12.4 Virtual Functions

Let’s look at the example first, what will be the output?

#include <iostream>
#include <string>
using namespace std;
class Person
{
public:
    string name;
    Person(string n) : name(n) {}
    virtual void print()
    {
        cout << "Name: " << name << endl;
    }
};
class Person2
{
public:
    string name;
    Person2(string n) : name(n) {}
    virtual void print() = 0;
};
class Student : public Person
{
public:
    string id;
    Student(string n, string i) : Person(n), id(i) {}
    void print()
    {
        cout << "Name: " << name;
        cout << ". ID: " << id << endl;
    }
};
void printObjectInfo(Person &p)
{
    p.print();
}
int main()
{
    {
        Student stu("yu", "2019");
        printObjectInfo(stu);
    }
    {
        Person *p = new Student("xue", "2020");
        p->print(); // if print() is not a virtual function, different output
        delete p;   // if its destructor is not virtual
    }
    // { //if you want to call a function in the base class
    //     Student stu("li", "2021");
    //     stu.Person::print();
    //     Person * p = new Student("xue", "2020");
    //     p->Person::print();
    //     delete p;
    // }
    return 0;
}

• But if we define print() function as a virtual function, the output will be different.
• Static binding: the compiler decides which function to call
• Dynamic binding: the called function is decided at runtime.
• Keyword virtual makes the function virtual for the base and all derived classes.

Virtual Destructors
If a virtual destructor is not virtual, only the destructor of the base class is executed in the follow examples.
析构函数一定是虚函数

Person * p = new Student("xue", "2020");
p->print(); 
...
...
delete p; //if its destructor is not virtual