C++中的继承小结(1)

• 概述

  C++中继承的方式有三种public,private,protected。子类根据继承方式继承父类的成员函数和数据成员。

• 书写格式

  class <新建子类名>: public <父类名>{}；

• 调用构造函数、析构函数顺序

  class Member {
  public:
  	Member() { cout << "create Member" << endl; }
  	~Member() { cout << "delete Member" << endl; }
  };
  class Base {
  public:
  	Base() { cout << "create Base " << endl; }
  	~Base() { cout << "delete Base " << endl; }
  };
  class Sub :public Base{
  public:
  	Sub() { cout << "create Sub " << endl; }
  	~Sub() { cout << "delete Sub" << endl; }
  private:
  	//Member m;
  protected:
  	Member m;
  };
  
  int main(void)
  {
  	Sub* a = new Sub();
  	delete a;
  	system("pause");
  	return 0;
  }

  输出：

  create Base
  create Member
  create Sub
  delete Sub
  delete Member
  delete Base

  先调用父类构造函数，然后生成数据成员调用成员的构造函数，最后调用子类的构造函数，析构函数顺序相反。

• 把基类的析构函数写成虚函数virtual

  上面定义基类Base时，析构函数不是虚函数。如果把main函数写成如下，左边对基类赋值。

int main(void)
{
	Base* a = new Sub();
	delete a;
	system("pause");
	return 0;
}

执行结果：

create Base
create Member
create Sub
delete Base

只释放了基类，没有对所有的资源回收。

把基类的析构函数写成：

  class Base {
  public:
	Base() { cout << "create Base " << endl; }
	virtual ~Base() { cout << "delete Base " << endl; }
};

create Base
create Member
create Sub
delete Sub
delete Member
delete Base

所以基类的析构函数写成虚函数

• 子类给父类赋值

  1. 子类赋值给父类引用
  2. 子类赋值给父类指针
  3. 子类直接赋值给父类

  前两种调用子类成员，第三种调用父类成员。

   class Base {
   public:
  virtual void show()
  {
  	cout << "this is Base !" << endl;
  }
   };
   class Sub :public Base{
   public:
   	void show()
   	{
   		cout << "this is Sub !" << endl;
   	}
   };
  
   int main(void)
   {
   	Sub s = Sub();
   	Base& a = s;
   	Base* b = &s;
   	Base c = s;
   	a.show();
   	b->show();
   	c.show();
   	system("pause");
   	return 0;
   }

  输出：

  this is Sub !
  
  this is Sub !
  
  this is Base !

• 纯虚函数 、 抽象类

• 多重继承

  class <新建子类名>: public <父类名1>，public <父类名2>，...{}；

  如果子类调用父类中的重名函数：

  Sub.Base1::show();
  Sub.Base2::show()