C++ Classes/Structures Virtual Inheritance
Example
When using inheritance, you can specify the virtual keyword:
struct A{};
struct B: public virtual A{};
When class B has virtual base A it means that A will reside in most derived class of inheritance tree, and thus that most derived class is also responsible for initializing that virtual base:
struct A
{
int member;
A(int param)
{
member = param;
}
};
struct B: virtual A
{
B(): A(5){}
};
struct C: B
{
C(): /*A(88)*/ {}
};
void f()
{
C object; //error since C is not initializing it's indirect virtual base `A`
}
If we un-comment /*A(88)*/ we won't get any error since C is now initializing it's indirect virtual base A.
Also note that when we're creating variable object, most derived class is C, so C is responsible for creating(calling constructor of) A and thus value of A::member is 88, not 5 (as it would be if we were creating object of type B).
It is useful when solving the diamond problem.:
A A A
/ \ | |
B C B C
\ / \ /
D D
virtual inheritance normal inheritance
B and C both inherit from A, and D inherits from B and C, so there are 2 instances of A in D! This results in ambiguity when you're accessing member of A through D, as the compiler has no way of knowing from which class do you want to access that member (the one which B inherits, or the one that is inherited byC?).
Virtual inheritance solves this problem: Since virtual base resides only in most derived object, there will be only one instance of A in D.
struct A
{
void foo() {}
};
struct B : public /*virtual*/ A {};
struct C : public /*virtual*/ A {};
struct D : public B, public C
{
void bar()
{
foo(); //Error, which foo? B::foo() or C::foo()? - Ambiguous
}
};
Removing the comments resolves the ambiguity.
