移情别恋c++ ദ്ദി˶ｰ̀֊ｰ́ ) ——11.多态

class Person {
public:
 virtual void BuyTicket() { cout << "买票-全价" << endl;}
};

class Person {
public:
 virtual void BuyTicket() { cout << "买票-全价" << endl; }
};


class Student : public Person {
public:
 virtual void BuyTicket() { cout << "买票-半价" << endl; }
 
 /*注意：在重写基类虚函数时，派生类的虚函数在不加virtual关键字时，虽然也可以构成重写(因
为继承后基类的虚函数被继承下来了在派生类依旧保持虚函数属性),但是该种写法不是很规范，不建议
这样使用*/
 /*void BuyTicket() { cout << "买票-半价" << endl; }*/
};

void Func(Person& p)
{ p.BuyTicket(); }

int main()
{
 Person ps;
 Student st;
 Func(ps);
 Func(st);
 return 0;
}

class A{};
class B : public A {};
class Person {
public:
 virtual A* f() {return new A;}
};
class Student : public Person {
public:
 virtual B* f() {return new B;}
};

class Person {
public:
 virtual ~Person() {cout << "~Person()" << endl;}
};

class Student : public Person {
public:
 virtual ~Student() { cout << "~Student()" << endl; }
};

// 只有派生类Student的析构函数重写了Person的析构函数，下面的delete对象调用析构函
数，才能构成多态，才能保证p1和p2指向的对象正确的调用析构函数。
int main()
{
 Person* p1 = new Person;
 Person* p2 = new Student;
 delete p1;
 delete p2;
 return 0;
}

class Car
{
public:
 virtual void Drive() final {}
};
class Benz :public Car
{
public:
 virtual void Drive() {cout << "Benz-舒适" << endl;}
};

class Car{
public:
 virtual void Drive(){}
};
class Benz :public Car {
public:
 virtual void Drive() override {cout << "Benz-舒适" << endl;}
};

class Car
{
public:
 virtual void Drive() = 0;  //无法实例出对象
};

class Benz :public Car
{
public:
 virtual void Drive()
 {
 cout << "Benz-舒适" << endl;
 }
};

class BMW :public Car
{
public:
 virtual void Drive()
 {
 cout << "BMW-操控" << endl;
 }
};

void Test()
{
 Car* pBenz = new Benz;
 pBenz->Drive();
 Car* pBMW = new BMW;
 pBMW->Drive();
}

// 这里常考一道笔试题：sizeof(Base)是多少？
class Base
{
public:
 virtual void Func1()
 {
 cout << "Func1()" << endl;
 }
private:
 int _b = 1;
};

// 针对上面的代码我们做出以下改造
// 1.我们增加一个派生类Derive去继承Base
// 2.Derive中重写Func1
// 3.Base再增加一个虚函数Func2和一个普通函数Func3

class Base
{
 public:
 virtual void Func1()
 {
  cout << "Base::Func1()" << endl;
 }

 virtual void Func2()
 {
 cout << "Base::Func2()" << endl;
 }

 void Func3()
 {
 cout << "Base::Func3()" << endl;
 }

private:
 int _b = 1;
};

class Derive : public Base
{
public:
 virtual void Func1()
 {
   cout << "Derive::Func1()" << endl;
 }

private:
 int _d = 2;
};

int main()
{
 Base b;
 Derive d;
 return 0;
}

class Person {
public:
 virtual void BuyTicket() { cout << "买票-全价" << endl; }
 virtual void func(){}
private:
 int a=0;
};

class Student : public Person {
public:
 virtual void BuyTicket() { cout << "买票-半价" << endl; }
private:
 int b=1;
};

void Func(Person& p)
{
 p.BuyTicket();
}

int main()
{
 Person Mike;
 Func(Mike);
 Student Johnson;
 Func(Johnson);
 return 0;
}

class Base { 
public :
 virtual void func1() { cout<<"Base::func1" <<endl;}
 virtual void func2() {cout<<"Base::func2" <<endl;}
private :
 int a;
};
class Derive :public Base { 
public :
 virtual void func1() {cout<<"Derive::func1" <<endl;}
 virtual void func3() {cout<<"Derive::func3" <<endl;}
 virtual void func4() {cout<<"Derive::func4" <<endl;}
private :
 int b;
};

typedef void(*VFPTR) ();
void PrintVTable(VFPTR vTable[])
{
 // 依次取虚表中的虚函数指针打印并调用。调用就可以看出存的是哪个函数
 cout << " 虚表地址>" << vTable << endl;
 for (int i = 0; vTable[i] != nullptr; ++i)
 {
 printf(" 第%d个虚函数地址 :0X%x,->", i, vTable[i]);
 VFPTR f = vTable[i];
 f();
 }
 cout << endl;
}

int main()
{
 Base b;
 Derive d;

 // 思路：取出b、d对象的头4bytes，就是虚表的指针，前面我们说了虚函数表本质是一个存虚函数
指针的指针数组，这个数组最后面放了一个nullptr
 // 1.先取b的地址，强转成一个int*的指针
 // 2.再解引用取值，就取到了b对象头4bytes的值，这个值就是指向虚表的指针
 // 3.再强转成VFPTR*，因为虚表就是一个存VFPTR类型(虚函数指针类型)的数组。
 // 4.虚表指针传递给PrintVTable进行打印虚表
 // 5.需要说明的是这个打印虚表的代码经常会崩溃，因为编译器有时对虚表的处理不干净，虚表最
后面没有放nullptr，导致越界，这是编译器的问题。我们只需要点目录栏的-生成-清理解决方案，再
编译就好了。
 VFPTR* vTableb = (VFPTR*)(*(int*)&b);
 PrintVTable(vTableb);
 VFPTR* vTabled = (VFPTR*)(*(int*)&d);
 PrintVTable(vTabled);
 return 0;
}

class Base1 {
public:
 virtual void func1() {cout << "Base1::func1" << endl;}
 virtual void func2() {cout << "Base1::func2" << endl;}
private:
 int b1;
};

class Base2 {
public:
 virtual void func1() {cout << "Base2::func1" << endl;}
 virtual void func2() {cout << "Base2::func2" << endl;}
private:
int b2;
};

class Derive : public Base1, public Base2 {
public:
 virtual void func1() {cout << "Derive::func1" << endl;}
 virtual void func3() {cout << "Derive::func3" << endl;}
private:
 int d1;
};

typedef void(*VFPTR) ();//函数指针的重命名
void PrintVTable(VFPTR vTable[])
{
 cout << " 虚表地址>" << vTable << endl;
 for (int i = 0; vTable[i] != nullptr; ++i)
 {
 printf(" 第%d个虚函数地址 :0X%x,->", i, vTable[i]);
 VFPTR f = vTable[i];
 f();
 }
 cout << endl;
}

int main()
{
 Derive d;
 VFPTR* vTableb1 = (VFPTR*)(*(int*)&d);
 PrintVTable(vTableb1);


 Base2* ptr=&d;//子类的地址可赋值给父类地址，相当于切片，可直接找到base2的地址
 VFPTR* vTableb2 = (VFPTR*)(*(int*)ptr));
 
 return 0;
}