C++純虛函數可以在類外實現

純虛函數的文法是:

class Y  {

public :

virtual void X () = 0;

};

在類內定義時是不能實現的，但是在類外是可以實現的：

void  Y:: X() {

cout << "amazing!" << endl;

}

#include <iostream>using namespace std;class Pet {public:    virtual void speak() const = 0;    virtual void eat() const = 0;};void Pet:: eat() const {    cout << "Pet:: eat()" << endl;}void Pet :: speak() const {    cout << "Pet::speak()" << endl;}class Dog : public Pet {public :    void speak() const {        Pet::speak();    }    void eat() const {        Pet::eat();    }};int main(){    Dog simba;    simba.speak();    simba.eat();    cout << "Hello world!" << endl;    return 0;}