objective-c - 在Cocoa中，Class类型是如何定义的？

Question

来自“objc.h”:

typedef struct objc_class *Class;

但是在“runtime.h”中:

struct objc_class {
    Class isa;

#if !__OBJC2__
    Class super_class                                        OBJC2_UNAVAILABLE;
    const char *name                                         OBJC2_UNAVAILABLE;
    long version                                             OBJC2_UNAVAILABLE;
    long info                                                OBJC2_UNAVAILABLE;
    long instance_size                                       OBJC2_UNAVAILABLE;
    struct objc_ivar_list *ivars                             OBJC2_UNAVAILABLE;
    struct objc_method_list **methodLists                    OBJC2_UNAVAILABLE;
    struct objc_cache *cache                                 OBJC2_UNAVAILABLE;
    struct objc_protocol_list *protocols                     OBJC2_UNAVAILABLE;
#endif

} OBJC2_UNAVAILABLE;
/* Use `Class` instead of `struct objc_class *` */

类到底是什么？

Answer 1

typedef struct objc_class *Class;

^ 这是一个 objc_class 指针的前向声明。它给它起了一个好记的名字Class。

现在让我们看一下 objc_class 结构: (删除 Objective-C 2.0 检查以缩短时间)

struct objc_class {
    Class isa;
};
//This is really saying
struct objc_class {
    struct objc_class *isa;
};

现在我们有一个指向它自己类型的结构。但你为什么这么问？

取自 Inside the Objective-C Runtime, Part 2

The Class pointed to by the Class' isa (the Class' Class) contains the Class' class methods in its objc_method_list. Got that? The terminology used in the runtime is that while an object's isa points to its Class, a Class's isa points to the object's "meta Class".

So what about a meta Class' isa pointer? Well, that points to the root class of the hierarchy (NSObject in most cases). (In the Foundation framework each meta class of a subclass of NSObject "isa" instance of NSObject.) And yes, by the way, NSObject's meta Class' isa points back to the same struct -- it's a circular reference so no Class' isa is ever NULL.

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因此，根据该描述，当您创建一个继承自 NSObject 的类时，您将拥有一个 isa ，它指向其类类型，该类类型指向其元类，该元类又指向其元类。根类 (NSObject) 包含对其自身的循环引用。这解释了它的原因requires two steps to determine if an object is an instance or a class .

假设您创建以下类:

@interface MyClass : NSObject
@end
@implementation MyClass
@end

如果您能够*遍历 isa 指针，您将得到以下结果:
*_{您不能，因为 isa 受到保护。请参阅下面的 Cocoa with Love 帖子，了解如何创建您自己的实现。}

Class c = myClassInstance->isa; //This would be the MyClass
c = c->isa; //This would be MyClass' meta class
c = c->isa; //This would be NSObjects meta class
c = c->isa; //This going forward would still be the NSObjects meta class
...

一旦c == c->isa，您就会知道您位于根对象。请记住，Objective-C 是 C 的超集，它本身不具有面向对象的结构，例如继承。这与其他实现细节(例如指向构成完整类层次结构的父类(super class)的指针)一起允许 Objective-C 提供面向对象的功能。有关类和元类的更多信息，请参阅 Cocoa with Love 上的帖子:What is a meta-class in Objective-C?