what is virtual inheritance in c++
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 ";
}
};
cl开发者_开发百科ass 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?
One - that's the intended use of virtual
inheritance. There is only one copy of A
.
You can actually verify that easily. Give A
a member and modify it using a D
instance. If there was more than one copy of A
, the compiler would say that the access is ambiguous.
Edit in the edited question, there will be two copies of A. Each time A
is inherited regularly (without virtual
), a fresh copy of A is made. If you want one copy, declare it virtual
every time.
From standard docs., 10.1.4,
... For each distinct base class that is specified virtual, the most derived object shall contain a single base class subobject of that type. ....
And so yes, just one.
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