Objective-C的类别构造或技术在C + +？ [英] Objective-C's category-like construct or technique in C++?

问题描述

Objective-C类别功能允许程序员添加未在原始类定义中定义的新方法。

Objective-C category feature allows programmer to add new method which was not defined in original class definition.

我可以在C ++上存档类似的功能（语言构造或某种技术）吗？

Can I archive similar functionality (language construct or some technique) on C++?

调用语法（。或 - > 运算符）。

Major concern is consistent method calling syntax (. or -> operator).

推荐答案

让我们考虑扩展下面的类：

Let's consider the following class to be extended:

struct A {
    int x, y;
    A(int x, int y) : x(x), y(y) {}
};

您可以继承此类或编写包含此类的实例的包装类。在大多数情况下，继承是要走的路，因为包装器类不是 A，而是包装（包含）A。

You can inherit from this class or write a wrapper class which contains an instance of this class. In most cases, inheritance is the way to go, as a wrapper class isn't an A but it wraps (contains) an A.

使用C ++ 11移动语义，将一个实例 A 推广到子类 B （继承 A ）将是高效的，并且不需要复制实例 A ：

With C++11 move semantics, promoting an instance A to a subclass B (inheriting A) will be efficient and doesn't require to copy the instance A:

class B : public A {
public:
    B (A &&a) : A(a), someOtherMember(a.x + a.y) {}

    // added public stuff:
    int someOtherFunction() const { return someOtherMember; }

private:
    // added private stuff:
    int someOtherMember;
};

完整代码示例： http://ideone.com/mZLLEu

当然，我添加的功能有点蠢（成员甚至更多，因为它不尊重原始成员 x 和 y 的进一步更改，但您应该了解

Of course the function I added is a bit silly (and the member even more, since it doesn't respect further changes of the original members x and y), but you should get an idea of what I want to demonstrate.

请注意构造函数 B（A& a）调用促销构造函数（这不是一个标准术语）。通常， B（B& b）是一个移动构造函数，其移动 B 实例转换为要构建的新 B 。我使用移动语义将 A （由另一个函数返回）的实例移动到超级-class A of B 。

Note the constructor B (A &&a) which is something I call "promote constructor" (this is not a standard term). Normally, B (B &&b) is a move constructor, which moves the contents of the provided B instance into a new B about to be constructed. I use the move semantics to move an instance of A (which has been returned by another function) into the super-class A of B.

可以促进 A 到 B ，同时可以使用 B 作为 A 。

Effectively, you can promote A to B while making it possible to use a B as an A.

与Soonts的回答相反，我的解决方案也适用于添加的虚拟表，因为它不依赖于不安全的指针转换。

In contrast to Soonts' answer, my solution also works with added virtual tables, since it doesn't rely on unsafe pointer casting.

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