一个关于多态之虚函数的例子

快来打我* 2022-08-01 14:56 140阅读 0赞

[程序实践][Link 1] [多态性][Link 2] [工资发放管理系统][Link 3]

目录[(?)][Link 4][\[+\]][Link 4]

## 引言    ##

本博文通过包含了一个公司支付系统中各种雇员情况的一个继承层次来讨论基类和派生类之间的关系.佣金雇员(表示为一个基类对象)的薪水完全是销售提成,带底薪佣雇员(表示为一个派生类的对象)的薪水由底薪和销售提成组成.以及创建一个增强的雇员类层次结构,以解决下面的问题:

*  假设某家公司按周支付雇员工资,雇员一共有有4类:
    
     *  **定薪雇员**:不管每周工作多长时间都领取固定的周薪
     *  **钟点雇员:**按工作的小时数领取工资,并且可以领取超过40个小时之外的加班费
     *  **佣金雇员:**工资完全是销售业绩提成
     *  **带薪佣雇员:**工资是基本工资和销售提成组成  
        
 *  在这次工资发放阶级,公司决定奖励带佣金雇员,把他们的基本工资提高10%.公司想实现一个C++程序多态地执行工资的计算.

我们使用抽象类Employee表示通常概念的雇员.直接从Employee类派生的是类SalariedEmployee,CommissionEmployee 和HourlyEmployee.而BasePlusCommissionEmployee类又是从CommissionEmployee类直接派生的,代表最后一种雇员类型.如图1所中的UML图显示多态的雇员工资应用程序中类的继承层次结构.

![Image 1][]

图1:雇员类型层次结构的UML图

抽象类Employee声明了类层次结构的"接口",即程序可以对所有的Employee类对象调用的一组成员函数集合.每个雇员,不论他的工资计算方式如何,都有名,姓及社会安全号码,因此在抽象基类Employee中含有private数据成员firstName ,lastName和 socialSecurityNumber.接下来的内容将实现上述的Employee类层次结构.在最后构建一个测试程序,它创建所有这些类的对象并多态地处理对象.

## 创建抽象基类: Employee类 ##

图2给出了图表类继承层次中的5个类,顶部显示的是earnings和print函数名.对于每个类,这张图表显示了每个函数期望的实现.注意在Employee类中earnings函数被指定"=0",表示它是一个纯virtual函数.每个派生类都重写earnings函数,提供合适的实现.

![Image 1][]

图2:Employee类层次结构的多态性接口

让我们考虑**Employee类头文件**(如下程序所示).public成员函数包括:一个构造函数,该构造函数以名,姓及社会安全号码为参数;设置函数名,姓及社会安全号码;获取函数,返回了名,姓及社会安全号码;以及纯virtual 函数earnings和virtual函数print;

### Employee类头文件    ###

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1.  // Employee.h: Employee abstract base class.
2.  \#ifndef EMPLOYEE\_H
3.  \#define EMPLOYEE\_H
4.  
5.  \#include <string> // C++ standard string class
6.  **using** std::string; 
7.  
8.  **class** Employee 
9.  \{ 
10. **public**: 
11.  Employee( **const** string &, **const** string &, **const** string & ); 
12. 
13. **void** setFirstName( **const** string & ); // set first name
14.  string getFirstName() **const**; // return first name
15. 
16. **void** setLastName( **const** string & ); // set last name
17.  string getLastName() **const**; // return last name
18. 
19. **void** setSocialSecurityNumber( **const** string & ); // set SSN
20.  string getSocialSecurityNumber() **const**; // return SSN
21. 
22. // pure virtual function makes Employee abstract base class
23. **virtual****double** earnings() **const** = 0; // pure virtual
24. **virtual****void** print() **const**; // virtual
25. **private**: 
26.  string firstName; 
27.  string lastName; 
28.  string socialSecurityNumber; 
29. \}; // end class Employee
30. 
31. \#endif // EMPLOYEE\_H

而 **Employee类的实现文件**中,没有为纯virtual 函数earnings提供任何实现.**请注意**: Employee类的构造函数并没有确认社会号码的有效性.通常应该提供这类有效性确认.

