Inheritance in C++ and expected behavior - c++

Question

I'm working on an assignment involving 4 classes and diamond inheritance. I've based all my answers on an assumption that may or may not be correct.
The diamond hierarchy is like this:
A
/ \
B C
\ /
D
and the code looks like this:
class A
{
private:
string s;
public:
A(string s);
virtual ~A;
virtual void set(string s);
virtual string get();
};
class B : public virtual A
{
public:
void set(string s);
string get();
}
class C : public virtual A
{
public:
void set(string s);
}
class D : public B, public C
{
public:
void set(string s);
}
Suppose I have a pointer that looks like this:C *obj = new D();
If get is called on obj, the pointer will actually end up looping back to D and then invoke B's get method. A C object alone would end up invoking A's get method. Assuming B and A's get methods return different results, which result would be expected?

Answer 1

The code you provided unfortunately won't compile as is.
We can create a similar example and just try it:
#include <cstdio>
struct A
{
virtual char const * get() {return "a";};
virtual ~A() = default;
};
struct B : public virtual A
{
char const * get() override {return "b";};
};
struct C : public virtual A
{
//char const * get() override {return "c";};
};
struct D : public B, public C
{
//char const * get() override {return "d";};
};
int main()
{
A* a = new A();
B* b = new B();
C* c = new C();
C* d = new D();
printf("%s\n", a->get());
printf("%s\n", b->get());
printf("%s\n", c->get());
printf("%s\n", d->get());
delete a;
delete b;
delete c;
delete d;
}
which gives:
a
b
a
b
isocpp calls this delegating to a sister class
As B and C both virtually inherit A, there is only a single instance of A, therefore in the virtual table, when B overrides an implementation of A, only a single member overrides get and this becomes also overridden for D.
Should you also uncomment the the get method in C, then we would get:
virtual.cc:19:8: error: no unique final overrider for 'virtual const char* A::get()' in 'D'
19 | struct D : public B, public C
|
However, further modifying the example and also uncommenting the get in D would make the code compile again. Now returning:
a
b
c
d

Answer 2

It will act like a D object.
Just like with single inheritance, it acts like the class that was created (not what the pointer to it is).