How to assign base class shared_ptr object to child class shared_ptr object

Question

in below scenario, I need to invoke child class B function (fun1) from Base class A shared pointer returned by setup function and for the same have used dynamic_cast_pointer so that derived class shared_ptr object can be assigned to Base class shared_ptr but during compilation I am not allowed to do so. Can anybody suggest how to achieve this behaviour

#include <iostream>
#include <memory>
using namespace std;

class A
{
  public: 
    void fun() {cout<<"A"<<endl;}    
};

class B : public A
{
   public:
   void fun1() {cout<<"B" <<endl;}
    
};

shared_ptr<A> setup(shared_ptr<A> ptr = nullptr )

{
    if(!ptr)
    {
        
        ptr = make_shared<A> ();
    }
    
    ptr.get()->fun();
    
    return ptr;
    
}

int main() {            
    auto ptr = std::dynamic_pointer_cast <B> (setup());
    ptr->fun1();   // Doesn't compile             
    }

Answer 1

I presume the question is cut down from the real problem.

Presuming you're not expecting this example to work at runtime (you are casting a pointer to A as a pointer to B when it's an instance of A - so you're in undefined behaviour territory), the issue is that you have no virtual methods. The compiler tells you this:

 error: 'A' is not polymorphic

When you intend to use pointers to a base class, you need to have virtual destructors (personally I always use them as a matter of course).

See demo: https://godbolt.org/z/qe6jPrhsa

Here you can see a modified example demonstrating that you're in Undefined Behaviour territory... https://godbolt.org/z/harzoP946