【C++ Virtual 有啥用啊？】

1.应用于多态中

代码示例如下

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#include <iostream>
using namespace std;

class Animal
{
public:
	Animal(){}
	~Animal(){}
	void shout()
	{
		cout << "Animal is shout!" << endl;
	}
};

class Dog:public Animal
{
public:
	Dog():Animal(){}
	~Dog(){}
	void shout()
	{
		cout << "Dog is shout!" << endl;
	}
};

class Cat:public Animal
{
public:
	Cat():Animal(){}
	~Cat(){}
	void shout()
	{
		cout << "Cat is shout!" << endl;
	}
};
int main()
{
   Animal *dog = new Dog();
   Animal *cat = new Cat();
   dog->shout();
   cat->shout();
   return 0;
}

以下是其输出

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Animal is shout!
Animal is shout!

解释：子类对象，使用父类指针进行声明，相当于'向上转换'，转换后的指针拥有且仅拥有父类属性方法！！！

虽然父类和子类都有各自的shout方法，但是如果这么写，他们一点关系都没有！！！这等同于，父类中拥有方法0，子类1中扩展出了方法1，子类2中扩展出了方法2。

所以，你想说，我不是在子类中重写了shout方法吗？虽然'向上转换了'，不应该用我重写后方法吗？抱歉，父类中的shout方法很直男！！！想让他脑瓜儿灵活点，重写请使用virtual ！
请不要混淆重载(同一个类中)和重写(用于继承关系中)
这里还说了另外两种用法：纯虚函数(抽象类)和虚拟继承(解决菱形继承问题)

修改后的代码示例如下：

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#include <iostream>
using namespace std;

class Animal
{
public:
	Animal(){}
	~Animal(){}
	virtual void shout()
	{
		cout << "Animal is shout!" << endl;
	}
};

class Dog:public Animal
{
public:
	Dog():Animal(){}
	~Dog(){}
	void shout()
	{
		cout << "Dog is shout!" << endl;
	}
};

class Cat:public Animal
{
public:
	Cat():Animal(){}
	~Cat(){}
	void shout()
	{
		cout << "Cat is shout!" << endl;
	}
};
int main()
{
   Animal *dog = new Dog();
   Animal *cat = new Cat();
   dog->shout();
   cat->shout();
   return 0;
}

输出如下

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Dog is shout!
Cat is shout!

所以想让父类指针聪明些，重写请使用virtual。(在父类中声明virtual即可)

virtual隔代都有效 ，

比如现在孙子类狼青继承自Dog，向上转换后的爷类指针和父类指针，也会按照狼青那样叫。

代码示例如下

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#include <iostream>
using namespace std;

class Animal
{
public:
	Animal(){}
	~Animal(){}
	virtual void shout()
	{
		cout << "Animal is shout!" << endl;
	}
};

class Dog:public Animal
{
public:
	Dog():Animal(){}
	~Dog(){}
	void shout()
	{
		cout << "Dog is shout!" << endl;
	}
};

class Cat:public Animal
{
public:
	Cat():Animal(){}
	~Cat(){}
	void shout()
	{
		cout << "Cat is shout!" << endl;
	}
};
class Langqing:public Dog
{
public:
	Langqing():Dog(){}
	~Langqing(){}
	void shout()
	{
		cout << "Langqing is shout!" << endl;
	}
};
int main()
{
   Animal *dog = new Dog();
   Animal *cat = new Cat();
   dog->shout();
   cat->shout();
   Animal *lq1 = new Langqing();
   lq1->shout();
   Dog *lq2 = new Langqing();
   lq2->shout();
   return 0;
}

输出如下

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Dog is shout!
Cat is shout!
Langqing is shout!
Langqing is shout!