iOS:系统宏说明(持续更新)

------__OBJC__：

#ifdef __OBJC__
    #import <UIKit/UIKit.h>
    #import <Foundation/Foundation.h>
#endif

          It means the objective C compiler is being used. So you can create hybrid header files that can be used when compiling objective C or C or C++.

          或者This would be the case if you were developing an objective-c / cocoa application. In other words, if you were developing a C++ / carbon application, the OBJC symbol would not be defined and those objective-c dependant frameworks would not be imported.

------__has_feature：某些特性验证

#if !__has_feature(objc_arc)
#error This library requires automatic reference counting
#endif