C中的抽象类是什么？

一个类成为抽象类的关键是它至少包含一个纯虚函数。当类中声明了纯虚函数（如 virtual void doSomething() = 0;），该类即成为抽象类，不能直接实例化对象，但可通过指针或引用实现多态；若派生类未实现所有纯虚函数，则其也保持为抽象类。抽象类常用于定义接口或共享行为，例如在绘图应用中设计 Shape 类并由 Circle、Rectangle 等派生类实现 draw() 方法。使用抽象类的场景包括：设计不应被直接实例化的基类、强制多个相关类遵循统一接口、提供默认行为的同时要求子类补充细节。此外，C 支持多重继承，允许类从多个抽象类派生，从而增强设计灵活性。

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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