converting base class pointer to unknown derived class pointer - c++

Question

I have 5 derived classes from an abstract base class. One function is overloaded which exists in every derived class, let's name it, print(). Example for Derived 4 class:
Derived4::print(*Derived1)
Derived4::print(*Derived2)
Derived4::print(*Derived3)
Derived4::print(*Base)
Like i said before, all derived classes have print function, but arguments are different, like
Derived1::print(*Derived2)
Derived1::print(*Derived3)
Derived1::print(*Derived4)
Derived1::print(*Base)
All objects are stored inside a vector like
vector<Base*> a
When i take one of them from vector and try to call print function, all calls are directed to print(*Base) function.I am not allowed to store the types, therefore don't have any idea what is coming from vector.Also, type checking is not allowed too.
An example:
#include <iostream>
#include <vector>
using namespace std;
class A{
public:
void print(){cout << "greetings from A" << endl;}
};
class C : public A{
public:
void print(){cout << "greetings from C" << endl;}
};
class D : public A{
public:
void print(){cout << "greetings from D" << endl;}
};
class B : public A{
public:
void print(C* c){c->print();}
void print(A* d){d->print();}
};
int main()
{
D d;
C c;
B b;
vector<A*> a; //B,C,D will be stored inside a vector like this.
a.push_back(&c);
a.push_back(&d);
b.print(a[0]);
b.print(a[1]);
return 0;
}
The result:
greetings from A
greetings from A
The desired result:
greetings from C
greetings from A

Answer 1

You need virtual functions. declaring A::print as virtual would make it so that calling print on a pointer of type A will call the print of which class the object was constructed as, instead of using pointer type to decide what print to call.
You also need to remove D::print as you expect A::print get called if the object is of type D
#include <iostream>
#include <vector>
using namespace std;
class A{
public:
virtual void print(){ cout << "This is printed twice." << endl; }
};
class C : public A{
public:
void print(){ cout << "This is desired output." << endl; }
};
class D : public A{
};
class B : public A{
public:
void print(C* c){ c->print(); }
void print(A* d){ d->print(); }
};
int main()
{
D d;
C c;
B b;
vector<A*> a; //B,C,D will be stored inside a vector like this.
a.push_back(&c);
a.push_back(&d);
b.print(a[0]);
b.print(a[1]);
return 0;
}
The result:
This is desired output.
This is printed twice.