MessageSend:

1,在方法没有实现的时候不会报错，是因为OC运行时机制，在运行的时候才会动态监测，而c或者C＋＋这类语言必须在定义的时候明确指明其类型，而不是等到运行过程动态监测，这类语言叫做静态语言

[cat play];

[cat playWithKid:@"zhangdanfeng"];

[IMCat play];

[IMCat playWithKid:@"zhangdanfeng"];

2,用稍微底层的方法performSeletor实现：

[cat performSelector:@selector(play)];

[cat performSelector:@selector(playWithKid:) withObject:@"zhangdanfeng"];

Class catClass = [cat class];

[catClass performSelector:@selector(play)];

[[IMCat class] performSelector:@selector(playWithKid:) withObject:@"zhangdanfeng"];

3,底层实际就是转成了messageSend方法,貌似iOS8之后底层方法被禁用了，如果需要打开，需要在Build Settings中禁用enable strict checking of Obj-c messageSend calls选项

objc_msgSend(cat, @selector(play));
objc_msgSend(cat, @selector(playWithKid:),@"zhangdanfeng");

objc_msgSend([IMCat class], @selector(play));
objc_msgSend([IMCat class], @selector(playWithKid:),@"zhangdanfeng");