Part III: Introduction to Classes in Visual Studio .NET

Making PDF417 in Visual Studio .NET Part III: Introduction to Classes

A class member can also be protected by declaring it private In this book, I use the protected keyword exclusively since it expresses the more generic concept

Now that you know a little more about how to use protected members in an actual class, I can replay the arguments for using protected members

Making the gpa member protected precludes the application from setting the grade point average to some arbitrary value The application can add courses, but it can t change the grade point average If the application has a legitimate need to set the grade point average directly, the class can provide a member function for that purpose, as follows:

class Student { public: // same as before float grade() { return gpa; } // here we allow the grade to be changed float grade(float newGPA) { float oldGPA = gpa; // only if the new value is valid if (newGPA > 0 && newGPA <= 40) { gpa = newGPA; } return oldGPA; } // other stuff is the same including the data members: protected: int semesterHours; // hours earned toward graduation float gpa; };

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The addition of the member function grade(float) allows the application to set the gpa Notice, however, that the class still hasn t given up control completely The application can t set gpa to any old value; only a gpa in the legal range of values (from 0 through 40) is accepted Thus, Student class has provided access to an internal data member without abdicating its responsibility to make sure that the internal state of the class is valid

A class provides a limited interface To use a class, all you need (or want) to know are its public members, what they do, and what their arguments are This can drastically reduce the number of things you need to master and remember to use the class As conditions change or as bugs are found, you want to be able to change the internal workings of a class Changes to those details are less likely to require changes in the external application code if you can hide the internal workings of the class A second, perhaps more important reason, lies in the limited ability of humans (I can t speak for dogs and cats) to keep a large number of things in their minds at any given instant Using a strictly defined class interface allows the pro grammer to forget the details that go on behind it Likewise, a programmer building the class need not concentrate to quite the same degree on exactly how each of the functions is being used

Occasionally, you want a non-member function to have access to the pro tected members of a class You do so by declaring the function to be a friend of the class by using the keyword friend Sometimes, an external function can use direct access to a data member I know this appears to break the strictly defined, well-sealed-off class interface position that I ve been advocating, but just consider the following First, including a friend function is, in effect, adding that function to the interface (that s why a class shouldn t have too many friends) You re okay as long as you attempt to treat this function as a normal function that, oh yeah, happens to have direct access Second, providing a public access method that acts as

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a thin veil over a data member doesn t do anything to abstract away class details Such a thin veneer function fulfills the letter of the law, but not the spirit The friend declaration appears in the class that contains the protected member The friend declaration is like a prototype declaration in that it includes the extended name and the return type In the following example, the function initialize() can now access anything it wants in Student:

class Student { friend void initialize(Student*); public: // same public members as before protected: int semesterHours; // hours earned toward graduation float gpa; }; // the following function is a friend of Student // so it can access the protected members void initialize(Student *pS) { pS->gpa = 0; // this is now legal pS->semesterHours = 0; // when it wasn t before }

A single function can be declared a friend of two classes at the same time Although this can be convenient, it tends to bind the two classes together This binding of classes is normally considered bad because it makes one class dependent on the other If the two classes naturally belong together, however, it s not all bad, as shown here:

class Student; // forward declaration class Teacher { friend void registration(Teacher& t, Student& s); public: void assignGrades(); protected: int noStudents; Student *pList[100]; }; class Student { friend void registration(Teacher& t, Student& s); public: // same public members as before protected: Teacher *pT; int semesterHours; // hours earned toward graduation float gpa; };

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