常用的继承有三种：公有继承、保护继承、私有继承
公有继承
1 运用protected声明类的属性时，只允许友元类和派生类访问它，禁止在类外部通过实例对象访问。
下面用一个例子解释公有继承的应用以及protected访问限定符的作用
#include "stdafx.h"
#include "iostream"
#include "string"
using namespace std;
class fish
{
protected :
bool Freshwaterproperty;
public:
void swim()
{
if (Freshwaterproperty == true)
cout << "the fish is in lake " << endl;
else
cout << "the fish is in sea" << endl;
}
};
class seafish:public fish
{
public:
seafish()
{
Freshwaterproperty =true;
}
};
class lakefish :public fish
{
public:
lakefish()
{
Freshwaterproperty =false;
}
};
int main()
{
seafish lunch;
lunch.swim();
lakefish dinner;
dinner.swim();
return 0;
}
2 基类的初始化——向基类传递参数
运用基类的初始化，在声明派生类时，直接赋予参数。
包含初始化列表的派生类函数构造函数
#include "stdafx.h"
#include "iostream"
#include "string"
using namespace std;
class fish
{
protected :
bool Freshwaterproperty;
public:
fish(bool intFreshwaterproperty):Freshwaterproperty(intFreshwaterproperty)
{}；
void swim()
{
if (Freshwaterproperty == true)
cout << "the fish is in lake " << endl;
else
cout << "the fish is in sea" << endl;
}
};
class seafish:public fish
{
public:
seafish():fish(true)
{
}
};
class lakefish :public fish
{
public:
lakefish():fish(false)
{
}
};
int main()
{
seafish lunch;
lunch.swim();
lakefish dinner;
dinner.swim();
return 0;
}
3 派生类覆盖基类函数及处理方法
#include "stdafx.h"
#include "iostream"
#include "string"
using namespace std;
class fish
{
protected :
bool Freshwaterproperty;
public:
fish(bool intFreshwaterproperty) :Freshwaterproperty(intFreshwaterproperty)
{};
void swim()
{
if (Freshwaterproperty == true)
cout << "the fish is in lake " << endl;
else
cout << "the fish is in sea" << endl;
}
};
class seafish:public fish
{
public:
seafish():fish(true)
{
}
void swim()
{
cout << "the seafish is fast " << endl;
}
};
class lakefish :public fish
{
public:
lakefish():fish(false)
{
}
void swim()
{
cout << "the lakefish is slow " << endl;
}
};
int main()
{
seafish lunch;
lunch.swim();
lakefish dinner;
dinner.swim();
return 0;
}
输出结果为派生类的方法，将基类方法覆盖：
解决方法：
1在派生类方法里面调用积累函数
//上面的例子的变化之处
class lakefish :public fish
{
public:
lakefish():fish(false)
{
}
void swim()
{
fish::swim();
cout << "the lakefish is slow " << endl;
}
};
2在主调函数中运用实例化对象访问时后面运用作用域符访问
int main()
{
seafish lunch;
lunch.swim();
lakefish dinner;
dinner.fish::swim();
return 0;
}
私有继承
私有继承与公有继承的不同之处：
1 在声明派生类时运用class seafish:private fish
2 运用私有继承后，其他seafish的派生类不能访问基类的成员，要想访问基类的成员，需要另外声明一个公有继承的类。更不能在类外部运用实例化对象对基类的成员（属性及方法）进行访问。
3 基类的成员只能在派生类声明里访问
4 一般情况下，尽量不使用私有继承，必要时，更好选择是将基类对象作为派生类的私有成员
#include "stdafx.h"
#include "iostream"
#include "string"
using namespace std;
class fish
{
protected :
bool Freshwaterproperty;
public:
fish(bool intFreshwaterproperty) :Freshwaterproperty(intFreshwaterproperty)
{};
void swim()
{
if (Freshwaterproperty == true)
cout << "the fish is in lake " << endl;
else
cout << "the fish is in sea" << endl;
}
};
class seafish:private fish
{
public:
seafish():fish(true)
{
}
void swim2()
{
cout << "the seafish is fast " << endl;
fish::swim();
}
};
class lakefish :public fish
{
public:
lakefish():fish(false)
{
}
void swim()
{
cout << "the lakefish is slow " << endl;
}
};
int main()
{
seafish lunch;
lunch.swim2();
lakefish dinner;
dinner.swim();
dinner.fish::swim();
return 0;
}
保护继承
保护继承与私有继承的不用之处在于：保护继承派生类的派生类可以访问基类。
多继承
class bird :public A,public B ,…
多态
1 使用虚函数实现多态行为
多态：运用函数参数调用类的方法时，将派生类对象视为基类对象，并执行派生的函数，无需关心引用指向的是那样一个类。
实现多态重要的一点是：在访问的基类函数前面添加virtual
#include "stdafx.h"
#include "iostream"
#include "string"
using namespace std;
class fish
{
protected :
bool Freshwaterproperty;
public:
fish(bool intFreshwaterproperty) :Freshwaterproperty(intFreshwaterproperty)
{};
virtual void swim()
{
if (Freshwaterproperty == true)
cout << "the fish is in lake " << endl;
else
cout << "the fish is in sea" << endl;
}
};
class seafish:public fish
{
public:
seafish():fish(true)
{
}
void swim()
{
cout << "the seafish is fast " << endl;
}
};
class lakefish :public fish
{
public:
lakefish():fish(false)
{
}
void swim()
{
cout << "the lakefish is slow " << endl;
}
};
void fishfuntion(fish& inputfishclass)
{
inputfishclass.swim();
};
int main()
{
seafish lunch;
fishfuntion(lunch);
lakefish dinner;
fishfuntion(dinner);
return 0;
}
输出结果如下：
2 纯虚函数——抽象基类
1 即与1的类似，在virtual void swim=0;即指定了派生类的方法及特征
2 抽象基类要求派生类必须对其方法swim进行定义，
#include "stdafx.h"
#include "iostream"
#include "string"
using namespace std;
class fish
{
protected :
bool Freshwaterproperty;
public:
fish(bool intFreshwaterproperty) :Freshwaterproperty(intFreshwaterproperty)
{}
virtual void swim() = 0;
};
class seafish:public fish
{
public:
seafish():fish(true)
{
}
void swim()
{
cout << "the fish is in sea " << endl;
cout << "the seafish is fast " << endl;
}
};
class lakefish :public fish
{
public:
lakefish():fish(false)
{
}
void swim()
{
cout << "the fish is in lake " << endl;
cout << "the lake fish is slow " << endl;
}
};
void fishfuntion(fish& inputfishclass)
{
inputfishclass.swim();
};
int main()
{
seafish lunch;
fishfuntion(lunch);
lakefish dinner;
fishfuntion(dinner);
return 0;
}
3使用虚函数释放内存
当类里面含有析构函数时，将析构函数表示为虚函数 virtual ~Base();确保当以基类作为函数的指针时，释放派生类的内存。防止资源为释放，内存泄漏问题。
4 运用虚继承解决菱形问题
在实现多继承时，为多继承的基类又来同一个基类，此时在用实例化对象访问基类的成员时，将产生二义性，即菱形问题。
解决的方法时在多继承的每一个类，在继承同一个基类时， public virtul 基类名