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 
     
       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() = 0; // 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 
      
        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 
       
         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';}