### Employee类的实现文件 ###

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1.  // Employee.cpp: Abstract-base-class Employee member-function definitions.
2.  // Note: No definitions are given for pure virtual functions.
3.  \#include <iostream>
4.  **using** std::cout; 
5.  
6.  \#include "Employee.h" // Employee class definition
7.  
8.  // constructor
9.  Employee::Employee( **const** string &first, **const** string &last, 
10. **const** string &ssn ) 
11.  : firstName( first ), lastName( last ), socialSecurityNumber( ssn ) 
12. \{ 
13. // empty body 
14. \} // end Employee constructor
15. 
16. // set first name
17. **void** Employee::setFirstName( **const** string &first ) 
18. \{ 
19.  firstName = first; 
20. \} // end function setFirstName
21. 
22. // return first name
23. string Employee::getFirstName() **const**
24. \{ 
25. **return** firstName; 
26. \} // end function getFirstName
27. 
28. // set last name
29. **void** Employee::setLastName( **const** string &last ) 
30. \{ 
31.  lastName = last; 
32. \} // end function setLastName
33. 
34. // return last name
35. string Employee::getLastName() **const**
36. \{ 
37. **return** lastName; 
38. \} // end function getLastName
39. 
40. // set social security number
41. **void** Employee::setSocialSecurityNumber( **const** string &ssn ) 
42. \{ 
43.  socialSecurityNumber = ssn; // should validate
44. \} // end function setSocialSecurityNumber
45. 
46. // return social security number
47. string Employee::getSocialSecurityNumber() **const**
48. \{ 
49. **return** socialSecurityNumber; 
50. \} // end function getSocialSecurityNumber
51. 
52. // print Employee's information
53. **void** Employee::print() **const**
54. \{ 
55.  cout << getFirstName() << ' ' << getLastName() 
56.  << "\\nsocial security number: " << getSocialSecurityNumber(); 
57. \} // end function print

注意:virtual函数print提供的实现会在每个派生类中被重写.可是,这些print函数都将使用这个抽象类中print函数的版本,输出Employee声明了类层次结构中所有类共有的信息.

## 创建具体的派生类: SalariedEmployee类 ##

SalariedEmployee类是从Employee类派生而来的,其public成员函数包括:一个以名, 姓,社会安全号码和周薪为参数的构造函数;给数据成员weeklySalary赋一个非负的设置函数;返回weeklySalary值的获取函数;计算一个SalariedEmployee雇员收入的virtual函数earnings;打印雇员信息的virtual函数print,它输出的内容依次是雇员类型(即:"salaried employee").以及由基类Employee的print函数和SalariedEmployee类的 getWeeklySalary函数产生的雇员的特定信息.

### SalariedEmployee类头文件 ###

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1.  // SalariedEmployee.h: SalariedEmployee class derived from Employee.
2.  \#ifndef SALARIED\_H
3.  \#define SALARIED\_H
4.  
5.  \#include "Employee.h" // Employee class definition
6.  
7.  **class** SalariedEmployee : **public** Employee 
8.  \{ 
9.  **public**: 
10.  SalariedEmployee( **const** string &, **const** string &, 
11. **const** string &, **double** = 0.0 ); 
12. 
13. **void** setWeeklySalary( **double** ); // set weekly salary
14. **double** getWeeklySalary() **const**; // return weekly salary
15. 
16. // keyword virtual signals intent to override
17. **virtual****double** earnings() **const**; // calculate earnings
18. **virtual****void** print() **const**; // print SalariedEmployee object
19. **private**: 
20. **double** weeklySalary; // salary per week
21. \}; // end class SalariedEmployee
22. 
23. \#endif // SALARIED\_H

SalariedEmployee类的成员函数的实现.类的构造函数把名,姓和社会安全号码传递给基类Employee的构造函数,从而对从基类继承但在派行类中不可访问的private数据成员进行了初始化.SalariedEmployee类的print函数重写了基类Employee的print函数.如果SalariedEmployee类不重写print函数,那么SalariedEmployee类将继承基类Employee的print函数.

