方法链+继承不能一起玩？ [英] Method chaining + inheritance don't play well together?

问题描述

请考虑：

  //为简洁省略成员数据
 
 //假设setAngle 需要单独实现
 //在Label和Image中，并且Button需要继承
 // Label，而不是说包含一个（etc）
 
 struct Widget {
 Widget& move（Point newPos）{pos = newPos; return * this; } 
}; 
 
 struct Label：Widget {
 Label& setText（string const& newText）{text = newText; return * this; } 
 Label& setAngle（double newAngle）{angle = newAngle; return * this; } 
}; 
 
 struct Button：Label {
 Button& setAngle（double newAngle）{
 backgroundImage.setAngle（newAngle）; 
 Label :: setAngle（newAngle）; 
 return * this; 
} 
}; 
 
 int main（）{
 Button btn; 
 
 // oops：Widget :: setText不存在
 btn.move（Point（0,0））。setText（Hey）; 
 
 // oops：调用Label :: setAngle而不是Button :: setAngle 
 btn.setText（Boo）。setAngle（.5）; 
} 
  

任何解决这些问题的技巧？

示例：使用模板魔法使Button :: move return Button&

$ b

编辑很明显，第二个问题可以通过setAngle virtual来解决。

但第一个问题仍然没有以合理的方式解决！

我想这是不可能在C ++中做正确。非常感谢你的努力。

解决方案

对于第二个问题，使setAngle virtual应该做的。
$ b

对于第一个，没有简单的解决方案。 Widget :: move返回一个Widget，它没有setText方法。你可以做一个纯虚拟的setText方法，但这将是一个非常丑的解决方案。你可以重载move（）在按钮类，但这是一个痛苦的维护。最后，你可能做一些模板。也许这样：

  //定义移动帮助函数
 template< typename T> 
 T& move（T& obj，Point& p）{return obj.move（p）; }; 
 
 //然后你的代码中有问题的行将如下所示：
 move（btn，Point（0,0））setText（Hey）; 
  

我会让你决定哪个解决方案是最干净的。但是有什么特别的原因，你需要能够链接这些方法？

Consider:

// member data omitted for brevity

// assume that "setAngle" needs to be implemented separately
// in Label and Image, and that Button does need to inherit
// Label, rather than, say, contain one (etc)

struct Widget {
    Widget& move(Point newPos) { pos = newPos; return *this; }
};

struct Label : Widget {
    Label& setText(string const& newText) { text = newText; return *this; }
    Label& setAngle(double newAngle) { angle = newAngle; return *this; }
};

struct Button : Label {
    Button& setAngle(double newAngle) {
        backgroundImage.setAngle(newAngle);
        Label::setAngle(newAngle);
        return *this;
    }
};

int main() {
    Button btn;

    // oops: Widget::setText doesn't exist
    btn.move(Point(0,0)).setText("Hey");

    // oops: calling Label::setAngle rather than Button::setAngle
    btn.setText("Boo").setAngle(.5); 
}

Any techniques to get around these problems?

Example: using template magic to make Button::move return Button& or something.

edit It has become clear that the second problem is solved by making setAngle virtual.

But the first problem remains unsolved in a reasonable fashion!

edit: Well, I guess it's impossible to do properly in C++. Thanks for the efforts anyhow.

解决方案

For the second problem, making setAngle virtual should do the trick.

For the first one, there are no easy solutions. Widget::move returns a Widget, which doesn't have a setText method. You could make a pure virtual setText method, but that'd be a pretty ugly solution. You could overload move() on the button class, but that'd be a pain to maintain. Finally, you could probably do something with templates. Perhaps something like this:

// Define a move helper function
template <typename T>
T& move(T& obj, Point& p){ return obj.move(p); };

// And the problematic line in your code would then look like this:
move(btn, Point(0,0)).setText("Hey");

I'll let you decide which solution is cleanest. But is there any particular reason why you need to be able to chain these methods?

这篇关于方法链+继承不能一起玩？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！