纯虚函数（pure virtual function ）和抽象类(abstract base class)

函数体=0的虚函数称为“纯虚函数”。包含纯虚函数的类称为“抽象类”

#include <string> class Animal // This Animal is an abstract base class

{

protected:

    std::string m_name;

public:

    Animal(std::string name)

        : m_name(name)

    {    }

    std::string getName() { return m_name; }

    virtual const char* speak() = ; // note that speak is now a pure virtual function

};

int main() {

    Animal a; //错：抽象类不能实例化(不能定义抽象类的对象(变量))

}

//从抽象类派生的类型如果没有继承实现所有的纯虚函数，则仍然是“抽象类”

#include <iostream> class Cow : public Animal

{

public:

    Cow(std::string name)

        : Animal(name)

    {

    }

    // We forgot to redefine speak

};

int main(){

    Cow cow("Betsy"); //仍然错：因为Cow仍然是抽象类

    std::cout << cow.getName() << " says " << cow.speak() << '\n';

}

像下面这样实现所有纯虚函数就没问题了，Cow不是一个抽象类

#include <iostream>

class Cow : public Animal

{

public:

    Cow(std::string name)

        : Animal(name)

    {

    }

    virtual const char* speak() { return "Moo"; }

};

int main()

{

    Cow cow("Betsy");

    std::cout << cow.getName() << " says " << cow.speak() << '\n';

}