为什么单独一个孩子班级错误？(Why one child class alone flings error ? does order matter?)

为什么单独一个孩子班级错误？(Why one child class alone flings error ? does order matter?)

我正在阅读关于java中的钻石继承问题。 所以想知道其他语言是如何解决的，并发现c ++已经解决了它。 我发现这个虚拟继承如何解决“钻石”（多重继承）歧义？ 。 然后我在在线c ++编译器中尝试了这段代码。 我试过了

#include <iostream> using namespace std; class A { public: void eat(){ cout<<"A";} }; class B: virtual public A { public: void eat(){ cout<<"B";} }; class C: virtual public A { public: void eat(){ cout<<"C";} }; class D: public B,C { public: void eat(){ cout<<"D";} }; int main() { A *a = new D(); a->eat(); }

有效 。 然后我想调用B的吃并改变A * a = new D（）; 至

B *a = new D();

这太有用了。但是当我尝试的时候

C *a = new D();

它扔了，错了

错误：'C'是'D'C * a = new D（）的难以接近的基础;

为什么单独使用C这个错误？ 它甚至是公开的。 那为什么它无法访问？

所以我做的是我改变了继承的顺序

class D: public C,B { public: void eat(){ cout<<"D";} };

现在C * a = new D（）; 但是B * a = new D（）; 失败了。

为什么会这样？

订单重要吗？

当我用Google搜索时，我发现了这个stackoverflow链接 ，但发现它是为构造函数。 我不明白为什么这个案子失败了。

如果我的问题很愚蠢，请原谅，因为我对C ++很陌生。我习惯于只有没有多重继承的java。

I was reading about diamond inheritance problem in java . So wanted to know how other languages have solved and found that c++ has solved it . And I found this How does virtual inheritance solve the "diamond" (multiple inheritance) ambiguity? . Then I tried this code in online c++ compiler . I tried

#include <iostream> using namespace std; class A { public: void eat(){ cout<<"A";} }; class B: virtual public A { public: void eat(){ cout<<"B";} }; class C: virtual public A { public: void eat(){ cout<<"C";} }; class D: public B,C { public: void eat(){ cout<<"D";} }; int main() { A *a = new D(); a->eat(); }

It worked . Then I wanted to invoke B's eat and changed A *a = new D(); to

B *a = new D();

and this too worked .But when I tried

C *a = new D();

It threw and error

error: 'C' is an inaccessible base of 'D' C *a = new D();

Why with C alone this error ? It is even public . Then why its not accessible ?

So what I did was I changed the order of inheritance to

class D: public C,B { public: void eat(){ cout<<"D";} };

And now C *a = new D(); passed but B *a = new D(); failed .

Why is it so ?

Is the order important ?

When I googled I found this stackoverflow link, but found that it is for constructors . I could not understand why this case fails .

Please forgive if my question is stupid , as I am damn new to C++ .I am used to only java where there is no multiple inheritance .

最满意答案

D从B公开继承，但私下从C继承。 如果要从两者公开继承，请使用此选项：

class D: public B, public C { public: void eat(){ cout<<"D";} };

D inherits publicly from B, but privately from C. Use this if you want to inherit publicly from both:

class D: public B, public C { public: void eat(){ cout<<"D";} };