C中的抽像類是什麼？

一個類成為抽像類的關鍵是它至少包含一個純虛函數。當類中聲明了純虛函數（如virtual void doSomething() = 0;），該類即成為抽像類，不能直接實例化對象，但可通過指針或引用實現多態；若派生類未實現所有純虛函數，則其也保持為抽像類。抽像類常用於定義接口或共享行為，例如在繪圖應用中設計Shape 類並由Circle、Rectangle 等派生類實現draw() 方法。使用抽像類的場景包括：設計不應被直接實例化的基類、強制多個相關類遵循統一接口、提供默認行為的同時要求子類補充細節。此外，C 支持多重繼承，允許類從多個抽像類派生，從而增強設計靈活性。

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 pointers 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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