What is an abstract class in C ?
The key to an abstract class is that it contains at least one pure virtual function. When a pure virtual function is declared in the class (such as virtual void doSomething() = 0;), the class becomes an abstract class and cannot be instantiated directly, but polymorphism can be achieved through pointers or references; if the derived class does not implement all pure virtual functions, it will also remain an abstract class. Abstract classes are often used to define interfaces or shared behaviors, such as designing Shape classes in drawing applications and implementing the draw() method by derived classes such as Circle and Rectangle. Scenarios using abstract classes include: designing base classes that should not be instantiated directly, forcing multiple related classes to follow a unified interface, providing default behavior, and requiring subclasses to supplement details. In addition, C supports multiple inheritance, allowing classes to derive from multiple abstract classes, thereby enhancing design flexibility.
An abstract class in C is a class that can't be used to create objects on its own — it's meant to be inherited by other classes. The key feature of an abstract class is that it contains at least one pure virtual function. This kind of function has no implementation in the base class and must be overridden in derived classes.
What makes a class abstract?
A class becomes abstract when it has one or more pure virtual functions . A pure virtual function is declared like this:
virtual void doSomething() = 0;
The = 0 part tells the compiler that this function has no body and must be implemented in a derived class.
Because of this, you can't create an instance of an abstract class directly. For example:
AbstractClass obj; // This will cause a compilation error
But you can use points or references to abstract classes, which is useful for polymorphism.
- If a derived class doesn't implement all pure virtual functions, it also becomes abstract.
- Abstract classes are often used as interfaces or base templates for related classes.
How are abstract classes used in practice?
Abstract classes are commonly used to define interfaces or shared behavior across multiple classes. Here's a simple example:
Let's say you're building a drawing application and have different shapes like Circle, Rectangle, and Triangle. You might create an abstract class called Shape with a pure virtual function draw() :
class Shape {
public:
virtual void draw() = 0;
}; Then each specific shape implements its own version of draw() :
class Circle : public Shape {
public:
void draw() override {
cout << "Drawing a circle" << endl;
}
};This allows your code to work with any shape through a common interface, without knowing exactly what type of shape it is until runtime.
- This pattern supports polymorphism and helps keep your code organized.
- It enforces a structure so that every derived class must follow certain rules.
When should I use an abstract class?
You'll want to use an abstract class if:
- You're designing a base class that shouldn't be instantiated directly.
- You need to enforce a common interface across several related classes.
- You want to provide some default behavior while still requiring subclasses to fill in specific details.
Abstract classes are especially useful in larger projects where clear structure and maintainable code matter.
- They help separate design from implementation.
- They encourage code reuse and reduce duplication.
One thing to note: unlike Java or C#, C allows multiple inheritance, so a class can inherit from more than one abstract class — which gives you more flexibility in how you design your class hierarchy.
That's basically how abstract classes work in C . Not too complicated once you get the idea, but very powerful when used right.
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