Use polymorphism instead of type switching

//Reference: Item90 Avoid type switching: prefer polymorphism C++ Coding Standards
//THIS IS GOOD: use polymorphism

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

class language
{
public:
virtual void knock_knock() = 0; //pure virtual
  virtual ~language() {}; //do not forget vdestructor!
};

class cpp : public language
{
public:
void knock_knock(){
 cout << "Knock Knock!" << endl;
 cout << "Hi, I am C++!" << endl;
 cout << "You are so fast!" << endl;
}
};

class python : public language
{
public:
void knock_knock(){
 cout << "Knock Knock!" << endl;
 cout << "Hi, I am Python!" << endl;
 cout << "You are batteries included!" << endl;
}
};

class java : public language

{
public:
void knock_knock(){
 cout << "Knock Knock!" << endl;

 cout << "(10 seconds later...)" << endl;
 cout << "Java: Who is this?" << endl;
}
};

void better_knock_knock(language* lan)
{
//dynamic cast (served as downcasting) is not necessary here
  if ( cpp *mycpp = dynamic_cast<cpp*>(lan)){

 mycpp->knock_knock();
}
else
 lan->knock_knock();
}

int main()
{
//cpp *my_cpp = (cpp*)malloc(sizeof(cpp));
  language *my_cpp = new cpp;
language *my_python = new python;
java *my_java = new java; //this works too

better_knock_knock(my_cpp);
better_knock_knock(my_python);
better_knock_knock(my_java);

free(my_cpp);
delete my_python;
delete my_java;
}
------------------------------------------------------------------------------------------------------------------

//The following is not good: type switching is hard to maintain

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

//p.314 programming B.S.
enum language { CPP, PYTHON, JAVA };

struct cpp
{
language type;
};

struct python
{
language type;
};
struct java
{
language type;
};

string knock_knock_cpp()
{
cout << "Knock Knock!" << endl;
cout << "Hi, I am C++!" << endl;
cout << "You are so fast!" << endl;
return "";
}

void knock_knock_python()
{
cout << "Knock Knock!" << endl;
cout << "Hi, I am Python!" << endl;
cout << "You are batteries included!" << endl;
}

void knock_knock_java()
{
cout << "Knock Knock!" << endl;
cout << "(10 seconds later...)" << endl;
cout << "Java: Who is this?" << endl;
}

void knock_language(language l_type)
{
switch (l_type){
case CPP: knock_knock_cpp(); break;

case PYTHON: knock_knock_python(); break;
case JAVA: knock_knock_java(); break;
}
}

int main()
{
struct cpp *my_cpp = (cpp*)malloc(sizeof(cpp));
my_cpp->type = CPP;
struct python *my_python = new python;
my_python->type = PYTHON;
struct java *my_java = new java;
my_java->type = JAVA;

knock_language(my_java->type);
knock_language(my_cpp->type);

free(my_cpp);
delete my_python;
delete my_java;
}