汉扬编程 编程大纲 怎样用C语言实现继承和多态?

怎样用C语言实现继承和多态?

1、引言继承和多态是面向对象语言最强大的功能。有了继承和多态,我们可以完成代码重用。在C中有许多技巧可以实现多态。本文的目的就是演示一种简单和容易的技术,在C中应用继承和多态。通过创建一个VTable(virtual table)和在基类和派生类对象之间提供正确的访问,我们能在C中实现继承和多态。VTable能通过维护一张函数表指针表来实现。为了提供基类和派生类对象之间的访问,我们可以在基类中维护派生类的引用和在派生类中维护基类的引用。

怎样用C语言实现继承和多态?

2、说明在C中实现继承和多态之前,我们应该知道类(Class)在C中如何表示。

怎样用C语言实现继承和多态?

2.1、类在C中的表示

怎样用C语言实现继承和多态?

考虑C++中的一个类\”Person\”。

怎样用C语言实现继承和多态?

//Person.hclass Person{private: char* pFirstName; char* pLastName; public: Person(const char* pFirstName, const char* pLastName); //constructor ~Person(); //destructor void displayInfo(); void writeToFile(const char* pFileName);};在C中表示上面的类,我们可以使用结构体,并用操作结构体的函数表示成员函数。

//Person.htypedef struct _Person{ char* pFirstName; char* pLastName;}Person;new_Person(const char* const pFirstName, const char* const pLastName); //constructordelete_Person(Person* const pPersonObj); //destructorvoid Person_DisplayInfo(Person* const pPersonObj);void Person_WriteToFile(Person* const pPersonObj, const char* const pFileName);这里,定义的操作结构体Person的函数没有封装。为了实现封装,即绑定数据、函数、函数指针。我们需要创建一个函数指针表。构造函数new_Person()将设置函数指针值以指向合适的函数。这个函数指针表将作为对象访问函数的接口。

下面我们重新定义C中实现类Person。

//Person.htypedef struct _Person Person;//declaration of pointers to functionstypedef void (*fptrDisplayInfo)(Person*);typedef void (*fptrWriteToFile)( Person*, const char*);typedef void (*fptrDelete)( Person *) ;//Note: In C all the members are by default public. We can achieve //the data hiding (private members), but that method is tricky. //For simplification of this article// we are considering the data members //public only.typedef struct _Person { char* pFName; char* pLName; //interface for function fptrDisplayInfo Display; fptrWriteToFile WriteToFile; fptrDelete Delete;}Person;person* new_Person(const char* const pFirstName, const char* const pLastName); //constructorvoid delete_Person(Person* const pPersonObj); //destructorvoid Person_DisplayInfo(Person* const pPersonObj);void Person_WriteToFile(Person* const pPersonObj, const char* pFileName);new_Person()函数作为构造函数,它返回新创建的结构体实例。它初始化函数指针接口去访问其它成员函数。这里要注意的一点是,我们仅仅定义了那些允许公共访问的函数指针,并没有给定私有函数的接口。让我们看一下new_Person()函数或C中类Person的构造函数。

//Person.cperson* new_Person(const char* const pFirstName, const char* const pLastName){ Person* pObj = NULL; //allocating memory pObj = (Person*)malloc(sizeof(Person)); if (pObj == NULL) { return NULL; } pObj->pFirstName = malloc(sizeof(char)*(strlen(pFirstName)+1)); if (pObj->pFirstName == NULL) { return NULL; } strcpy(pObj->pFirstName, pFirstName); pObj->pLastName = malloc(sizeof(char)*(strlen(pLastName)+1)); if (pObj->pLastName == NULL) { return NULL; } strcpy(pObj->pLastName, pLastName); //Initializing interface for access to functions pObj->Delete = delete_Person; pObj->Display = Person_DisplayInfo; pObj->WriteToFile = Person_WriteToFile; return pObj;}创建完对象之后,我们能够访问它的数据成员和函数。

