__OBJC__ in objective C

Question

What does __OBJC__ mean in Objective C?

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

Answer 1

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++.

You could use it in a header file like this, if you wanted to publish a header file that defined an objective c object that you wanted to make available to c and c++ programmers/code :

#ifndef MYHEADER_H #define MYHEADER_H  #ifdef __OBJC__ // Put objective C things in this block // This is an objc object implemented in a .m or .mm file @implementation some_objc_object { } @end #endif  #ifdef __cplusplus #define CLINKAGE "C" // c++ things that .m or .c files wont understand go in here // This class, in a .mm file, would be able to call the obj-c objects methods // but present a c++ interface that could be called from c++ code in .cc or .cpp  // files class SomeClassThatWrapsAnObjCObject {   id idTheObject; public:   // ... }; #endif // and here you can declare c functions and structs // this function could be used from a .c file to call to a .m file and do something // with the object identified by id obj extern CLINKAGE somefunction(id obj, ...);  #endif // MYHEADER_H 

Answer 2

This looks like your precompiled header file.

The precompiled header is shared between all C-dialect files in your project. It's as if all your .c, .cpp, .m and .mm files have an invisible #include directive as the first line. But the Cocoa header files are pure Objective C - trying to include them in a C/C++ source will yield nothing but syntax errors aplenty. Thus the #ifdef.

If your project only contains Objective C files (.m/.mm), which is the typical case, the #ifdef is not really necessary. But Xcode, which generated this header in the first place, protects you all the same.

Even if it's not a PCH file, this #ifdef only makes sense if the file is to be included from both Objective C and plain C/C++. But it does not hurt regardless.