c++ 中的虚拟继承是什么？
假设我有这样的代码：
class A {
};
class B: virtual public A {
};
class C: virtual public A {
};
class D: public B,public C, virtual public A {
};
如果D继承B和C，虚拟继承可以保证A只有一份副本code> 包含在 D 中；但是如果 D 再次使用 virtual public A 继承 A 会怎样，就像上面的代码一样？
是否会有一个或两个 A 类型的子对象？
我仍然对一些带有虚拟继承的表达式感到困惑。 例如：
#include <iostream>
using namespace std;
class A {
public:
A() {std::cout<<"A ";}
};
class B: A {
public:
B() {std::cout<<"B ";}
};
class AToo: virtual A {
public:
AToo() {
std::cout<<"AToo ";
}
};
class ATooB: virtual AToo, virtual B {
public:
ATooB() {
std::cout<<"ATooB ";
}
};
virtual关键字能否保证ATooB中只有一份A的副本？如果 AToo 使用虚拟继承从 A 继承，但 B 没有，会发生什么？ ATooB 中会有两份副本吗？这是否意味着 B 和 AToo 都应该对 A 使用虚拟继承，以确保 ATooB 仅具有一份？
Suppose I have this code:
class A {
};
class B: virtual public A {
};
class C: virtual public A {
};
class D: public B,public C, virtual public A {
};
If D inherits B and C, virtual inheritance can ensure there is only one copy of A contained in D; but what if D inherits A using virtual public A again, like in the code above?
Will there be one sub-object of type A, or two?
I am still confused about some expressions with virtual inheritance.
for example:
#include <iostream>
using namespace std;
class A {
public:
A() {std::cout<<"A ";}
};
class B: A {
public:
B() {std::cout<<"B ";}
};
class AToo: virtual A {
public:
AToo() {
std::cout<<"AToo ";
}
};
class ATooB: virtual AToo, virtual B {
public:
ATooB() {
std::cout<<"ATooB ";
}
};
can the virtual keyword ensure that there is only one copy of A in ATooB? if AToo inherits from A using virtual inheritance, but B does not, what will happen? Will there be two copies in ATooB? Does this imply that both B and AToo should use virtual inheritance for A in order to ensure that ATooB has only one copy?
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评论（2）
一 - 这是虚拟继承的预期用途。
A只有一份副本。您实际上可以轻松验证这一点。为
A提供一个成员并使用D实例修改它。如果A有多个副本，编译器会说访问不明确。编辑在编辑的问题中，将有 A 的两个副本。每次定期继承
A时（没有虚拟），都会有一个新的 A 副本A已制作完成。如果您想要一份副本，请每次都将其声明为虚拟。One - that's the intended use of
virtualinheritance. There is only one copy ofA.You can actually verify that easily. Give
Aa member and modify it using aDinstance. If there was more than one copy ofA, the compiler would say that the access is ambiguous.Edit in the edited question, there will be two copies of A. Each time
Ais inherited regularly (withoutvirtual), a fresh copy of A is made. If you want one copy, declare itvirtualevery time.来自标准文档，10.1.4，
所以是的，只有一个。
From standard docs., 10.1.4,
And so yes, just one.