Person* pPersonObj = new_Person(\”Anjali\”, \”Jaiswal\”);//displaying person infopPersonObj->Display(pPersonObj);//writing person info in the persondata.txt filepPersonObj->WriteToFile(pPersonObj, \”persondata.txt\”);//delete the person objectpPersonObj->Delete(pPersonObj);pPersonObj = NULL;注意:不像C++,在C中我们不能在函数中直接访问数据成员。在C++中,可以隐式通过“this”指针直接访问数据成员。我们知道C中是没有“this”指针的,通过显示地传递对象给成员函数。在C中为了访问类的数据成员,我们需要把调用对象作为函数参数传递。上面的例子中,我们把调用对象作为函数的第一个参数,通过这种方法,函数可以访问对象的数据成员。

3、在C中类的表现Person类的表示——检查初始化接口指向成员函数:

3.1、继承和多态的简单例子

继承-Employee类继承自Person类:

在上面的例子中,类Employee继承类Person的属性。因为DisplayInfo()和WriteToFile()函数是virtual的,我们能够从Person的实例访问Employee对象中的同名函数。为了实现这个,我们创建Person实例的时候也初始化Employee类。多态使这成为可能。 在多态的情况下,去解析函数调用,C++使用VTable——即一张函数指针表。

前面我们在结构体中维护的指向函数的指针接口的作用类似于VTable。

//Polymorphism in C++Person PersonObj(\”Anjali\”, \”Jaiswal\”);Employee EmployeeObj(\”Gauri\”, \”Jaiswal\”, \”HR\”, \”TCS\”, 40000);Person* ptrPersonObj = NULL; //preson pointer pointing to person objectptrPersonObj = &PersonObj;//displaying person infoptrPersonObj ->Display();//writing person info in the persondata.txt fileptrPersonObj ->WriteToFile(\”persondata.txt\”);//preson pointer pointing to employee objectptrPersonObj = &EmployeeObj;//displaying employee infoptrPersonObj ->Display();//writing empolyee info in the employeedata.txt fileptrPersonObj ->WriteToFile(\”employeedata.txt\”);在C中,继承可以通过在派生类对象中维护一个基类对象的引用来完成。在基类实例的帮助下,women可以访问基类的数据成员和函数。然而,为了实现多态,街垒对象应该能够访问派生类对象的数据。为了实现这个,基类应该有访问派生类的数据成员的权限。

为了实现虚函数,派生类的函数签名应该和基类的函数指针类似。即派生类函数将以基类对象的一个实例为参数。我们在基类中维护一个派生类的引用。在函数实现上,我们可以从派生类的引用访问实际派生类的数据。

3.2、在C中结构体中的等效表示

C中的继承-Person和Employee结构体:

如图所示,我们在基类结构体中声明了一个指针保存派生类对像,并在派生类结构体中声明一个指针保存基类对象。

在基类对象中,函数指针指向自己的虚函数。在派生类对象的构造函数中,我们需要使基类的接口指向派生类的成员函数。这使我们可以通过基类对象(多态)灵活的调用派生类函数。更多细节,请检查Person和Employee对象的构造函数。

当我们讨论C++中的多态时,有一个对象销毁的问题。为了正确的清楚对象,它使用虚析构函数。在C中,这可以通过使基类的删除函数指针指向派生类的析构函数。派生类的析构函数清楚派生类的数据和基类的数据和对象。注意:检查例子的源码中,实现须构造函数和虚函数的实现细节。

