Menu with Polymorphism

Question

I’m creating some examples to learn polymorphism more deeply, but in the original code, all functions work correctly.

Original code:

#include <cstdlib>
#include <iostream>

class Mamifero
{
 protected:
   int idade;

 public:
   Mamifero(){}
   ~Mamifero(){}

   virtual void somMamifero() const
   {
     std::cout<<"\n\tSom de mamifero.\n";
  }
};

class Boi: public Mamifero
{
 public:
   void somMamifero() const
   {
    std::cout<<"\n\tMuu ..! Muu..!!\n";
   }
};

class Gato: public Mamifero
{
 public:
   void somMamifero() const
   {
    std::cout<<"\n\tMiAu ..! MiAu..!!\n";
   }
};

class Porco: public Mamifero
{
 public:
   void somMamifero() const
   {
    std::cout<<"\n\tOinc ..! Oinc..!!\n";
   }
};

class Cachorro: public Mamifero
{
 public:
   void somMamifero() const
   {
    std::cout<<"\n\tAu ..! Au..!!\n";
   }
};

int main()
{
  Mamifero* mamPtr;
  int op;
  while(op != 5)
  {
    std::cout<<"\n\t(1) Boi"
             <<"\n\t(2) Gato"
             <<"\n\t(3) Porco"
             <<"\n\t(4) Cachorro"
             <<"\n\t(5) Sair"
         <<"\n\tDigite: ";
     std::cin>>op;
   switch(op)
    {
     case 1:{
       mamPtr = new Boi();
       mamPtr->somMamifero();
       break;
      }
     case 2:{
       mamPtr = new Gato();
       mamPtr->somMamifero();
       break;
      }
     case 3:{
       mamPtr = new Porco();
       mamPtr->somMamifero();
       break;
      }
     case 4:{
       mamPtr = new Cachorro();
       mamPtr->somMamifero();
       break;
      }
     case 5:{
       std::cout<<"\n\tGood Bye\n\n";
       exit(0);
       break;
      }
     default:
       std::cout<<"\n\tOpção Inválida ..!!!\n";
    }
  }
}

Because of that, I thought of creating a function called menu, and it would be virtual and redefine itself as it was executed. But I’m not succeeding, the code compiled but gives the error Segmentation fault.

It has to do something similar so that it works in principle in this sense, the same as what I’m trying to do on the menu without being what this in the first example?

Modified with polymorphic menu giving error:

#include <cstdlib>
#include <iostream>

class Mamifero
{
 public:
   Mamifero(){}
   ~Mamifero(){}

   virtual void somMamifero() const
   {
     std::cout<<"\n\tSom de mamifero.\n";
  }

  virtual void menu() const
  {
   Mamifero* mamPtr;
  int op;
  while(op != 5)
  {
    std::cout<<"\n\t(1) Boi"
             <<"\n\t(2) Gato"
             <<"\n\t(3) Porco"
             <<"\n\t(4) Cachorro"
             <<"\n\t(5) Sair"
         <<"\n\tDigite: ";
     std::cin>>op;
   switch(op)
    {
     case 1:{
       mamPtr = new Mamifero();
       mamPtr->somMamifero();
       break;
      }
     case 2:{
       mamPtr = new Mamifero();
       mamPtr->somMamifero();
       break;
      }
     case 3:{
       mamPtr = new Mamifero();
       mamPtr->somMamifero();
       break;
      }
     case 4:{
       mamPtr = new Mamifero();
       mamPtr->somMamifero();
       break;
      }
     case 5:{
       std::cout<<"\n\tGood Bye\n\n";
       exit(0);
       break;
      }
     default:
       std::cout<<"\n\tOpção Inválida ..!!!\n";
    }
  }
  }

};

class Boi: public Mamifero
{
 public:
   void somMamifero() const
   {
    std::cout<<"\n\tMuu ..! Muu..!!\n";
   }

   void menu() const
   {
    Mamifero* mamPtr;
    mamPtr = new Boi();
    mamPtr->somMamifero();
   }
};

class Gato: public Mamifero
{
 public:
   void somMamifero() const
   {
    std::cout<<"\n\tMiAu ..! MiAu..!!\n";
   }

   void menu() const
   {
    Mamifero* mamPtr;
    mamPtr = new Gato();
    mamPtr->somMamifero();
   }
};

class Porco: public Mamifero
{
 public:
   void somMamifero() const
   {
    std::cout<<"\n\tOinc ..! Oinc..!!\n";
   }

   void menu() const
   {
    Mamifero* mamPtr;
    mamPtr = new Porco();
    mamPtr->somMamifero();
  }
};

class Cachorro: public Mamifero
{
 public:
   void somMamifero() const
   {
    std::cout<<"\n\tAu ..! Au..!!\n";
   }

   void menu() const
   {
    Mamifero* mamPtr;
    mamPtr = new Cachorro();
    mamPtr->somMamifero();
  }
};

int main()
{
  Mamifero *m;
  m->menu();
}

Answer 1

It’s very simple, don’t do it. It doesn’t make any sense to put the Menu() inside Mamifero. It makes even less sense for the method (not function) to be virtual and to use polymorphism. The example looks great as it was originally designed.

I suggest you learn one concept at a time, and after mastering one you must pass to another. I’m sorry, but the way you’re trying, you’re "unlearning" and it’s not going to evolve. Learning should be a ladder where you can’t jump steps. I know everybody wants to see it work, which means they want to see the practice. But without mastering the theory, it cannot program right, mainly in object orientation which is an essentially theoretical concept. You have to learn to do right, not see it working. What works but is wrong serves for nothing. Needs to appeal in a structured way and with good sources.

If you still want to see "working", just initialize the variable in Main() so that it can have an instance and call the method Menu(). But this is still very wrong. I will not correct, nor list all the problems, including in the original, but it would be this:

Mamifero *m = new Mamifero();

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference.