可以为间接基类共享基类的实例吗?

问题描述

我必须承认我在提出这个问题时遇到了麻烦，但我会尽力而为.我试图寻找问题的答案，但我怀疑我找不到所需的内容，因为我不确定该怎么称呼.

I must admit I'm having trouble formulating this question, but I'll try my best to be precise. I have tried to search for an answer to my question, but I suspect I have been unable to find what I'm looking for, as I'm not exactly sure what to call this.

我有一个基类A，还有几个从该基类继承的子类.然后，我使另一个类X继承自某些提到的子类.我现在面临的问题是，每个X继承的类都有各自的类A实例.下面的代码应更好地理解我的意思.

I have a base class A, and several child classes inheriting from this base class. I then make another class X that inherits from some of the mentioned child classes. The problem I'm now facing is that each of the classes X inherits, have their own instance of class A. The code below should give a better understanding of what I mean.

class A;
class B : public A;
class C : public A;

class X : public B, public C;

当它们都充当同一类的间接基类时，是否有办法使B类和C类共享A类的相同实例?

Is there a way to make class B and C share the same instance of class A, when they are both acting as indirect base classes for the same class?

举一个为什么我想要这样做的例子，让我们看一下这段代码.

To give an example of why I want this, lets look at this code.

    class A
    {
        int _x;
    };

    class B : public A
    {
        void outputX(){std::cout << A::_x << std::endl;
    };

    class C : public A
    {
       void setX(int x){A::_x=x;}
    };

    class X : public B, public C
    {
        C::setX(5);

        // this will output an un-initialized _x,
        // as B and C have their own version of A
        B::outputX()
    };

现在我意识到在此示例中这似乎没有必要，但是在我的实际情况下，我想如果B和C在类X中共享A的实例将是一个很好的解决方案.

Now I realize this seems rather unnecessary in this example here, but in my real situation I like to think it would be a good solution if B and C shared instance of A in class X.

这有可能吗?

推荐答案

您可以通过使用虚拟继承来解决此问题:

You can solve this by using virtual inheritance:

class B : virtual public A;
class C : virtual public A;

class X : virtual public B, virtual public C;

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