创建Person对象

//Person.htypedef struct _Person Person;//pointers to functiontypedef void (*fptrDisplayInfo)(Person*);typedef void (*fptrWriteToFile)(Person*, const char*);typedef void (*fptrDelete)(Person*) ;typedef struct _person{ void* pDerivedObj; char* pFirstName; char* pLastName; fptrDisplayInfo Display; fptrWriteToFile WriteToFile; fptrDelete Delete;}person;Person* new_Person(const char* const pFristName, const char* const pLastName); //constructorvoid delete_Person(Person* const pPersonObj); //destructorvoid Person_DisplayInfo(Person* const pPersonObj);void Person_WriteToFile(Person* const pPersonObj, const char* const pFileName); //Person.c//construction of Person objectPerson* new_Person(const char* const pFirstName, const char* const pLastName){ Person* pObj = NULL; //allocating memory pObj = (Person*)malloc(sizeof(Person)); if (pObj == NULL) { return NULL; } //pointing to itself as we are creating base class object pObj->pDerivedObj = pObj; pObj->pFirstName = malloc(sizeof(char)*(strlen(pFirstName)+1)); if (pObj->pFirstName == NULL) { return NULL; } strcpy(pObj->pFirstName, pFirstName); pObj->pLastName = malloc(sizeof(char)*(strlen(pLastName)+1)); if (pObj->pLastName == NULL) { return NULL; } strcpy(pObj->pLastName, pLastName); //Initializing interface for access to functions //destructor pointing to destrutor of itself pObj->Delete = delete_Person; pObj->Display = Person_DisplayInfo; pObj->WriteToFile = Person_WriteToFile; return pObj;}Person对象的结构

创建Employee对象

//Employee.h#include \”Person.h\”typedef struct _Employee Employee;//Note: interface for this class is in the base class//object since all functions are virtual.//If there is any additional functions in employee add//interface for those functions in this structure typedef struct _Employee{ Person* pBaseObj; char* pDepartment; char* pCompany; int nSalary; //If there is any employee specific functions; add interface here.}Employee;Person* new_Employee(const char* const pFirstName, const char* const pLastName, const char* const pDepartment, const char* const pCompany, int nSalary); //constructorvoid delete_Employee(Person* const pPersonObj); //destructorvoid Employee_DisplayInfo(Person* const pPersonObj);void Employee_WriteToFile(Person* const pPersonObj, const char* const pFileName); //Employee.cPerson* new_Employee(const char* const pFirstName, const char* const pLastName, const char* const pDepartment, const char* const pCompany, int nSalary){ Employee* pEmpObj; //calling base class construtor Person* pObj = new_Person(pFirstName, pLastName); //allocating memory pEmpObj = malloc(sizeof(Employee)); if (pEmpObj == NULL) { pObj->Delete(pObj); return NULL; } pObj->pDerivedObj = pEmpObj; //pointing to derived object //initialising derived class members pEmpObj->pDepartment = malloc(sizeof(char)*(strlen(pDepartment)+1)); if(pEmpObj->pDepartment == NULL) { return NULL; } strcpy(pEmpObj->pDepartment, pDepartment); pEmpObj->pCompany = malloc(sizeof(char)*(strlen(pCompany)+1)); if(pEmpObj->pCompany== NULL) { return NULL; } strcpy(pEmpObj->pCompany, pCompany); pEmpObj->nSalary = nSalary; //Changing base class interface to access derived class functions //virtual destructor //person destructor pointing to destrutor of employee pObj->Delete = delete_Employee; pObj->Display = Employee_DisplayInfo; pObj->WriteToFile = Employee_WriteToFile; return pObj;}Employee对象的结构

注意:从基类函数到派生类函数改变了接口(VTable)中指针位置。现在我们可以从基类(多态)访问派生类函数。我们来看如何使用多态。

Person* PersonObj = new_Person(\”Anjali\”, \”Jaiswal\”);Person* EmployeeObj = new_Employee(\”Gauri\”, \”Jaiswal\”,\”HR\”, \”TCS\”, 40000);//accessing person object//displaying person infoPersonObj->Display(PersonObj);//writing person info in the persondata.txt filePersonObj->WriteToFile(PersonObj,\”persondata.txt\”);//calling destructorPersonObj->Delete(PersonObj);//accessing to employee object//displaying employee infoEmployeeObj->Display(EmployeeObj);//writing empolyee info in the employeedata.txt fileEmployeeObj->WriteToFile(EmployeeObj, \”employeedata.txt\”);//calling destrutorEmployeeObj->Delete(EmployeeObj);结论使用上面描述的简单的额外代码能是过程式C语言有多态和继承的特性。我们简单的使用函数指针创建一个VTable和在基类和派生类对象中交叉维护引用。用这些简单的步骤,我们在C中可以实现继承和多态。

声明:本文为译文,原文通道-Inheritance and Polymorphism in C

例子代码下载:/skynet/PolymorphisminC.zip

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