方法链接 + 继承不能很好地协同工作？

发布于 2024-07-14 10:13:49 字数 1181 浏览 8 评论 0原文

考虑一下：

// 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); 
}

有什么技术可以解决这些问题吗？

示例：使用模板魔法使 Button::move 返回 Button& 或者其他的东西。

编辑很明显，第二个问题可以通过将 setAngle 设置为虚拟来解决。

但是第一个问题仍然没有以合理的方式解决！

编辑：嗯，我想在 C++ 中这是不可能正确完成的。无论如何，感谢您的努力。

原文

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.

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北凤男飞 2024-07-21 10:13:50

对于第二个问题，将 setAngle 设置为虚拟应该可以解决问题。

对于第一个问题，没有简单的解决方案。 Widget::move 返回一个 Widget，它没有 setText 方法。您可以创建一个纯虚拟 setText 方法，但这将是一个非常丑陋的解决方案。您可以在按钮类上重载 move() ，但这会很难维护。最后，您可能可以使用模板做一些事情。也许是这样的：

// 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");

我会让你决定哪种解决方案是最干净的。但是有什么特殊原因需要链接这些方法吗？

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?

方法链接 + 继承不能很好地协同工作？

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