21天学通C++——多态专题

多态专题

- 11.1 多态基础
- - 11.1.1 为何需要多态行为
  - 11.1.2 使用虚函数实现多态行为
  - 11.1.3 为何需要构造虚函数
  - 11.1.4 虚函数的工作原理——理解虚函数表
  - - 使用sizeof证明虚函数表指针存在
  - 11.1.5 抽象基类和纯虚函数
- 11.2 使用虚继承解决菱形问题
- 11.4 使用final来禁止覆盖函数

11.1 多态基础 11.1.1 为何需要多态行为

#include 
using namespace std;

class Fish
{
public:
	void Swim()
	{
		cout << "Fish swims! " << endl;
	}
};

class Tuna :public Fish
{
public:
	// override Fish :: Swim
	void Swim()
	{
		cout << "Tuna swims! " << endl;
	}
};

void MakeFishSwim(Fish& inputFish)
{
	// calling Fish :: Swim
	inputFish.Swim();
}
int main()
{
	Tuna myDinner;
	// calling Tuna: :Swim
	myDinner.Swim();
	// sending Tuna as Fish
	MakeFishSwim(myDinner);
	return 0;
}

结果如下：
Tuna swims!
Fish swims!
理想情况下，用户希望Tuna对象表现出金枪鱼的行为，即便通过外部函数的Fish参数调用Swim()时亦如此。因此，在MakeFishSwim(Fish& inputFish)函数中调用inputFish.Swim()时，用户希望执行的是Tuna::Swim()。要实现这种多态行为，可将Fish::Swim()声明为虚函数。

11.1.2 使用虚函数实现多态行为

调用语法：

class Base
{
virtual ReturnType FunctionName (Parameter List);
};
class Derived
{
ReturnType FunctionName (Parameter List);
};

通过使用关键字 virtual，可确保编译器调用覆盖版本。也就是说，如果 Swim( )被声明为虚函数，则将参数 myFish（其类型为 Fish&）设置为一个 Tuna 对象时， myFish.Swim( )将执行 Tuna::Swim( )。
示例如下：

#include 
using namespace std;

class Fish
{
public:
	virtual void Swim()  //加入virtual关键字，声明为虚函数
	{
		cout << "Fish swims! " << endl;
	}
};

class Tuna :public Fish
{
public:
	// override Fish :: Swim
	void Swim()
	{
		cout << "Tuna swims! " << endl;
	}
};

void MakeFishSwim(Fish& inputFish)
{
	// calling Fish :: Swim
	inputFish.Swim();
}
int main()
{
	Tuna myDinner;
	// calling Tuna: :Swim
	myDinner.Swim();
	// sending Tuna as Fish
	MakeFishSwim(myDinner);
	return 0;
}

总结：多态就是将派生类对象视为基类对象，并执行派生类的 Swim( )实现。

11.1.3 为何需要构造虚函数

对于使用new在自由存储区中实例化的派生类对象，如果将其赋给基类指针，并通过该指针调用delete，将不会调用派生类的析构函数。这可能导致资源未释放、内存泄露等问题。
示例如下：

#include 
using namespace std;
class Fish
{
public:
	Fish()
	{
		cout << "Constructed Fish" << endl;
	}
	~Fish()
	{
		cout << "Destroyed Fish" << endl;
	}
};
class Tuna :public Fish
{
public:
	Tuna()
	{
		cout << "Constructed Tuna" << endl;
	}
	~Tuna()
	{
		cout << "Destroyed Tuna" << endl;
	}
};
void DeleteFishMemory(Fish* pFish)
{
	delete pFish;
}

int main()
{
	cout << "Allocating a Tuna on the free store:" << endl;
	Tuna* pTuna = new Tuna;
	cout << "Deleting the Tuna: "<< endl ;
	DeleteFishMemory(pTuna);
	cout <<" Instantiating a Tuna on the stack:" << endl;
	Tuna myDinner;
	cout << "Automatic destruction as it goes out of scope: " << endl;
	return 0;
}

结果如下：
Allocating a Tuna on the free store:
Constructed Fish
Constructed Tuna
Deleting the Tuna:
Destroyed Fish //仅仅释放了基类对象
Instantiating a Tuna on the stack:
Constructed Fish
Constructed Tuna
Automatic destruction as it goes out of scope:
Destroyed Tuna
Destroyed Fish

要避免上述问题，可将基类中的析构函数声明为虚函数：

#include 
using namespace std;
class Fish
{
public:
	Fish()
	{
		cout << "Constructed Fish" << endl;
	}
	virtual ~Fish()
	{
		cout << "Destroyed Fish" << endl;
	}
};
class Tuna :public Fish
{
public:
	Tuna()
	{
		cout << "Constructed Tuna" << endl;
	}
	~Tuna()
	{
		cout << "Destroyed Tuna" << endl;
	}
};
void DeleteFishMemory(Fish* pFish)
{
	delete pFish;
}

int main()
{
	cout << "Allocating a Tuna on the free store:" << endl;
	Tuna* pTuna = new Tuna;
	cout << "Deleting the Tuna: "<< endl ;
	DeleteFishMemory(pTuna);
	cout <<" Instantiating a Tuna on the stack:" << endl;
	Tuna myDinner;
	cout << "Automatic destruction as it goes out of scope: " << endl;
	return 0;
}

