What is the definition of an abstract class in C++?

In C++, an abstract class is a special kind of class that cannot be instantiated directly, but can only be used as a base class for other classes. Abstract classes are typically used to define interfaces and shared behaviors without concerning themselves with specific implementation details.

Abstract classes are defined by declaring pure virtual functions within the class definition. Pure virtual functions are functions that do not have an actual implementation, only the function declaration, for example:

class AbstractClass {
public:
    virtual void doSomething() = 0; // 纯虚函数
    virtual void doAnotherThing() = 0; // 纯虚函数
};

In the above code, AbstractClass is an abstract class that includes two pure virtual functions, doSomething() and doAnotherThing(). These pure virtual functions do not have actual implementations and are only used to define an interface.

For any derived class that inherits from an abstract class, it must implement all pure virtual functions. Otherwise, the derived class will also become an abstract class. For example:

class ConcreteClass : public AbstractClass {
public:
    void doSomething() override {
        // 实现具体的行为
    }
    
    void doAnotherThing() override {
        // 实现具体的行为
    }
};

ConcreteClass is a concrete class that inherits from AbstractClass and implements all the pure virtual functions, so it can be instantiated and used.

The main purpose of abstract classes is to provide a generic interface and behavior definition, requiring derived classes to adhere to certain constraints when implementing them. They are commonly used in object-oriented design to achieve polymorphism and code reuse.