C語言實現繼承和多型
繼承和多型是面嚮物件語言最強大的功能。有了繼承和多型,我們可以完成程式碼重用。在C中有許多技巧可以實現多型。本文的目的就是演示一種簡單和容易的技術,在C中應用繼承和多型。通過建立一個VTable(virtual table)和在基類和派生類物件之間提供正確的訪問,我們能在C中實現繼承和多型。VTable能通過維護一張函式表指標表來實現。為了提供基類和派生類物件之間的訪問,我們可以在基類中維護派生類的引用和在派生類中維護基類的引用。
2、說明
在C中實現繼承和多型之前,我們應該知道類(Class)在C中如何表示。
2.1、類在C中的表示
考慮C++中的一個類"Person"。
//Person.h
class 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中表示上面的類,我們可以使用結構體,並用操作結構體的函式表示成員函式。
//Person.h
typedef struct _Person
{
char* pFirstName;
char* pLastName;
}Person;
new_Person(const char* const pFirstName, const char* const pLastName); //constructor
delete_Person(Person* const pPersonObj); //destructor
void Person_DisplayInfo(Person* const pPersonObj);
void Person_WriteToFile(Person* const pPersonObj, const char* const pFileName);
這裡,定義的操作結構體Person的函式沒有封裝。為了實現封裝,即繫結資料、函式、函式指標。我們需要建立一個函式指標表。建構函式new_Person()將設定函式指標值以指向合適的函式。這個函式指標表將作為物件訪問函式的介面。
下面我們重新定義C中實現類Person。
//Person.h
typedef struct _Person Person;
//declaration of pointers to functions
typedef 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); //constructor
void delete_Person(Person* const pPersonObj); //destructor
void Person_DisplayInfo(Person* const pPersonObj);
void Person_WriteToFile(Person* const pPersonObj, const char* pFileName);
new_Person()函式作為建構函式,它返回新建立的結構體例項。它初始化函式指標介面去訪問其它成員函式。這裡要注意的一點是,我們僅僅定義了那些允許公共訪問的函式指標,並沒有給定私有函式的介面。讓我們看一下new_Person()函式或C中類Person的建構函式。
//Person.c
person* 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;
}
建立完物件之後,我們能夠訪問它的資料成員和函式。
Person* pPersonObj = new_Person("Anjali", "Jaiswal");
//displaying person info
pPersonObj->Display(pPersonObj);
//writing person info in the persondata.txt file
pPersonObj->WriteToFile(pPersonObj, "persondata.txt");
//delete the person object
pPersonObj->Delete(pPersonObj);
pPersonObj = NULL;
注意:不像C++,在C中我們不能在函式中直接訪問資料成員。在C++中,可以隱式通過“this”指標直接訪問資料成員。我們知道C中是沒有“this”指標的,通過顯示地傳遞物件給成員函式。在C中為了訪問類的資料成員,我們需要把呼叫物件作為函式引數傳遞。上面的例子中,我們把呼叫物件作為函式的第一個引數,通過這種方法,函式可以訪問物件的資料成員。
3、在C中類的表現
Person類的表示——檢查初始化介面指向成員函式:
3.1、繼承和多型的簡單例子
繼承-Employee類繼承自Person類:
在上面的例子中,類Employee繼承類Person的屬性。因為DisplayInfo()和WriteToFile()函式是virtual的,我們能夠從Person的例項訪問Employee物件中的同名函式。為了實現這個,我們建立Person例項的時候也初始化Employee類。多型使這成為可能。 在多型的情況下,去解析函式呼叫,C++使用VTable——即一張函式指標表。
前面我們在結構體中維護的指向函式的指標介面的作用類似於VTable。
//Polymorphism in C++
Person PersonObj("Anjali", "Jaiswal");
Employee EmployeeObj("Gauri", "Jaiswal", "HR", "TCS", 40000);
Person* ptrPersonObj = NULL;
//preson pointer pointing to person object
ptrPersonObj = &PersonObj;
//displaying person info
ptrPersonObj ->Display();
//writing person info in the persondata.txt file
ptrPersonObj ->WriteToFile("persondata.txt");
//preson pointer pointing to employee object
ptrPersonObj = &EmployeeObj;
//displaying employee info
ptrPersonObj ->Display();
//writing empolyee info in the employeedata.txt file
ptrPersonObj ->WriteToFile("employeedata.txt");
在C中,繼承可以通過在派生類物件中維護一個基類物件的引用來完成。在基類例項的幫助下,women可以訪問基類的資料成員和函式。然而,為了實現多型,街壘物件應該能夠訪問派生類物件的資料。為了實現這個,基類應該有訪問派生類的資料成員的許可權。
為了實現虛擬函式,派生類的函式簽名應該和基類的函式指標類似。即派生類函式將以基類物件的一個例項為引數。我們在基類中維護一個派生類的引用。在函式實現上,我們可以從派生類的引用訪問實際派生類的資料。
3.2、在C中結構體中的等效表示
C中的繼承-Person和Employee結構體:
如圖所示,我們在基類結構體中聲明瞭一個指標儲存派生類對像,並在派生類結構體中宣告一個指標儲存基類物件。
在基類物件中,函式指標指向自己的虛擬函式。在派生類物件的建構函式中,我們需要使基類的介面指向派生類的成員函式。這使我們可以通過基類物件(多型)靈活的呼叫派生類函式。更多細節,請檢查Person和Employee物件的建構函式。
當我們討論C++中的多型時,有一個物件銷燬的問題。為了正確的清楚物件,它使用虛解構函式。在C中,這可以通過使基類的刪除函式指標指向派生類的解構函式。派生類的解構函式清楚派生類的資料和基類的資料和物件。注意:檢查例子的原始碼中,實現須建構函式和虛擬函式的實現細節。
建立Person物件
//Person.h
typedef struct _Person Person;
//pointers to function
typedef 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); //constructor
void delete_Person(Person* const pPersonObj); //destructor
void Person_DisplayInfo(Person* const pPersonObj);
void Person_WriteToFile(Person* const pPersonObj, const char* const pFileName);
//Person.c
//construction of Person object
Person* 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物件的結構
建立Employee物件
//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); //constructor
void delete_Employee(Person* const pPersonObj); //destructor
void Employee_DisplayInfo(Person* const pPersonObj);
void Employee_WriteToFile(Person* const pPersonObj, const char* const pFileName);
//Employee.c
Person* 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物件的結構
注意:從基類函式到派生類函式改變了介面(VTable)中指標位置。現在我們可以從基類(多型)訪問派生類函式。我們來看如何使用多型。
Person* PersonObj = new_Person("Anjali", "Jaiswal");
Person* EmployeeObj = new_Employee("Gauri", "Jaiswal","HR", "TCS", 40000);
//accessing person object
//displaying person info
PersonObj->Display(PersonObj);
//writing person info in the persondata.txt file
PersonObj->WriteToFile(PersonObj,"persondata.txt");
//calling destructor
PersonObj->Delete(PersonObj);
//accessing to employee object
//displaying employee info
EmployeeObj->Display(EmployeeObj);
//writing empolyee info in the employeedata.txt file
EmployeeObj->WriteToFile(EmployeeObj, "employeedata.txt");
//calling destrutor
EmployeeObj->Delete(EmployeeObj);
結論
使用上面描述的簡單的額外程式碼能是過程式C語言有多型和繼承的特性。我們簡單的使用函式指標建立一個VTable和在基類和派生類物件中交叉維護引用。用這些簡單的步驟,我們在C中可以實現繼承和多型。
https://blog.csdn.net/changyourmind/art