polymorphism c++

Polymorphism is a c++ skill to make many classes that are related to each other by being derived from the same class. Polymorphism means "many forms".

Polymorphism let different objects to respond the same message differently.

Note that the above description is just the original polymorphism, which is inheritance polymorphism. c++ has many ways to implement polymorphism, not only inheritance.

This article talks inheritance polymorphism only.

step-by-step

Step-by-step to write a simple inheritance polymorphism c++ program.

Step 1: Write base class: base_class

Write the base class, and its method: echo.

echo will be used as the polymorphism response action.

#include <iostream>

class base_class
{
public:
	virtual void echo() const
	{
		std::cout << "echo echo, I am base" << std::endl;
	}
};

virtual method

Specifier: virtual

You have noticed that the method echo is marked virtual.

To make polymorphism c++ work, the related method of the base class must be marked virtual .

Step 2: Write the derived class: derived_A

Write derived class: derived_A, and its method echo,

Note that the method name echo is the same as the related method name of its base class.

And the arguments of echo have to be the same too. In this example, it has no arguments.

And the method const must be the same.

class derived_A: public base_class
{
public:
	void echo() const override
	{
		std::cout << "echo echo, I am derived_A" << std::endl;
	}
};

Specifier: override

To make it polymorphism, the same name method of the derived class and the method of that name of the base class must meet the following conditions:

And also because c++ overloading feature, so if they have a little difference, it makes a different method and it will not polymorphism.

To avoid that mistake, you can add the keyword override at the end of the method_name() declaration, then the compiler will look up that if an overrode method exists on the base class, if not found or mismatched, the compiling will report errors, then you will know what's up. No keyword override, no compiling checking.

Step 3: do the same to derived_B

Do the same to derived_B.

Note that derived_B and derived_A are derived from the same class.

class derived_B: public base_class
{
public:
	void echo() const override
	{
		std::cout << "echo echo, I am derived_B" << std::endl;
	}
};

Step 4: Create relation object !

Make a polymorphism object:

How to implement point 1 and point 2 simultaneously ?

There are two ways:

c++ reference

derived_A a_obj;
derived_B b_obj;

base_class & base_r = a_obj;
base_class & base_s = b_obj;

base_r.echo();
base_s.echo();

You see, the type of base_r and base_s are base class type: base_class, but they are bound to the objects of derived class derived_A and derived_B.

And base_r.echo(); will call the method echo from derived_A,
base_s.echo(); will call the method echo from derived_B.

c++ pointer

Similar to reference, pointer can be used for polymorphism too.

base_class * base_ptr = new derived_A;
base_ptr->echo();

delete base_ptr;
base_ptr = new derived_B;
base_ptr->echo();

delete base_ptr;

Step 5: Parameterize

// reference
void call_echo_1(base_class & object)
{
	object.echo();
}

// pointer
void call_echo_2(base_class * object)
{
	object->echo();
}

derived_A aa_obj;
derived_B bb_obj;

call_echo_1(aa_obj);
call_echo_1(bb_obj);

call_echo_2(&aa_obj);
call_echo_2(&bb_obj);

Exercise

Write a complete inheritance polymorphism c++ program that can be compiled, use both reference and pointer to do the polymorphism response action. Check, modify and compile the code by yourself.

You can also send your exercise code to the discord server to let others check for you. If you send this exercise code to discord server, the following rules must be followed:
(Discord server can be found on the foot of this page)

Written on Nov 05, 2024

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