Tag: polymorphism

Achieve polymorphism using pointer or reference

It is very common to use pointer to achieve polymorphism. For example,

#include <iostream>
 
using namespace std;

// Base class
class A {
public:
  virtual void output() = 0;
};

// Derived class B
class B : public A {
public:
  void output() override {
    cout << "From derived class B" << endl;
  }
};

// Derived class C
class C : public A {
public:
  void output() override {
    cout << "From derived class C" << endl;
  }
};

// Client code
int main(int argc, char** argv) 
{
  A *b = new B;
  A *c = new C;
  
  int flag = 0;
  cin >> flag;
  
  auto t = flag == 0 ? b : c;
  t->output();
  
  delete b;
  delete c;
  
  return 0;
}

Build the above code, and test it.

g++ -o testrefer main.cpp  --std=c++14

./testrefer
0
From the derived class B

./testrefer
1
From the derived class C

What if there are two objects of derived classes instead of two pointers of them in the client code? The answer is using reference.

// Client code
int main(int argc, char** argv) 
{
  B b;
  C c;
 
  int flag = 0;
  cin >> flag;
  
  auto& t = flag == 0 ? static_cast< A& >( b ) : static_cast< A& >( c );
  
  t.output();
  
  return 0;
}

Build and test it, and get the same output.

g++ -o testrefer main.cpp  --std=c++14

./testrefer
0
From the derived class B

./testrefer
1
From the derived class C