使用点表示法作为实例方法 [英] Using dot notation for instance methods

问题描述

我今天正在看一段代码，注意到这个特定的编码器使用点表示法来访问实例方法(这些方法不使用值，它们只是返回值).

I was looking at a piece of code today and notice that this particular coder use dot notation to access instance methods (these methods don't take values, they just return value).

例如:

@interface MyClass : NSObject {

}
-(double)add;
@end

@implementation MyClass
-(double)valueA {
    return 3.0;
}

-(double)valueB {
    return 7.0;
}

-(double)add {
    return self.valueA + self.valueB;
}    
@end

他在整个代码中都做到了这一点，并且编译器没有抱怨，但是当我像上面的示例一样在我的代码中尝试该操作时，出现以下错误:在不是结构或联合的内容中请求成员"valueA"".我想念什么，知道吗?

He did this through out his code and the compiler doesn't complain, but when I try it in my code like the example above I get the following error: "Request for member "valueA" in something not a structure or union". What am I missing, any idea?

推荐答案

点语法通常应用于声明的属性，但这不是强制性的.使用 obj.valueA 和 obj.valueB 确实可以.

The dot syntax is usually applied to declared properties but that’s not mandatory. Using obj.valueA and obj.valueB does work.

您收到的错误消息可能是由于对象没有明确的类型 MyClass * .例如，以下作品:

The error message you’re getting is probably due to the object not having explicit type MyClass *. For example, the following works:

MyClass *obj1 = [MyClass new];
NSLog(@"%f %f %f", obj1.valueA, obj1.valueB, [obj1 add]);

另一方面:

MyClass *obj1 = [MyClass new];
NSLog(@"%f %f %f", obj1.valueA, obj1.valueB, [obj1 add]);

id obj2 = obj1;
NSLog(@"%f %f %f", obj2.valueA, obj2.valueB, [obj2 add]);

给予:

error: request for member ‘valueA’ in something not a structure or union
error: request for member ‘valueB’ in something not a structure or union

因为 obj2 的类型为 id ，所以编译器没有足够的信息来知道 .valueA 和 .valueB实际上是getter方法 -valueA 和 -valueB .如果将类型为 MyClass 的对象放置在 NSArray 中，然后通过 -objectAtIndex:检索它们，则可能会发生这种情况，因为此方法返回通用对象类型为 id .

because obj2 has type id, so the compiler doesn’t have enough information to know that .valueA and .valueB are actually the getter methods -valueA and -valueB. This can happen if you place objects of type MyClass in an NSArray and later retrieve them via -objectAtIndex:, since this method returns a generic object of type id.

要安抚编译器，您需要将对象强制转换为 MyClass * ，然后才使用点语法.您可以通过以下方式完成此操作:

To appease the compiler, you need to cast the object to MyClass * and only then use the dot syntax. You could accomplish this by:

MyClass *obj2 = obj1;
// or
MyClass *obj2 = [someArray objectAtIndex:someIndex];
// and then
obj2.valueA

，或者，如果将 obj2 声明为 id :

or, if obj2 is declared as id:

((MyClass *)obj2).valueA

，或者，如果对象是由返回类型为 id 的方法返回的:

or, if the object is returned by a method whose return type is id:

((MyClass *)[someArray objectAtIndex:someIndex]).valueA

或者，您可以完全摆脱点语法(我的最爱):

Alternatively, you could simply get rid of the dot syntax altogether (my favourite):

[obj2 valueA]
[[someArray objectAtIndex:someIndex] valueA]

这篇关于使用点表示法作为实例方法的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！