runtime——Class——create

runtime——Class——interface implementation

• class_addIvar不支持对已存在class object add，不支持对meta-class object add，只能对objc_allocateClassPair和objc_registerClassPair之间新创建class object add
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• OBJC_EXPORT BOOL class_addIvar(Class cls, const char *name, size_t size, 
                                 uint8_t alignment, const char *types) 
       __OSX_AVAILABLE_STARTING(__MAC_10_5, __IPHONE_2_0);