CODEWITHSUNDEEP
c++oopabstract-class
mreff555
mreff555

Reputation: 1121

When does it make sense to use an abstract class

I'm transitioning from c to c++ and of course, OOP which is proving more difficult than expected. The difficulty isn't understanding the core mechanics of classes and inheritance, but how to use it. I've read book on design patterns but they only show the techniques and paint a vague picture of why the techniques should be used. I am really struggling to find a use for abstract classes. Take the code below for example.

class baseClass {
public:
    virtual void func1() = 0;
    virtual void func2() = 0;
};
class inhClass1 : baseClass {
public:
    void func1();
    void func2();
};
class inhClass2 : baseClass{
public:
    void func1();
    void func2();
};
int main() {}

I frequently see abstract classes set up like this in design books. I understand that with this configuration the inherited classes have access to the public members of the base class. I understand that virtual functions are placeholders for the inherited classes. The problem is I still don't understand how this is useful. I'm trying to compare it to overloading functions and I'm just not seeing a practical use.

What I would really like is for someone to give the simplest example possible to illustrate why an abstract class is actually useful and the best solution for a situation. Don't get me wrong. I'm not saying there isn't a good answer. I just don't understand how to use them correctly.

Upvotes: 5

Views: 6147

Answers (3)

ravi
ravi

Reputation: 10733

Abstract classes are used to provide abstract representation of some concept you want to implement hiding the details.

For example let's say I want to implement File system interface :-

At abstract level what I can think of?

class FileSystemInterface
{
    virtual void openFile();
    virtual void closeFile();
    virtual void readFile();
    virtual void writeFile();
};

At this point of time I am not thinking of anything specific like how they will be handled in windows or linux rather I am focusing on some abstract idea.

Upvotes: 0

duffymo
duffymo

Reputation: 308988

Abstract classes and interfaces both allow you to define a method signature that subclasses are expected to implement: name, parameters, exceptions, and return type.

Abstract classes can provide a default implementation if a sensible one exists. This means that subclasses do not have to implement such a method; they can use the default implementation if they choose to.

Interfaces do not have such an option. Classes that implement an interface are required to implement all its methods.

In Java the distinction is clear because the language includes the keyword interface.

C++ interfaces are classes that have all virtual methods plus a pure virtual destructor.

Abstract classes and interfaces are used when you want to decouple interface from implementation. They're useful when you know you'll have several implementations to choose from or when you're writing a framework that lets clients plug in their own implementation. The interface provides a contract that clients are expected to adhere to.

Upvotes: 8

Code-Apprentice
Code-Apprentice

Reputation: 83567

One use of abstract classes is to be able to easily switch between different concrete implementations with minimal changes to your code. You do this by declaring a reference variable to the base class type. The only mention of the derived class is during creation. All other code uses the base class reference.

Upvotes: 4

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