objc_getClass/object_getClass/cl

1. objc_getClass

通过类名来获取一个类对象，传入 const char * 返回对应的类对象。

• 源码

/***********************************************************************
* objc_getClass.  Return the id of the named class.  If the class does
* not exist, call _objc_classLoader and then objc_classHandler, either of 
* which may create a new class.
* Warning: doesn't work if aClassName is the name of a posed-for class's isa!
**********************************************************************/
Class objc_getClass(const char *aClassName)
{
    if (!aClassName) return Nil;

    // NO unconnected, YES class handler
    return look_up_class(aClassName, NO, YES);
}

示例：

  Class c1 = objc_getClass((const char *)"NSObject");
  Class c2 = objc_getClass((const char *)"NSObject"
  Class c3 = [NSObject class];
                
  NSObject *ojb = [[c1 alloc] init];

  NSLog(@"%p, %p, %p", c1, c2 , c3 );
  //0x7fff94d1c118, 0x7fff94d1c118, 0x7fff94d1c118
  NSLog(@"%p",  ojb );
  //0x10071c950

示例：

        NSObject *instanceClass = [[NSObject alloc] init];
        Class classObj = object_getClass(instanceClass);
        Class metaclassObj = object_getClass(classObj);
        
        NSLog(@"%p %p %p",instanceClass, classObj, metaclassObj);
      //0x100743440 0x7fff94d1c118 0x7fff94d1c0f0
      
        NSLog(@"%p ", [NSObject class]);//0x7fff94d1c118

        bool isMeta = class_isMetaClass(metaclassObj);
        NSLog(@"metaclassObj : %d ", isMeta);// 1

2. object_getClass

传入一个对象，获取该对象isa指针指向的对象。

• 源码

Class object_getClass(id obj)

{
    if (obj) return obj->getIsa();
    else return Nil;
}

如果传入的是 实例对象instance，则返回 类对象class，
如果传入的是 类对象class，则返回 元类对象meta-class,
如果传入的是 元类对象meta-class，则返回NSObject（基类）的元类对象meta-class。

3. class

实例对象、类对象调用class方法都返回类对象

- (Class)class {
    //self->isa
    return object_getClass(self);
}

+ (Class)class {

    return self;
}

示例：

NSObject *object1 = [[NSObject alloc] init];
Class object2 = object_getClass(object1);
// 0x7fff94d1c118 0x7fff94d1c118 0x7fff94d1c118
NSLog(@"%p %p %p",[NSObject class], [object1 class], [object2 class]);```