我们将类定义和使用此类的客户代码分离,增强了软件的可复用性.而接口定义并标准化了人和系统等诸如此类事物彼此交互的方式.每个类定义包含了类的公有成员函数的完整定义及其私有数据成员声明.可是更好的软件工程是在类定义的外部定义成员函数,这样这些成员函数的实现细节对客户代码而言隐藏的,这种方式保证程序员不会写出依赖于类的实现细节的客户代码.否则,若类的实现更改,则客户代码将更可能"遭到损坏".

GradeBook.h:使用函数原型定义类接口

// GradeBook.h GradeBook class definition. This file presents GradeBook‘s public 
// interface without revealing the implementations of GradeBook‘s member
// functions, which are defined in GradeBook.cpp.
#include <string> // class GradeBook uses C++ standard string class
using std::string;

// GradeBook class definition
class GradeBook
{
public:
   GradeBook( string ); // constructor that initializes courseName
   void setCourseName( string ); // function that sets the course name
   string getCourseName(); // function that gets the course name
   void displayMessage(); // function that displays a welcome message
private:
   string courseName; // course name for this GradeBook
}; // end class GradeBook  

GradeBook.cpp:在独立的源代码文件中定义成员函数

源代码GradeBook.cpp定义了GradeBook类的成员函数,这些函数声明位于GradeBook.h.

// GradeBook.cpp GradeBook member-function definitions. This file contains
// implementations of the member functions prototyped in GradeBook.h.
#include <iostream>
using std::cout; 
using std::endl;

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

// constructor initializes courseName with string supplied as argument
GradeBook::GradeBook( string name )
{
   setCourseName( name ); // call set function to initialize courseName
} // end GradeBook constructor

// function to set the course name
void GradeBook::setCourseName( string name )
{
   courseName = name; // store the course name in the object
} // end function setCourseName

// function to get the course name
string GradeBook::getCourseName()
{
   return courseName; // return object‘s courseName
} // end function getCourseName

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

测试GradeBook类

// GradeBook class demonstration after separating its interface from its implementation.
#include <iostream>
using std::cout; 
using std::endl;

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

// function main begins program execution
int main()
{
   // create two GradeBook objects
   GradeBook gradeBook1( "CS101 Introduction to C++ Programming" );
   GradeBook gradeBook2( "CS102 Data Structures in C++" );

   // display initial value of courseName for each GradeBook
   cout << "gradeBook1 created for course: " << gradeBook1.getCourseName()
      << "\ngradeBook2 created for course: " << gradeBook2.getCourseName() 
      << endl;
   return 0; // indicate successful termination
} // end main

编译和连接过程

下图所示是显示了生成可供教师使用的,可行的GradeBook应用程序的编译和连接过程.通常由一个程序员创建和编译类的接口和实现,而由不同的实现类客户代码的程序员使用它们.因此这个示意图显示了类实现程序员和客户代码程序员需要做的部分.图中虚线划分了类实现程序员,客户代码程序员和GradeBook应用程序用户各自需要做的部分.

负责创建可复用GradeBook类的类实现程序员首先创建两个文件,一个是头文件GradeBook.h,另一个是包含(#include)头文件的源代码文件GradeBook.cpp.然后,编译源代码文件,创建GradeBook对象的目标代码.

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程序实践:接口与实现分离