os << "Scores for " << stu.name << ":\n";
...
}
Because stu.name is a string object, it invokes the operator<<(ostream &, const string &) function, which is provided as part of the string class package. Note that the operator<<(ostream & os, const Student & stu) function has to be a friend to the Student class so that it can access the name member. (Alternatively, the function could use the public Name() method instead of the private name data member.)
Similarly, the function could use the valarray implementation of << for output; unfortunately, there is none. Therefore, the class defines a private helper method to handle this task:
// private method
ostream & Student::arr_out(ostream & os) const
{
int i;
int lim = scores.size();
if (lim > 0)
{
for (i = 0; i < lim; i++)
{
os << scores[i] << " ";
if (i % 5 == 4)
os << endl;
}
if (i % 5 != 0)
os << endl;
}
else
os << " empty array ";
return os;
}
Using a helper like this gathers the messy details together in one place and makes the coding of the friend function neater:
// use string version of operator<<()
ostream & operator<<(ostream & os, const Student & stu)
{
os << "Scores for " << stu.name << ":\n";
stu.arr_out(os); // use private method for scores
return os;
}
The helper function could also act as a building block for other user-level output functions, should you choose to provide them.
Listing 14.2 shows the class methods file for the Student class. It includes methods that allow you to use the [] operator to access individual scores in a Student object.
Listing 14.2. studentc.cpp
// studentc.cpp -- Student class using containment
#include "studentc.h"
using std::ostream;
using std::endl;
using std::istream;
using std::string;
//public methods
double Student::Average() const
{
if (scores.size() > 0)
return scores.sum()/scores.size();
else
return 0;
}
const string & Student::Name() const
{
return name;
}
double & Student::operator[](int i)
{
return scores[i]; // use valarray
}
double Student::operator[](int i) const
{
return scores[i];
}
// private method
ostream & Student::arr_out(ostream & os) const
{
int i;
int lim = scores.size();
if (lim > 0)
{
for (i = 0; i < lim; i++)
{
os << scores[i] << " ";
if (i % 5 == 4)
os << endl;
}
if (i % 5 != 0)
os << endl;
}
else
os << " empty array ";
return os;
}
// friends
// use string version of operator>>()
istream & operator>>(istream & is, Student & stu)
{
is >> stu.name;
return is;
}
// use string friend getline(ostream &, const string &)
istream & getline(istream & is, Student & stu)
{
getline(is, stu.name);
return is;
}
// use string version of operator<<()
ostream & operator<<(ostream & os, const Student & stu)
{
os << "Scores for " << stu.name << ":\n";
stu.arr_out(os); // use private method for scores
return os;
}
Aside from the private helper method, Listing 14.2 doesn’t require much new code. Using containment allows you to take advantage of the code you or someone else has already written.
Using the New Student Class
Let’s put together a small program to test the new Student class. To keep things simple, it should use an array of just three Student objects, each holding five quiz scores. And it should use an unsophisticated input cycle that doesn’t verify input and that doesn’t let you cut the input process short. Listing 14.3 presents the test program. Be sure to compile it along with studentc.cpp.
Listing 14.3. use_stuc.cpp
// use_stuc.cpp -- using a composite class
// compile with studentc.cpp
#include
#include "studentc.h"
using std::cin;
using std::cout;
using std::endl;
void set(Student & sa, int n);
const int pupils = 3;
const int quizzes = 5;
int main()
{
Student ada[pupils] =
{Student(quizzes), Student(quizzes), Student(quizzes)};
int i;
for (i = 0; i < pupils; ++i)
set(ada[i], quizzes);
cout << "\nStudent List:\n";
for (i = 0; i < pupils; ++i)
cout << ada[i].Name() << endl;
cout << "\nResults:";
for (i = 0; i < pupils; ++i)
{
cout << endl << ada[i];
cout << "average: " << ada[i].Average() << endl;
}
cout << "Done.\n";
return 0;
}
void set(Student & sa, int n)
{
cout << "Please enter the student's name: ";
getline(cin, sa);
cout << "Please enter " << n << " quiz scores:\n";
for (int i = 0; i < n; i++)
cin >> sa[i];
while (cin.get() != '\n')
continue;
}
Here is a sample run of the program in Listings 14.1, 14.2, and 14.3:
Please enter the student's name: Gil Bayts
Please enter 5 quiz scores:
92 94 96 93 95
Please enter the student's name: Pat Roone
Please enter 5 quiz scores:
83 89 72 78 95
Please enter the student's name: Fleur O'Day
Please enter 5 quiz scores:
92 89 96 74 64
Student List:
Gil Bayts
Pat Roone
Fleur O'Day
Results:
Scores for Gil Bayts:
92 94 96 93 95
average: 94
Scores for Pat Roone:
83 89 72 78 95
average: 83.4
Scores for Fleur O'Day:
92 89 96 74 64
average: 83
Done.
Private Inheritance
