2019独角兽企业重金招聘Python工程师标准>>>

"Import" links the header file it contains. Everything in the header, including property definitions, method declarations and any imports in the header are made available. Import provides the actual definitions to the linker.

@class by contrast just tells the linker not to complain it has no definition for a class. It is a "contract" that you will provide a definition for the class at another point.

Most often you use @class to prevent a circular import i.e. ClassA refers to ClassB so it imports ClassB.h in its own ClassA.h but ClassB also refers to ClassA so it imports ClassA.h in ClassB.h. Since the import statement imports the imports of a header, this causes the linker to enter an infinite loop.

Moving the import to the implementation file (ClassA.m) prevents this but then the linker won't recognize ClassB when it occurs in ClassA.h. The@class ClassB;directive tells the linker that you will provide the header for ClassB later before it is actually used in code.

当有两个类A,类B。在类A.h中#import类B.h。然后当我们写类B的时候，我们又需要用到类A的时候。使用类A.h包含在类B.h中，这样当在编译的时候，编译器就会重复、循环地在A.h 和B.h中来回的加载。此时，我们可以使用这样来避免出现这种情况。

在类A.h中，我们可以包含类B.h,但在类B.h中当我们需要使用类A时，我们可以使用@class A，这只是告诉编译器，你为类B提供了一个类A的定义，其他的都没做。这样就不会存在重复循环调用了。

其实，在类A.h中如果不会用到类B时，只是在A.m中用到类B时，你可以在A.m中#import B.h，只有当需要在类A.h中要用到类B时，才使用@class B.

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