### SalariedEmployee类实现文件 ###

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1.  // SalariedEmployee.cpp :SalariedEmployee class member-function definitions.
2.  \#include <iostream>
3.  **using** std::cout; 
4.  
5.  \#include "SalariedEmployee.h" // SalariedEmployee class definition
6.  
7.  // constructor 
8.  SalariedEmployee::SalariedEmployee( **const** string &first, 
9.  **const** string &last, **const** string &ssn, **double** salary ) 
10.  : Employee( first, last, ssn ) 
11. \{ 
12.  setWeeklySalary( salary ); 
13. \} // end SalariedEmployee constructor
14. 
15. // set salary
16. **void** SalariedEmployee::setWeeklySalary( **double** salary ) 
17. \{ 
18.  weeklySalary = ( salary < 0.0 ) ? 0.0 : salary; 
19. \} // end function setWeeklySalary
20. 
21. // return salary
22. **double** SalariedEmployee::getWeeklySalary() **const**
23. \{ 
24. **return** weeklySalary; 
25. \} // end function getWeeklySalary
26. 
27. // calculate earnings; 
28. // override pure virtual function earnings in Employee
29. **double** SalariedEmployee::earnings() **const**
30. \{ 
31. **return** getWeeklySalary(); 
32. \} // end function earnings
33. 
34. // print SalariedEmployee's information 
35. **void** SalariedEmployee::print() **const**
36. \{ 
37.  cout << "salaried employee: "; 
38.  Employee::print(); // code reuse
39.  cout << "\\nweekly salary: " << getWeeklySalary(); 
40. \} // end function print

## 创建具体的派生类: HourlyEmployee类 ##

### HourlyEmployee类头文件 ###

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1.  // HourlyEmployee.h
2.  // HourlyEmployee class definition.
3.  \#ifndef HOURLY\_H
4.  \#define HOURLY\_H
5.  
6.  \#include "Employee.h" // Employee class definition
7.  
8.  **class** HourlyEmployee : **public** Employee 
9.  \{ 
10. **public**: 
11.  HourlyEmployee( **const** string &, **const** string &, 
12. **const** string &, **double** = 0.0, **double** = 0.0 ); 
13. 
14. **void** setWage( **double** ); // set hourly wage
15. **double** getWage() **const**; // return hourly wage
16. 
17. **void** setHours( **double** ); // set hours worked
18. **double** getHours() **const**; // return hours worked
19. 
20. // keyword virtual signals intent to override
21. **virtual****double** earnings() **const**; // calculate earnings
22. **virtual****void** print() **const**; // print HourlyEmployee object
23. **private**: 
24. **double** wage; // wage per hour
25. **double** hours; // hours worked for week
26. \}; // end class HourlyEmployee
27. 
28. \#endif // HOURLY\_H

### HourlyEmployee类实现文件 ###

// HourlyEmployee.cpp: HourlyEmployee class member-function definitions.
    #include <iostream>
    using std::cout;
    
    #include "HourlyEmployee.h" // HourlyEmployee class definition
    
    // constructor
    HourlyEmployee::HourlyEmployee(const string &first, const string &last,
    	const string &ssn, double hourlyWage, double hoursWorked)
    	: Employee(first, last, ssn)
    {
    	setWage(hourlyWage); // validate hourly wage
    	setHours(hoursWorked); // validate hours worked
    } // end HourlyEmployee constructor
    
    // set wage
    void HourlyEmployee::setWage(double hourlyWage)
    {
    	wage = hourlyWage < 0.0 ? 0.0 : hourlyWage;
    } // end function setWage
    
