C++多重继承

1.多重继承定义
class derived_class_name: public base_class_name1,...{//...};
说明:继承列表中的访问级别可以是public,protected,private
2.大多重继承下,派生类的对象包含每个基类的基类子对象
3.构造派生类型的对象包括构造和初始化它的所有基类子对象
4.基类构造函数按照构造在类派生列表中的出现次序调用

示例:

/*
file:animal.h
author:longsy
*/
#include <iostream>

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

class Bear : public ZooAnimal {
public:
	Bear(){
		std::cout << "Bear" <<std::endl;
	}
	virtual	~Bear(){
		std::cout << "~Bear" << std::endl;
	}
};

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

class Panda : public Bear, public Endangered {
public:
	Panda() {
		std::cout << "Panda" << std::endl;
	}
	virtual	~Panda() {
		std::cout << "~Panda" << std::endl;
	}
};

测试代码

/*
file:test.cpp
author:longsy
*/
#include "animal.h"

int main()
{
	Panda p;
	return 0;
}
/*
ZooAnimal
Bear
Endangered
Panda
~Panda
~Endangered
~Bear
~ZooAnimal
*/

5.转换:派生类的指针或引用可以转换为其任意基类的指针或引用
在转换过程中是向上逐渐转换,对于多重继承要注意转换二义性
示例:

/*
file:test1.cpp
author:longsy
*/
#include "animal.h"
#include <iostream>

void shout(const ZooAnimal &animal) {
	std::cout << "animal begins to shout!" << std::endl;	
}
void shout(const Bear &bear) {
	std::cout << "the bear is shouting!" << std::endl;
}
int main()
{
	Panda p;
	shout(p); //shout(const Bear &bear)
	return 0;
}

二义性示例:

/*
file:test2.cpp
author:longsy
*/
#include "animal.h"
#include <iostream>

void shout(const Bear &bear) {
	std::cout << "the bear is shouting!" << std::endl;
}
void shout(const Endangered&) {
	std::cout << "the endangered is shouting!" << std::endl;
}
int main()
{
	Panda p;
         shout(p); //error:存在二义性
	return 0;
}

6.多重继承下的多态(虚函数的调用)
同时多重继承下也会出现二义性
示例:

/*
file:animal.h
author:longsy
*/
#include <iostream>

class ZooAnimal {
public:
	ZooAnimal(){
		std::cout << "ZooAnimal" << std::endl;
	}
	virtual	~ZooAnimal(){
		std::cout << "~ZooAnimal" << std::endl;
	}
	
	virtual void shout() {std::cout <<"animal shout" << std::endl;}
};

class Bear : public ZooAnimal {
public:
	Bear(){
		std::cout << "Bear" <<std::endl;
	}
	virtual	~Bear(){
		std::cout << "~Bear" << std::endl;
	}
        //如果基类指明了vritual,这里的vritual可以省略
	virtual void shout() {
	std::cout <<"bear shout" << std::endl;
	}
};

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

class Panda : public Bear, public Endangered {
public:
	Panda() {
		std::cout << "Panda" << std::endl;
	}
	virtual	~Panda() {
		std::cout << "~Panda" << std::endl;
	}
	
};

测试代码

/*
file:test3.cpp
author:longsy
*/
#include "animal.h"

int main()
{
	ZooAnimal *animal = new Panda();
	animal->shout(); //bear shout
	delete animal; //对于new出现的对象一定要手动地delete
	return 0;
}

多态二义性示例代码
在上面Endangered类也添加如下代码:

class Endangered {
//...
  virtual void shout() {
	std::cout <<"endangered shout" << std::endl;
  }
};

测试代码

Panda p;
p.shout(); //存在二义性

解决在多重继承下,对不同基类而同一数据多次继承的问题:使用虚继承