1>虛函數（Virtual Function）

1.1>Base Class

#ifndef Animal_h #define Animal_h #include <string> class Animal { protected:std::string m_strName;// We're making this constructor protected because// we don't want people creating Animal objects directly,// but we still want derived classes to be able to use it.Animal(std::string strName): m_strName(strName){}public:std::string GetName() { return m_strName; }virtual const char* Speak() { return "???"; }//virtual const char* Speak()=0; };#endif

1.2>Derived Class 1

#ifndef Cat_h #define Cat_h #include"Animal.h" class Cat: public Animal { public:Cat(std::string strName): Animal(strName){}virtual const char* Speak() { return "Meow"; } }; #endif

1.3>Derived Class 2

#ifndef Dog_h #define Dog_h #include"Animal.h" class Dog: public Animal { public:Dog(std::string strName) : Animal(strName){}virtual const char* Speak() { return "Woof"; } }; #endif

1.4>Main Class

#include<iostream> #include"Cat.h" #include"Dog.h" using namespace std; int main(){Cat cCat("cat");cout<<cCat.GetName()<<" says "<<cCat.Speak()<<endl;Dog cDog("dog");cout<<cDog.GetName()<<" says "<<cDog.Speak()<<endl;return 0; }

Result is:

運行結果正常。

1.5>Derived Class 3

#ifndef Cow_h #define Cow_h #include"Animal.h" class Cow: public Animal { public:Cow(std::string strName): Animal(strName){}// We forgot to redefine Speak//virtual const char* Speak(){ return "Moo";} }; #endif

在這里我們忘記定義Base Class Animal的Speak()函數了。

而Main函數改為下面的形式：

#include<iostream> #include"Cow.h" using namespace std; int main(){Cow cCow("cow");cout<<cCow.GetName()<<" says "<<cCow.Speak()<<endl;return 0; }

Result is:

What happened? We forgot to redefine Speak, so cCow.Speak() resolved to Animal.Speak(), which isn’t what we wanted.(我們忘記重新定義Speak函數了，所以cCow.Speak()解析為基類Animal的Speak()函數了，這不是我們所期望的。)

A better solution to this problem is to use a pure virtual function:
（解決這個問題的一個更好的方法是使用純虛函數）

2>純虛函數（Pure Virtual Function）

將Animal中的Speak()聲明改成如下純虛函數形式：

virtual const char* Speak()=0;

然后重新編譯1.5中的程序，出現如下錯誤：

1>d:\visual studio 2008\projects\cnn_finite_wordlength_emulator\purevirtualfunctiontest\purevirtualfunctiontest\main.cpp(5) : error C2259: 'Cow' : cannot instantiate abstract class 1> due to following members: 1> 'const char *Animal::Speak(void)' : is abstract 1> d:\visual studio 2008\projects\cnn_finite_wordlength_emulator\purevirtualfunctiontest\purevirtualfunctiontest\animal.h(19) : see declaration of 'Animal::Speak'

大致意思是：

error C2259 ‘Cow’:不能實例化抽象類? , 因為'const char *Animal::Speak(void)' 是抽象的，請查看'Animal::Speak'的聲明。

也就是說，Cow類只有提供Speak的函數實現才可以實例化實例化。

2.1>提供Speak的函數實現

#ifndef Cow_h #define Cow_h #include"Animal.h" class Cow: public Animal { public:Cow(std::string strName): Animal(strName){}// Cow provides a body for Speak().virtual const char* Speak(){ return "Moo";} }; #endif

再次編譯并運行，結果如下：

A pure virtual function is useful when we have a function that we want to put in the base class, but only the derived classes know what it should return. A pure virtual function makes it so the base class can not be instantiated, and the derived classes are forced to define these function before they can be instantiated. This helps ensure the derived classes do not forget to redefine functions that the base class was expecting them to.

URL：http://www.learncpp.com/cpp-tutorial/126-pure-virtual-functions-abstract-base-classes-and-interface-classes/



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