为什么我们必须在Objective-C中执行[MyClass类]? [英] Why do we have to do [MyClass class] in Objective-C?

问题描述

在Objective-C中，您可以使用以下方法调用类方法:

In Objective-C, you can invoke class methods with:

[MyClass aClassMethod];

您可以使用以下方法查询实例的种类:

And you can query an instance's kind with:

[someInstance isKindOfClass:[MyClass class]];

但是，为什么我们需要做[MyClass class]，而不是像这样简单地提供MyClass:

But, why do we need to do [MyClass class], and not simply provide MyClass like this:

[someInstance isKindOfClass:MyClass];

是否有理由让编译器可以将MyClass作为接收方(指针类型)而不是作为参数来进行处理?解析语言是否有局限性?还是编译器的局限性?

Is there a reason that the compiler is fine with encountering MyClass as a receiver (a pointer type) but not as an argument? Is it a limitation of parsing the language? Or perhaps a limitation of the compiler?

推荐答案

噢...好玩的问题.答案是c-ism.

Ooooh... fun question. The answer is a c-ism.

考虑:

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

现在，说您有:

...
MyClass *m = nil;
...

在这种情况下，编译器将MyClass视为类型定义. *说变量m是pointer to a hunk o' memory that contains one (or many -- don't forget your C pointer-fu) MyClass instances.

In that context, the compiler sees MyClass as a type definition. The * says that the variable m is a pointer to a hunk o' memory that contains one (or many -- don't forget your C pointer-fu) MyClass instances.

换句话说，MyClass是一种类型.

In other words, MyClass is a type.

但是，在类似这样的上下文中:

But, in the context of something like:

[someInstance isKindOfClass: x ];

x必须是右值，或者用人类的话来说，是表达式的值.但是，类型不能用作右值.

x must be an rvalue or, in human terms, the value of an expression. A type, however, cannot be used as an rvalue.

[MyClass class]的工作实际上在语言和编译器上都是有点破绽，因为语法专门允许类型名称作为消息接收者(成为方法调用的目标).

That [MyClass class] works is actually a bit of a hack, both in the language and the compiler in that the grammar specifically allows a type name to be the message receiver (to be the target of a method call).

事实上，您可以执行以下操作:

And, as a matter of fact, you can do:

typedef MyClass Foo;
....
[MyClass class];
[Foo Class];

一切正常.但是，您不能执行以下，但是错误消息会显示出来:

It'll all work. However, you can't do the following but the error message is illuminating:

[NSUInteger class];

错误:"NSUInteger"不是Objective-C类名称或别名

现在，为什么不把它作为裸名在任何地方都进行特殊处理?

Now, why not special case it everywhere as a bare name?

这会混淆类型名和右值，您很快就不得不吞下[foo isKindOfClass: (MyClass)];之类的东西，而对[foo isKindOfClass: (MyClass *)];进行倒刺，然后以一种相当不舒服的方式侵犯了类型转换区域.

That colludes type names and rvalues and you quickly end up having to swallow something like [foo isKindOfClass: (MyClass)]; while barfing on [foo isKindOfClass: (MyClass *)]; which then encroaches upon typecasting territory in a rather uncomfortable fashion.

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