前言

为了保证代码的可复用性、可扩展性、可维护性，我们提出了面向对象的思想。

面向对象的核心特性有以下几个

• 封装特性
  
  信息隐藏或者数据访问保护。类通过暴露有限的访问接口，授权外部仅能通过类提供的方式来访问内部信息或者数据。
  
  封装用来提升代码的可扩展性、可维护性
• 继承特性
  
  继承是用来表示类之间的 is-a 关系，分为两种模式：单继承和多继承。单继承表示一个子类只继承一个父类，多继承表示一个子类可以继承多个父类。为了实现继承这个特性，编程语言需要提供特殊的语法机制来支持。继承主要是用来解决代码可复用性的问题
• 多态特性
  
  多态是指子类可以替换父类，在实际的代码运行过程中，调用子类的方法实现。
  
  多态可以提高代码的可扩展性和可复用性

C++提供语法来实现以上三个特性，而C语言则可以使用函数指针来实现以上三个特性。

封装

C++实现

class Person{ 
private:char *pFName;char *pLName;
public:Person(const char * const str1,const char * const str2):pFName(str1),pLNname(str2){ }~Person();void Person_DisplayInfo();void Person_WriteToFile(const char* pFileName);
}void Person::Person_DisplayInfo()
{ cout <<"pFName is " << this->pFName<< endl;cout <<"pLName is " << this -> pLName << endl;
}void Person::Person_WriteToFile(const char* pFileName)
{ if(!pFileName) { cout <<"file name is null" << endl;return;}FILE *fp;printf("write ot file name is: %s
",pFileName);fp = fopen(pFileName, "a+");char *buff = NULL;buff = (char *)malloc(sizeof(DATA_SIZE));if(!buff) { printf("malloc failed
");return;}int count;strcpy(buff,this->pFName);count = fwrite(buff,sizeof(char),strlen(buff),fp);printf("write to %s's result is %d 
",pFileName,count);fclose(fp);fp = NULL;
}int main()
{ Person obj("lili","chuchu");obj.Person_DisplayInfo();obj.Person_WriteToFile("test.txt");return 0;
}

C 实现

person.h

#include 
#include typedef struct _Person Person;//declaration of pointers to functions
typedef void    (*fptrDisplayInfo)(Person*);
typedef void    (*fptrWriteToFile)( Person*, const char*);
typedef void    (*fptrDelete)( Person *) ;typedef struct _Person { char* pFName;char* pLName;//interface for functionfptrDisplayInfo   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);

person.c

#include 
#include 
#include "person.h"#define DATA_SIZE 1024 Person* new_person(const char* const pFirstName, const char* const pLastName)
{ Person *obj = NULL;obj = (Person *)malloc(sizeof(Person));if(!obj){ return NULL;}obj -> pFName =  malloc(sizeof(char)*(strlen(pFirstName)+1));if(!obj -> pFName) { return NULL;}strcpy(obj -> pFName, pFirstName);obj -> pLName = malloc(sizeof(char) * (strlen(pLastName) + 1));if(!obj -> pLName) { return NULL;}strcpy(obj -> pLName, pLastName);/*construct the function pointer*/obj->Delete = delete_Person;obj->Display = Person_DisplayInfo;obj->WriteToFile = Person_WriteToFile;printf("finish constructor Person
");return obj;
}void Person_WriteToFile(Person* const pPersonObj, const char* pFileName)
{ if(!pPersonObj || !pFileName) { return;}FILE *fp;printf("write ot file name is: %s
",pFileName);fp = fopen(pFileName, "a+");char *buff = NULL;buff = (char *)malloc(sizeof(DATA_SIZE));if(!buff) { printf("malloc failed
");return;}int count;strcpy(buff,pPersonObj->pFName);count = fwrite(buff,sizeof(char),strlen(buff),fp);printf("write to %s's result is %d 
",pFileName,count);fclose(fp);fp = NULL;
}void delete_Person(Person* const pPersonObj)
{ if(!pPersonObj) { return;}if(pPersonObj -> pFName) { free(pPersonObj->pFName);pPersonObj -> pFName = NULL;}if(pPersonObj -> pLName) { free(pPersonObj -> pLName);pPersonObj -> pLName = NULL;}}void Person_DisplayInfo(Person* const pPersonObj) { if(!pPersonObj) { printf("obj is not exists, please construct
");}printf("pFName is %s
",pPersonObj -> pFName);printf("pLName is %s
",pPersonObj -> pLName);
}