    // return wage
    double HourlyEmployee::getWage() const
    {
    	return wage;
    } // end function getWage
    
    // set hours worked
    void HourlyEmployee::setHours(double hoursWorked)
    {
    	hours = hoursWorked < 0.0 ? 0.0 : hoursWorked;
    }
    double HourlyEmployee::getHours() const
    {
    	return hours;
    }
    double HourlyEmployee::earnings() const
    {
    	return getWage() * getHours();
    }
    void HourlyEmployee::print() const
    {
    	cout << "Hourly employee: ";
    	Employee::print(); // code reuse
    	cout << "\nwage: " << getWage()
    		<< "; hours: " << getHours();
    }

## 创建具体的派生类: CommissionEmployee类 ##

### CommissionEmployee类头文件 ###

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1.  // CommissionEmployee.h: CommissionEmployee class derived from Employee.
2.  \#ifndef COMMISSION\_H
3.  \#define COMMISSION\_H
4.  
5.  \#include "Employee.h" // Employee class definition
6.  
7.  **class** CommissionEmployee : **public** Employee 
8.  \{ 
9.  **public**: 
10.  CommissionEmployee( **const** string &, **const** string &, 
11. **const** string &, **double** = 0.0, **double** = 0.0 ); 
12. 
13. **void** setCommissionRate( **double** ); // set commission rate
14. **double** getCommissionRate() **const**; // return commission rate
15. 
16. **void** setGrossSales( **double** ); // set gross sales amount
17. **double** getGrossSales() **const**; // return gross sales amount
18. 
19. // keyword virtual signals intent to override
20. **virtual****double** earnings() **const**; // calculate earnings
21. **virtual****void** print() **const**; // print CommissionEmployee object
22. **private**: 
23. **double** grossSales; // gross weekly sales
24. **double** commissionRate; // commission percentage
25. \}; // end class CommissionEmployee
26. 
27. \#endif // COMMISSION\_H

### CommissionEmployee类实现文件 ###

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1.  // CommissionEmployee.cpp: CommissionEmployee class member-function definitions.
2.  \#include <iostream>
3.  **using** std::cout; 
4.  
5.  \#include "CommissionEmployee.h" // CommissionEmployee class definition
6.  
7.  // constructor 
8.  CommissionEmployee::CommissionEmployee( **const** string &first, 
9.  **const** string &last, **const** string &ssn, **double** sales, **double** rate ) 
10.  : Employee( first, last, ssn ) 
11. \{ 
12.  setGrossSales( sales ); 
13.  setCommissionRate( rate ); 
14. \} // end CommissionEmployee constructor
15. 
16. // set commission rate
17. **void** CommissionEmployee::setCommissionRate( **double** rate ) 
18. \{ 
19.  commissionRate = ( rate > 0.0 && rate < 1.0 ) ? rate : 0.0; 
20. \} // end function setCommissionRate
21. 
22. // return commission rate
23. **double** CommissionEmployee::getCommissionRate() **const**
24. \{ 
25. **return** commissionRate; 
26. \} // end function getCommissionRate
27. 
28. // set gross sales amount
29. **void** CommissionEmployee::setGrossSales( **double** sales ) 
30. \{ 
31.  grossSales = ( sales < 0.0 ) ? 0.0 : sales; 
32. \} // end function setGrossSales
33. 
34. // return gross sales amount
35. **double** CommissionEmployee::getGrossSales() **const**
36. \{ 
37. **return** grossSales; 
38. \} // end function getGrossSales
39. 
40. // calculate earnings;
41. // override pure virtual function earnings in Employee
42. **double** CommissionEmployee::earnings() **const**
43. \{ 
44. **return** getCommissionRate() \* getGrossSales(); 
45. \} // end function earnings
46. 
47. // print CommissionEmployee's information 
48. **void** CommissionEmployee::print() **const**
49. \{ 
50.  cout << "commission employee: "; 
51.  Employee::print(); // code reuse
52.  cout << "\\ngross sales: " << getGrossSales() 
53.  << "; commission rate: " << getCommissionRate(); 
54. \} // end function print

