sgdb List has-a

This page is a follow-up of solution discussed in the forward list solution page where studentNode is implemented with inheritance (the is-a relationship). Here, we will implement the studentNode with composition (i.e. has-a relationship) It is highly recommended that you get to know the other page first.

The student (aka studentGrade in code) class remains unchanged. We just add studentNode class as follows:

class studentGrade

{

friend ostream& operator<< (ostream&, const studentGrade&);

friend istream& operator>> (istream&, studentGrade&);

public:

string name;

int grade;

};

class studentNode

{

studentGrade sg;

studentNode *next; // link to next

};

Building the link list from file now use dynamic allocations.

bool studentGradeDB::load(string fn)

{

ifstream fin(fn.c_str());

if (fin.fail()) return false;

while(!fin.eof()){

studentNode *sgp = new studentNode;

// fin >> *sgp; this is replaced because it is composition now

fin >> (sgp->sg);

sgp->next = head; // add it to the beginning of linklist

head = sgp;

}

fin.close();

return true;

}

Note that because it is NOT inheritance any more, studentNode is NOT a student. studentGrade operators will not work with studentNode. That's why we can NOT directly use fin >> *sgp; But, since studentNode has a studentGrade member, we can use fin >> (sgp->sg); to read in a studentGrade object directly.