main.c

#include 
#include 
#include "person.h"
//#include "employee.h"int main(int argc, char *argv[]) { Person *obj;obj = new_person("pfirst_name","plast_name");obj -> Display(obj);obj -> WriteToFile(obj,"F:\test_write.txt");obj -> Delete(obj);return 0;
}

继承

C++ 实现

class Person{ 
private:char *pFName;char *pLName;
public:Person(const char * const str1,const char * const str2):pFName(str1),pLNname(str2){ }~Person(){ }void Person_DisplayInfo();void Person_WriteToFile(const char* pFileName);
}class Empolyee:public Person{ 
private:char* pDepartment;char* pCompany;int nSalary;
public:Empolyee(const char * const str1,const char * conststr2,int num):Person(str1,str2),pDepartment(str1),pCompany(str2),nSalary(num){ }~Empolyee(){ }void Employee_DisplayInfo();
}
void Empolyee::Employee_DisplayInfo()
{ Person_DisplayInfo();//可以访问基类的公有成员cout << "pDepartment is " << pDepartment << endl;cout << "pCompany is " << pCompany<< endl;cout << "nSalary is " << nSalary << endl;
}

C实现

person.c如上，person.h增加万能指针void *

person.h

#include 
#include /* run this program using the console pauser or add your own getch, system("pause") or input loop */typedef struct _Person Person;//declaration of pointers to functions
typedef void    (*fptrDisplayInfo)(Person*);
typedef void    (*fptrWriteToFile)( Person*, const char*);
typedef void    (*fptrDelete)( Person *) ;typedef struct _Person { void *pDerivedObj; char* pFName;char* pLName;//interface for functionfptrDisplayInfo   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);

employee.h

#include "Person.h"typedef struct _Employee Employee;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);    //destructorvoid Employee_DisplayInfo(Person* const pPersonObj);

employee.c

#include 
#include #include "employee.h"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 construtorPerson* pObj = new_person(pFirstName, pLastName);//allocating memorypEmpObj = malloc(sizeof(Employee));if (pEmpObj == NULL){ pObj->Delete(pObj);return NULL;}pObj->pDerivedObj = pEmpObj; //pointing to derived object//initialising derived class memberspEmpObj->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 employeepObj->Delete = delete_Employee;pObj->Display = Employee_DisplayInfo;//pObj->WriteToFile = Employee_WriteToFile;printf("finish construct employee
");return pObj;
}void delete_Employee(Person* const pPersonObj)
{ if(!pPersonObj){ return;}Employee* pEmpObj;pEmpObj = pPersonObj->pDerivedObj;if(pEmpObj->pDepartment) { free(pEmpObj->pDepartment);pEmpObj->pDepartment = NULL;}if(pEmpObj->pCompany) { free(pEmpObj->pCompany);pEmpObj->pCompany = NULL;}pPersonObj->Delete(pPersonObj);
}void Employee_DisplayInfo(Person* const pPersonObj) { if(!pPersonObj) { printf("pPersonObj is null
");return;}Employee* pEmpObj;pEmpObj = pPersonObj->pDerivedObj;printf("pEmpObj's pDepartment is %s
",pEmpObj->pDepartment);printf("pEmpObj's pCompany is %s
",pEmpObj->pCompany);printf("pEmpObj's nSalary is %d
",pEmpObj->nSalary);
}

main.c

#include 
#include 
#include "person.h"
//#include "employee.h"int main(int argc, char *argv[]) { Person *obj;obj = new_person("pfirst_name","plast_name");obj -> Display(obj);obj -> WriteToFile(obj,"F:\test_write.txt");obj -> Delete(obj);Person *pobj;pobj = new_Employee("Gauri", "Jaiswal","HR", "TCS", 40000);pobj -> Display(pobj);pobj -> Delete(pobj);return 0;
}