simulator SDKs:	Simulator - watchOS 4.1       	-sdk watchsimulator4.1复制代码

3.具体情况

//指定真机xcrun -sdk iphoneos clang -rewrite-objc main.m  //指定模拟器xcrun -sdk iphonesimulator clang -rewrite-objc main.m //指定SDK版本xcrun -sdk iphonesimulator10.3 clang -rewrite-objc main.m  //指定frameworkxcrun -sdk iphonesimulator11.1 clang -rewrite-objc -F /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/System/Library/Frameworks main.m //可以同时指定多个源文件、多个framework复制代码