Abstract class vs Interface in C++

It is often confusing when it comes to interface and abstract class in C++. There are no keyword to define an interface and abstract classes in C++, as in other programming languages like Java or C#.

Yet, the use of interface and abstract class can be achieved in C++ similar to other languages.

First let us compare the concept of interface and abstract class

1. Interface class does not have any method implementation. It only has method declarations and the class that implements an interface implement the methods.
2. Interface does not have defined variables. It does exist in java but then the variables are stated as final and static.
3. The class which implements an interface must implement all the methods of the interface.
4. Abstract class can have variable declaration and method implementation/declarations. Moreover one can inherit the abstract class without implementing the abstract methods.
5. An abstract class cannot be instantiated but rather inherited by another class. Instantiating and abstract class will give compilation error.

Before looking at how we define abstract and interface, lets understand what is a virtual and a pure virtual method in C++.

A virtual method in C++ is a method which is to be redefined in the derived class, using the virtual keyword tells the compiler to perform dynamic linkage or late binding on the method.

A pure virtual method in C++ is a virtual method which does not need to be defined but only declared. It is declared by assigning 0 to a virtual method.

An abstract class in C++
a) must have at least one pure virtual method.
b) can have implemented methods.
c) can have variables declaration.

This is how we declare an interface class, in C++ an interface class
a) has all methods declared as pure virtual methods.
b) has no variable declaration.

When to use an interface:
Use interface when the features you are implementing are not related to each other, every object is kind of separate.

When to use abstract class:
Use abstract class, when you are trying to create multiple versions of a feature, we use abstract class when there is some relation between the created objects.