## 创建具体的派生类: BasePlusCommissionEmployee类 ##

### BasePlusCommissionEmployee类头文件 ###

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1.  // BasePlusCommissionEmployee.h
2.  // BasePlusCommissionEmployee class derived from Employee.
3.  \#ifndef BASEPLUS\_H
4.  \#define BASEPLUS\_H
5.  
6.  \#include "CommissionEmployee.h" // CommissionEmployee class definition
7.  
8.  **class** BasePlusCommissionEmployee : **public** CommissionEmployee 
9.  \{ 
10. **public**: 
11.  BasePlusCommissionEmployee( **const** string &, **const** string &, 
12. **const** string &, **double** = 0.0, **double** = 0.0, **double** = 0.0 ); 
13. 
14. **void** setBaseSalary( **double** ); // set base salary
15. **double** getBaseSalary() **const**; // return base salary
16. 
17. // keyword virtual signals intent to override
18. **virtual****double** earnings() **const**; // calculate earnings
19. **virtual****void** print() **const**; // print BasePlusCommissionEmployee object
20. **private**: 
21. **double** baseSalary; // base salary per week
22. \}; // end class BasePlusCommissionEmployee
23. 
24. \#endif // BASEPLUS\_H

### BasePlusCommissionEmployee类实现文件 ###

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1.  // BasePlusCommissionEmployee.cpp: BasePlusCommissionEmployee member-function definitions.
2.  \#include <iostream>
3.  **using** std::cout; 
4.  
5.  // BasePlusCommissionEmployee class definition
6.  \#include "BasePlusCommissionEmployee.h"
7.  
8.  // constructor 
9.  BasePlusCommissionEmployee::BasePlusCommissionEmployee( 
10. **const** string &first, **const** string &last, **const** string &ssn, 
11. **double** sales, **double** rate, **double** salary ) 
12.  : CommissionEmployee( first, last, ssn, sales, rate ) 
13. \{ 
14.  setBaseSalary( salary ); // validate and store base salary
15. \} // end BasePlusCommissionEmployee constructor
16. 
17. // set base salary
18. **void** BasePlusCommissionEmployee::setBaseSalary( **double** salary ) 
19. \{ 
20.  baseSalary = ( salary < 0.0 ) ? 0.0 : salary; 
21. \} // end function setBaseSalary
22. 
23. // return base salary
24. **double** BasePlusCommissionEmployee::getBaseSalary() **const**
25. \{ 
26. **return** baseSalary; 
27. \} // end function getBaseSalary
28. 
29. // calculate earnings;
30. // override pure virtual function earnings in Employee
31. **double** BasePlusCommissionEmployee::earnings() **const**
32. \{ 
33. **return** getBaseSalary() + CommissionEmployee::earnings(); 
34. \} // end function earnings
35. 
36. // print BasePlusCommissionEmployee's information 
37. **void** BasePlusCommissionEmployee::print() **const**
38. \{ 
39.  cout << "base-salaried "; 
40.  CommissionEmployee::print(); // code reuse
41.  cout << "; base salary: " << getBaseSalary(); 
42. \} // end function print

## 演示多态性的执行过程 ##

为了测试Employee类层次结构,如下所示程序中的程序为4个具体类 SalariedEmployee, HourlyEmployee, CommissionEmployee和BasePlusCommissionEmployee的每一个都创建了一个对象.程序首先使用静态绑定方式对这些对象进行了操作,然后使用Employee指针的vector多态地对这些对象进行操作.

