13: Making Classes Work in Visual Studio .NET

Drawer PDF417 in Visual Studio .NET 13: Making Classes Work

A member function is a lot like a member of a family The full name of the function addCourse(int, float) is Student::addCourse(int, float), just as my full name is Stephen Davis The short name of the function is addCourse(int, float), just as my short name is Stephen The class name at the beginning of the full name indicates that the function is a member of the class Student (The :: between the class name and the function name is simply a separator) The name Davis on the end of my name indicates that I am a member of the Davis family Another name for a full name is extended name You can define an addCourse(int, float) function that has nothing to do with Student there are Stephens out there who have nothing to do with my family (I mean this literally: I know several Stephens who want nothing to do with my family) You could have a function Teacher::addCourse(int, float) or even Golf::addCourse() A function addCourse(int, float) without a class name is just a plain ol conventional non-member function The extended name for the non-member function is ::addCourse(int, float) (Note the colon without a family name in front)

Before you look at how to call a member function, remember how to access a data member:

class Student { public: int semesterHours; float gpa; }; Student s; void fn(void) { // access data members of s ssemesterHours = 10; sgpa = 30; }

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Notice that you have to specify an object along with the member name In other words, the following makes no sense:

Student s; void fn(void) { // neither of these is legal semesterHours = 10; // member of what object of what // class Student::semesterHours = 10; // okay, I know the class // but I still don t know // the object }

Remember that member functions function like data members functionally The following CallMemberFunction shows how to invoke the member func tion addCourse():

// // CallMemberFunction - define and invoke a function that s // a member of the class Student // #include <cstdio> #include <cstdlib> #include <iostream> using namespace std; class Student { public: // add a completed course to the record float addCourse(int hours, float grade) { // calculate the sum of all courses times // the average grade float weightedGPA; weightedGPA = semesterHours * gpa; // now add in the new course semesterHours += hours; weightedGPA += grade * hours; gpa = weightedGPA / semesterHours; // return the new gpa return gpa;

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} int semesterHours; float gpa; }; int main(int nNumberofArgs, char* pszArgs[]) { Student s; ssemesterHours = 10; sgpa = 30; // the values before the call cout << Before: s = ( << ssemesterHours << , << s gpa << endl; // the following subjects the data members of the s // object to the member function addCourse() saddCourse(3, 40); // call the member function // the values are now changed cout << After: s = ( << ssemesterHours << , << s gpa << ) << endl; // access another object just for the heck of it Student t; tsemesterHours = 6; tgpa = 10; // not doing so good taddCourse(3, 15); // things aren t getting any better // wait until user is ready before terminating program // to allow the user to see the program results system( PAUSE ); return 0; }

The syntax for calling a member function looks like a cross between the syntax for accessing a data member and that used for calling a function The right side of the dot looks like a conventional function call, but an object is on the left of the dot We say that addCourse() operates on the object s or, said another way, s is the student to which the course is to be added You can t fetch the number of semester hours without knowing from which student you can t add a stu dent to a course without knowing which student to add Calling a member func tion without an object makes no more sense than referencing a data member without an object

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