结果如下：
Allocating a Tuna on the free store:
Constructed Fish
Constructed Tuna
Deleting the Tuna:
Destroyed Tuna
Destroyed Fish
Instantiating a Tuna on the stack:
Constructed Fish
Constructed Tuna
Automatic destruction as it goes out of scope:
Destroyed Tuna
Destroyed Fish

11.1.4 虚函数的工作原理——理解虚函数表

在编译时，编译器将为Base和Derived类都将有自己的虚函数表（VFT）。实例化这些类的对象时，将创建一个隐藏的指针（VFT*），它指向相应的VFT，可将VFT视为一个包含函数指针的静态数组，其中每个指针都指向相应的虚函数实现。

如上图所示，Derived派生类中除了Func2()外，其它虚函数都有Derived类的本地虚函数实现，Derived 没有覆盖Base::Func2( )，因此相应的函数指针指向 Base 类的 Func2( )实现。

使用sizeof证明虚函数表指针存在

#include 
using namespace std;
class SimpleClass
{
	int a, b;
public:
	void DoSomething() { }
};
class Base
{
	int a, b;
public:
	virtual void DoSomething() { }
};
int main()
{
	cout << "sizeof (SimpleClass) = " << sizeof(SimpleClass) << endl;
	cout << "sizeof (Base) ="<< sizeof (Base) << endl ;
	return 0;
}

结果如下：
sizeof (SimpleClass) = 8
sizeof (Base) =16
添加关键字virtual带来的影响是，编译器将为Base类生成一个虚函数表，并为其虚函数表指针预留空间。

11.1.5 抽象基类和纯虚函数

抽象基类：不能实例化的基类，这样的基类只有一个用途，那就是从它派生出其他类。要创建抽象基类，可声明纯虚函数。
语法如下：

class AbstractBase
{
public:
	virtual void DoSomething() = 0;
};

改声明告诉编译器，AbstractBase的派生类必须实现方法DoSomething()
示例代码：

#include 
using namespace std;
class Fish
{
public:
	// define a pure virtual function Swim
	virtual void Swim() = 0;
};
class Tuna :public Fish
{
public:
	void Swim()
	{
		cout << "Tuna swims fast in the sea! " << endl;
	}
};

class Carp :public Fish
{
public:
	void Swim()
	{
		cout << "Carp swims slow in the lake! " << endl;
	}
};

void MakeFishSwim(Fish& inputFish) {
	inputFish.Swim();
}

int main()
{
	// Fish myFish; // Fails, cannot instantiate an ABC
	Carp myLunch;
	Tuna myDinner;
	MakeFishSwim(myLunch);
	MakeFishSwim(myDinner);
	return 0;
}

结果如下：
Carp swims slow in the lake!
Tuna swims fast in the sea!
编译器不允许创建抽象基类（ABC）Fish的实例，但是抽象基类可以作为指针或者引用的类型输入函数。

11.2 使用虚继承解决菱形问题

按照如下关系图实例化一个鸭嘴兽的类：

示例代码如下：

#include 
using namespace std;
class Animal
{
public:
	Animal()
	{
		cout << "Animal constructor" << endl;
	}
	// sample member
	int age;
};
class Mammal :public Animal
{
};
class Bird :public Animal
{
};
class Reptile :public Animal
{
};
class Platypus:public Mammal, public Bird, public Reptile
{
public:
	Platypus()
	{
		cout << "Platypus constructor" << endl;
	}
};

int main()
{
	Platypus duckBilledP;
	return 0;
}

结果如下：
Animal constructor
Animal constructor
Animal constructor
Platypus constructor
由于采用了多继承，且鸭嘴兽的全部三个基类都是从Animal类派生而来的，因此自动创建了3个Animal实例。
下面采用虚继承方式解决这个问题：

#include 
using namespace std;
class Animal
{
public:
	Animal()
	{
		cout << "Animal constructor" << endl;
	}
	// sample member
	int age;
};
class Mammal :public virtual Animal
{
};
class Bird :public virtual Animal
{
};
class Reptile :public virtual Animal
{
};
class Platypus final:public Mammal, public Bird, public Reptile
{
public:
	Platypus()
	{
		cout << "Platypus constructor" << endl;
	}
};

int main()
{
	Platypus duckBilledP;
	duckBilledP.age = 25;
	return 0;
}

结果如下：
Animal constructor
Platypus constructor
从 Animal 类派生 Mammal、Bird 和 Reptile 类时，使用了关键字 virtual，这样 Platypus 继承这些类时，每个 Platypus 实例只包含一个 Animal 实例。同时，使用了关键字 final 以禁止将 Platypus 类用作基类。

11.4 使用final来禁止覆盖函数

class Tuna :public Fish
{
public:
	// override Fish :: Swim
	void Swim() override final
	{
		cout << "Tuna swims! " << endl;
	}
};

class BluefinTuna final : public Tuna
{
public:
	void Swim()   //Error: Swim() was final in Tuna, cannot override
	{

	}
};

21天学通C++——多态专题

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