类和对象-3

第3个版本用浮点数计算平均值，并且添加了小数位固定和强制类型转换

3个文件

GradeBook.h

// GradeBook.h
// Definition of class GradeBook that determines a class average.
// Member functions are defined in GradeBook.cpp

#ifndef GRADEBOOK_H
#define GRADEBOOK_H

#include<string>    // program uses C++ standard string class
using std::string;

// GradeBook class definition
class GradeBook
{
    public:
        GradeBook( string );    // constructor initialize course name
        void setCourseName( string );   // function to set the course name
        string getCourseName(); // function to retrieve the course name
        void displayMessage();  // display a welcome message
        void determineClassAverage();   // averages grades entered by the user

    private:
        string courseName;  // course name for this GradeBook
};  // end class GradeBook

#endif // GRADEBOOK_H

GradeBook.cpp

// GradeBook.cpp
// Member-function definitions for class GradeBook that solves the
// class average program with sentinel-controlled repetition
#include<iostream>
using std::cout;
using std::cin;
using std::endl;
using std::fixed;   // ensures that decimal point is displayed

#include<iomanip>   // parameterized stream manipulators
using std::setprecision;    // sets numeric output precision

// include definition of class GradeBook from GradeBook.h
#include "GradeBook.h"

// constructor initialize course name with string supplied as argument
GradeBook::GradeBook( string name )
{
    setCourseName( name );  // validate and store courseName
}   // end GradeBook constructor

// function to set the course name
// ensures that the course name has at most 25 characters
void GradeBook::setCourseName( string name )
{
    if( name.length() <= 25 )   // if name has 25 or fewer characters
        courseName = name;
    if( name.length() > 25 )    // if name has more than 25 characters
    {
        courseName = name.substr( 0, 25 );  // select first 25 characters

        cout << "Name "" << name << "" exceeds maximum length(25).
"
        << "Limiting courseName to first 25 characters.
" << endl;
    }   // end if...else
}   // end function setCourseName

// function to retrieve the course name
string GradeBook::getCourseName()
{
    return courseName;
}   // end function getCourseName

// display a welcome message to the GradeBook user
void GradeBook::displayMessage()
{
    cout << "Welcome to the grade book for
" << getCourseName()
    << "!
" << endl;
}   // end function displayMessage

// determine class average of grades entered by the user
void GradeBook::determineClassAverage()
{
    int total;  // sum of the grades entered by user
    int gradeCounter;   // number of grades entered
    int grade;  // grade value
    double average; // number with decimal point for average

    // initialization phase
    total = 0;  // initialize total
    gradeCounter = 0;   // initialize loop counter

    // processing phase
    // prompt for input and read grade from user
    cout << "Enter grade or -1 to quit:";
    cin >> grade;   // input grade or sentinel value

    // loop until sentinel value read from user
    while( grade != -1 )
    {
        total += grade; // add grade to total
        gradeCounter++; // increment counter

        // prompt for input and read next grade from user
        cout << "Enter grade or -1 to quit:";
        cin >> grade;  // input grade or sentinel value
    }   // end while

    // termination phase
    if( gradeCounter != 0 ) // if user entered at least one grade
    {
        // calculate average of all grades entered
        average = static_cast< double >( total ) / gradeCounter;

        // display total and average ( with 2 digits of precision )
        cout << "
Total of all " << gradeCounter << " grades entered is "
        << total << endl;

        cout << "
Class average is " << setprecision( 2 ) << fixed << average
        << endl;
    }   // end if
    else    // no grades were entered, so output appropriate message
        cout << "No grades were entered" << endl;
}   // end function determineClassAverage

测试文件 main.cpp

// Create GradeBook object and invoke its determineClassAverage function

// include definition of class GradeBook from GradeBook.h
#include "GradeBook.h"

int main()
{
    // Create GradeBook object myGradeBook and
    // pass course name to constructor
    GradeBook myGradeBook( "CS101 C++ Programming!" );

    myGradeBook.displayMessage();   // display welcome message
    myGradeBook.determineClassAverage();    // find average of grades
    return 0;   // indicate successful termination
}   // end main

常见的编程错误

• 忽略了定义一个块的花括号对会导致一个逻辑错误
• 把浮点数当做准确值来用（比较两个浮点数是否相等），很可能导致错误的结果，浮点数在大多数计算机中只表示近似值
• 强制类型转换运算符可以用于基本数据类型之间的转换，如果进行非法的转换，可能产生变异错误或运行时错误

良好的编程习惯

• 在每次用户输入之前最好给出相应的提示，指出输入数据的形式和所有的特殊输入值（如标记值-1）