std::enable_shared_from_this<> and multiple inheritance [duplicate] - c++

Question

This question already has answers here:
Double inheritance of enable_shared_from_this
(3 answers)
Closed 2 years ago.
When I try to run this code I get bad_weak_ptr exception:
using namespace std;
class O {
public:
O() {}
};
class A : public std::enable_shared_from_this<A>, virtual public O {
public:
A() {}
};
class B : public std::enable_shared_from_this<B>, virtual public O {
public:
B() {}
void GetShared() {
shared_from_this();
}
};
class C : public A, public B {
public:
C() {}
};
int main()
{
std::shared_ptr<B> pt = std::make_shared<C>();
pt->GetShared();
}
I would like to create instance of C class but then return B shared_ptr because that is what other interface requires. Is there a way to fix this code ? I cannot seem to do that.

Answer 1

Well as far as I know you are trying to do the undoable:
Inheritance in C++ means that you also include the same members of the parent classes. However for class C that would mean 2 classes that are already supposed to be held by a std::shared_ptr. But by the inheritance you tell the compiler that C should contain sub instances of A and B not held by a std::shared_ptr. shared_from_this will complain about this fact at run time by a bad weak ptr exception.
Probably you want C to be held only by a std::shared_ptr, then you can to the attached code.
If you simply want to "up-cast" a instance of C i.e. use it as it would be a class of B you can simply use a B& or use std::shared_ptr with out std::enables_shared_from_this or only on the up most class.
#include <memory>
using namespace std;
class O {
public:
O() {}
};
struct A : virtual public O {
};
struct B : virtual O {
B() {}
void GetShared() {
}
};
struct C : std::enable_shared_from_this<C>, A, B {
C() = default;
};
int main()
{
std::shared_ptr<B> pt = std::make_shared<C>();
pt->GetShared();
}