Objective-C @protocol等同于C ++ [英] Objective-C @protocol equivalent in C++

问题描述

A类具有作为成员的B类实例。有时B类的实例想和A类交谈。在Objective-C中我可以做：

  // Ah 
 @interface A：NSObject< BDelegate> 
 @property（非原子，保留）B * b; 
 @end 
 
 // Am 
  - （void）classBsays {
 
} 
 
 
 // b 
 @protocol BDelegate 
  - （void）classBsays; 
 @end 
 
 @interface B：NSObject 
 @property（nonatomic，assign）id< BDelegate>代表; 
 @end 
 
 // B.m 
 @implementation B 
  - （void）f {
 [delegate classBsays]; 
} 
 @end 
  

我在C ++中使用在B类上的void指针。但是，这错过了B类代表应该实现这样的方法的部分。

如何在C ++中模仿Objective-C的协议？

解决方案

您的示例的C ++等价物看起来像这样：

  // A.hpp 
 #includeB.hpp
 
 class A：public BDelegate {
 public：
 void classBSays（）{} 
 B * b; 
} 
 
 // B.hpp 
 class BDelegate {
 public：
 virtual void classBSays（）= 0; 
} 
 class B {
 public：
 void f（）{delegate-> classBSays（）; } 
 BDelegate * delegate; 
} 
  

请注意，我在这里使用了成员函数的内联实现，为了简洁 - 您可以在单独的 A中同样实现 A.classBSays（）和 Bf（） .cpp 和 B.cpp 文件。

在这个例子中，类 BDelegate 是一个抽象基类（ABC），相当于您的 BDelegate 协议。通过仅包含纯粹的虚拟成员函数（前缀为关键字 virtual 和 = 0 后缀）的函数，它强制其子类提供了这些方法的实现，就像在Objective-C协议中使用 @required 标记（或没有标记）一样。事实上， BDelegate 只包含这样的功能是什么使它成为一个ABC。

您可以模拟Objective-C通过为ABC中的函数指定一个空体，这意味着子类不需要实现（因为它在ABC中实现） @optional 标签。例如，您可以通过如下修改 BDelegate 来模拟可选的 foo 方法：

  @protocol BDelegate 
  - （void）classBsays; 
 @optional 
  - （void）foo; 
 @end 
 //大致相当于：
 class BDelegate {
 public：
 virtual void classBSays（）= 0; 
 virtual void foo（）{} 
} 
  

使用该定义，类 A 可以根据需要选择是否提供 foo 的定义。请注意，这并不完全等同于Objective-C @optional 符号，因为 A 仍然会继承 BDelegate 的 foo 方法，如果它不提供自己的覆盖。另一方面，使用Objective-C协议， A 根本没有这样的方法，除非它明确地实现了它。

可以更全面地了解该主题。这里。

Class A has an instance of class B as a member. Sometimes the instance of class B wants to talk to class A. In Objective-C I can do:

// A.h
@interface A : NSObject <BDelegate>
@property (nonatomic, retain) B *b;
@end 

// A.m
- (void) classBsays {

}


// B.h
@protocol BDelegate
- (void) classBsays;
@end

@interface B : NSObject
@property (nonatomic, assign) id<BDelegate> delegate;
@end

// B.m
@implementation B
- (void) f {
    [delegate classBsays];
}
@end

I've done something similar in C++ using a void pointer on class B. But this misses the part that says "class B's delegate should implement such and such methods".

How can I imitate Objective-C's protocol in C++?

解决方案

The C++ equivalent of your example looks something like this:

// A.hpp
#include "B.hpp"

class A : public BDelegate {
    public:
        void classBSays ( ) { }
        B* b;
}

// B.hpp
class BDelegate {
    public:
        virtual void classBSays( ) = 0;
}
class B {
    public:
        void f ( ) { delegate->classBSays( ); }
        BDelegate* delegate;
}

Note that I've used inline implementation of the member functions here, for brevity's sake - you could equally implement A.classBSays() and B.f() in separate A.cpp and B.cpp files if you wanted.

In this example, the class BDelegate is an abstract base class (ABC), equivalent to your BDelegate protocol. By containing only pure virtual member functions (functions preceded with the keyword virtual and with the =0 suffix), it forces its subclasses to provide implementations for those methods, much as using a @required tag (or no tag) does in an Objective-C protocol. The fact that BDelegate contains only such functions is what makes it an ABC.

You can emulate the Objective-C @optional tag by specifying an empty body for the function in your ABC, which means that subclasses are not required to implement it (since it is implemented in the ABC). For example, you could emulate an optional foo method by modifying the BDelegate as follows:

@protocol BDelegate
- (void) classBsays;
@optional
- (void) foo;
@end
// Is approximately equivalent to:
class BDelegate {
    public:
        virtual void classBSays( ) = 0;
        virtual void foo( ) { }
}

Using that definition, the class A could choose whether to provide a definition for foo or not, as is desired. Note however that this is not exactly equivalent to the Objective-C @optional notation, because A will still inherit BDelegate's foo method if it doesn't provide it's own override. With the Objective-C protocol, on the other hand, A would have no such method at all unless it explicitly implements it itself.

A more thorough introduction to the subject is available here.

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