isa

1.官方图片：

isa.jpg

2.几个概念：
a.oc中的对象就是objc_object的结构体
b.oc中的类就是objc_class的结构体, 而且objc_class继承objc_object，所以说oc中类也是对象
c. objc_object中都会有个isa指针，objc_object和对应isa地址存在一一对应关系（objc_object地址>>3就是isa的地址）
d. isMemberOfClass，isKindOfClass

+ (BOOL)isMemberOfClass:(Class)cls {
    return object_getClass((id)self) == cls;
}

- (BOOL)isMemberOfClass:(Class)cls {
    return [self class] == cls;
}

+ (BOOL)isKindOfClass:(Class)cls {
    for (Class tcls = object_getClass((id)self); tcls; tcls = tcls->superclass) {
        if (tcls == cls) return YES;
    }
    return NO;
}

- (BOOL)isKindOfClass:(Class)cls {
    for (Class tcls = [self class]; tcls; tcls = tcls->superclass) {
        if (tcls == cls) return YES;
    }
    return NO;
}

object_getClass源码如下：

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