### Employee类层次结构的驱动程序 ###

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1.  // Demonstrating downcasting and run-time type information.
2.  // NOTE: For this example to run in Visual C++ .NET, 
3.  // you need to enable RTTI (Run-Time Type Info) for the project.
4.  \#include <iostream>
5.  **using** std::cout; 
6.  **using** std::endl; 
7.  **using** std::fixed; 
8.  
9.  \#include <iomanip>
10. **using** std::setprecision; 
11. 
12. \#include <vector>
13. **using** std::vector; 
14. 
15. \#include <typeinfo>
16. 
17. // include definitions of classes in Employee hierarchy
18. \#include "Employee.h"
19. \#include "SalariedEmployee.h" 
20. \#include "HourlyEmployee.h"
21. \#include "CommissionEmployee.h" 
22. \#include "BasePlusCommissionEmployee.h" 
23. 
24. **int** main() 
25. \{ 
26. // set floating-point output formatting
27.  cout << fixed << setprecision( 2 ); 
28. 
29. // create vector of four base-class pointers
30.  vector < Employee \* > employees( 4 ); 
31. 
32. // initialize vector with various kinds of Employees
33.  employees\[ 0 \] = **new** SalariedEmployee( 
34. "John", "Smith", "111-11-1111", 800 ); 
35.  employees\[ 1 \] = **new** HourlyEmployee( 
36. "Karen", "Price", "222-22-2222", 16.75, 40 ); 
37.  employees\[ 2 \] = **new** CommissionEmployee( 
38. "Sue", "Jones", "333-33-3333", 10000, .06 ); 
39.  employees\[ 3 \] = **new** BasePlusCommissionEmployee( 
40. "Bob", "Lewis", "444-44-4444", 5000, .04, 300 ); 
41. 
42. // polymorphically process each element in vector employees
43. **for** ( **size\_t** i = 0; i < employees.size(); i++ ) 
44.  \{ 
45.  employees\[ i \]->print(); // output employee information
46.  cout << endl; 
47. 
48. // downcast pointer
49.  BasePlusCommissionEmployee \*derivedPtr = 
50. **dynamic\_cast** < BasePlusCommissionEmployee \* > 
51.  ( employees\[ i \] ); 
52. 
53. // determine whether element points to base-salaried 
54. // commission employee
55. **if** ( derivedPtr != 0 ) // 0 if not a BasePlusCommissionEmployee
56.  \{ 
57. **double** oldBaseSalary = derivedPtr->getBaseSalary(); 
58.  cout << "old base salary: $" << oldBaseSalary << endl; 
59.  derivedPtr->setBaseSalary( 1.10 \* oldBaseSalary ); 
60.  cout << "new base salary with 10% increase is: $"
61.  << derivedPtr->getBaseSalary() << endl; 
62.  \} // end if
63. 
64.  cout << "earned $" << employees\[ i \]->earnings() << "\\n\\n"; 
65.  \} // end for 
66. 
67. // release objects pointed to by vector抯 elements
68. **for** ( **size\_t** j = 0; j < employees.size(); j++ ) 
69.  \{ 
70. // output class name
71.  cout << "deleting object of "
72.  << **typeid**( \*employees\[ j \] ).name() << endl; 
73. 
74. **delete** employees\[ j \]; 
75.  \} // end for
76. 
77. **return** 0; 
78. \} // end main

### 输出结果: ###

![Image 1][]

[Link 1]: http://www.csdn.net/tag/%e7%a8%8b%e5%ba%8f%e5%ae%9e%e8%b7%b5
[Link 2]: http://www.csdn.net/tag/%e5%a4%9a%e6%80%81%e6%80%a7
[Link 3]: http://www.csdn.net/tag/%e5%b7%a5%e8%b5%84%e5%8f%91%e6%94%be%e7%ae%a1%e7%90%86%e7%b3%bb%e7%bb%9f
[Link 4]: http://blog.csdn.net/songzitea/article/details/42298567#
[Image 1]: 
[CODE]: https://code.csdn.net/assets/CODE_ico.png
[CODE_CODE]: https://code.csdn.net/snippets/569204
[ico_fork.svg]: https://code.csdn.net/assets/ico_fork.svg
[ico_fork.svg 1]: https://code.csdn.net/snippets/569204/fork