abstract类中可以有private的成员_Miles的C++学习笔记#5.2 受保护的成员和类访问

╰+哭是因爲堅強的太久メ 2023-01-10 09:55 27阅读 0赞

**概念：基类的受保护成员就像私有成员一样，但是可以通过派生类访问它们。基类访问规范确定派生类继承私有，公共和受保护的基类成员时如何访问它们**

到现在为止，我们在类中有两个关键的标示符：private和public。实际上，在C++中还提供了第三个访问限制符叫protected。这个protected的成员基类就和之前谈到的private成员一样，只不过private成员只能通过函数来在一个派生类中访问，但是对于protected来说，其成员是能且只能在当前scoop中进行访问，一旦离开了这个scoop就没有其它任何方法来访问其中的成员。看下面这个GradedActivity.h

#ifndef GRADEDACTIVITY_H
    #define GRADEDACTIVITY_H
    
    // GradedActivity class declaration
    
    class GradedActivity
    {
         
    protected:
       double score; // To hold the numeric score
    public:
       // Default constructor
       GradedActivity()
       {
         
          score = 0.0;
       }
    
       // Constructor
       GradedActivity(double s)
       {
         
          score = s;
       }
    
       // Mutator function
       void setScore(double s)
       {
         
          score = s;
       }
    
       // Accessor functions
       double getScore() const
       {
         
          return score;
       }
    
       char getLetterGrade() const;
    };
    #endif

有点头绪了? 好，我们来看这个FinalExam的头文件

#ifndef FINALEXAM_H
    #define FINALEXAM_H
    #include "GradedActivity.h"
    
    class FinalExam : public GradedActivity
    {
         
    private:
       int numQuestions;  // Number of questions
       double pointsEach; // Points for each question
       int numMissed;     // Number of questions missed
    public:
       // Default constructor
       FinalExam()
       {
         
          numQuestions = 0;
          pointsEach = 0.0;
          numMissed = 0;
       }
    
       // Constructor
       FinalExam(int questions, int missed)
       {
         
          set(questions, missed);
       }
    
       // Mutator functions
       void set(int, int); // Defined in FinalExam.cpp
       void adjustScore(); // Defined in FinalExam.cpp
    
       // Accessor functions
       double getNumQuestions() const
       {
         
          return numQuestions;
       }
    
       double getPointsEach() const
       {
         
          return pointsEach;
       }
    
       int getNumMissed() const
       {
         
          return numMissed;
       }
    };
    #endif

这个头文件实际上是从上面那个叫GradedActivity.h的头文件中派生而来的。这个头文件中有一个全新的成员函数叫做adjustScore，这个函数直接访问GradedActivity类的成员变量。如果score变量里面的内容有大于0.5的小数部分，则该函数将舍入为下一个整数。然后在程序计算完成数字分数以后，set函数就会调用adjustScore函数。下面是主程序

#include "FinalExam.h"
    
    //********************************************************
    // set function                                          *
    // The parameters are the number of questions and the    *
    // number of questions missed.                           *
    //********************************************************
    
    void FinalExam::set(int questions, int missed)
    {
         
       double numericScore; // To hold the numeric score
    
       // Set the number of questions and number missed.
       numQuestions = questions;
       numMissed = missed;
    
       // Calculate the points for each question.
       pointsEach = 100.0 / numQuestions;
    
       // Calculate the numeric score for this exam.
       numericScore = 100.0 - (missed * pointsEach);
    
       // Call the inherited setScore function to set
       // the numeric score.
       setScore(numericScore);
    
       // Call the adjustScore function to adjust
       // the score.
       adjustScore();
    }
    
    //*****************************************************************
    // Definition of Test::adjustScore. If score is within 0.5 points *
    // of the next whole point, it rounds the score up and            *
    // recalculates the letter grade.                                 *
    //*****************************************************************
    
    void FinalExam::adjustScore()
    {
         
       double fraction = score - static_cast<int>(score);
    
       if (fraction >= 0.5)
       {
         
          // Adjust the score variable in the GradedActivity class.
          score += (1.0 - fraction);
       }
    }

然后下面这个程序展示了一个基础的带有protected成员的类

// This program demonstrates a base class with a
    // protected member.
    #include <iostream>
    #include <iomanip>
    #include "FinalExam.h"
    using namespace std;
    
    int main()
    {
         
         int questions; // Number of questions on the exam
         int missed;    // Number of questions missed by the student
    
         // Get the number of questions on the final exam.
         cout << "How many questions are on the final exam? ";
         cin >> questions;
    
         // Get the number of questions the student missed.
         cout << "How many questions did the student miss? ";
         cin >> missed;
    
         // Define a FinalExam object and initialize it with
         // the values entered.
         FinalExam test(questions, missed);
    
         // Display the adjusted test results.
         cout << setprecision(2) << fixed;
         cout << "nEach question counts "
              << test.getPointsEach() << " points.n";
         cout << "The adjusted exam score is "
              << test.getScore() << endl;
         cout << "The exam grade is "
              << test.getLetterGrade() << endl;
    
         return 0;
    }

下面这个表格详细讲述了private, public和protected这三个访问限制符的用法

![ccb96c70dd974df24fc8871f5c827c66.png][]

![629e780ce0393b21905bfc616d2f65e5.png][]

[ccb96c70dd974df24fc8871f5c827c66.png]: /images/20221119/b35d7a47823d446cb3c6b688c40b8d5a.png
[629e780ce0393b21905bfc616d2f65e5.png]: /images/20221119/e977842dc61d4e15b54652c2e79b43f1.png