A New Version of the Student Class Example
To get private inheritance, use the keyword private instead of public when defining the class. (Actually, private is the default, so omitting an access qualifier also leads to private inheritance.) The Student class should inherit from two classes, so the declaration should list both:
class Student : private std::string, private std::valarray<double>
{
public:
...
};
Having more than one base class is called multiple inheritance (MI). In general, MI, particularly public MI, can lead to problems that have to be resolved with additional syntax rules. We’ll talk about such matters later in this chapter. But in this particular case, MI causes no problems.
Note that the new class doesn’t need private data. That’s because the two inherited base classes already provide all the needed data members. The containment version of this example provides two explicitly named objects as members. Private inheritance, however, provides two nameless subobjects as inherited members. This is the first of the main differences in the two approaches.
Initializing Base-Class Components
Having implicitly inherited components instead of member objects affects the coding of this example because you can no longer use name and scores to describe the objects. Instead, you have to go back to the techniques you used for public inheritance. For example, consider constructors. Containment uses this constructor:
Student(const char * str, const double * pd, int n)
: name(str), scores(pd, n) {} // use object names for containment
The new version should use the member initializer list syntax for inherited classes, which uses the class name instead of a member name to identify a constructor:
Student(const char * str, const double * pd, int n)
: std::string(str), ArrayDb(pd, n) {} // use class names for inheritance
Here, as in the preceding example, ArrayDb is a typedef for std::valarray<double>. Be sure to note that the member initializer list uses terms such as std::string(str) instead of name(str). This is the second main difference in the two approaches
Listing 14.4 shows the new class declaration. The only changes are the omission of explicit object names and the use of class names instead of member names in the inline constructors.
// studenti.h -- defining a Student class using private inheritance
#ifndef STUDENTC_H_
#define STUDENTC_H_
#include <iostream>
#include <valarray>
#include <string>
class Student : private std::string, private std::valarray<double>
{
private:
typedef std::valarray<double> ArrayDb;
// private method for scores output
std::ostream & arr_out(std::ostream & os) const;
public:
Student() : std::string("Null Student"), ArrayDb() {}
explicit Student(const std::string & s)
: std::string(s), ArrayDb() {}
explicit Student(int n) : std::string("Nully"), ArrayDb(n) {}
Student(const std::string & s, int n)
: std::string(s), ArrayDb(n) {}
Student(const std::string & s, const ArrayDb & a)
: std::string(s), ArrayDb(a) {}
Student(const char * str, const double * pd, int n)
: std::string(str), ArrayDb(pd, n) {}
~Student() {}
double Average() const;
double & operator[](int i);
double operator[](int i) const;
const std::string & Name() const;
// friends
// input
friend std::istream & operator>>(std::istream & is,
Student & stu); // 1 word
friend std::istream & getline(std::istream & is,
Student & stu); // 1 line
// output
friend std::ostream & operator<<(std::ostream & os,
const Student & stu);
};
#endif
Accessing Base-Class Methods
Private inheritance limits the use of base-class methods to within derived-class methods. Sometimes, however, you might like to make a base-class facility available publicly. For example, the Student class declaration suggests the ability to use an Average() function. As with containment, the technique for doing this is to use the valarray size() and sum() methods within a public Student::average() function (see Figure 14.2). Containment invoked the methods with an object:
double Student::Average() const
{
if (scores.size() > 0)
return scores.sum()/scores.size();
else
return 0;
}
Figure 14.2. Objects within objects: private inheritance.
Here, however, inheritance lets you use the class name and the scope-resolution operator to invoke base-class methods:
double Student::Average() const
{
if (ArrayDb::size() > 0)
return ArrayDb::sum()/ArrayDb::size();
else
return 0;
}
In short, the containment approach uses object names to invoke a method, whereas private inheritance uses the class name and the scope-resolution operator instead.
Accessing Base-Class Objects
The scope-resolution operator allows you access to a base-class method, but what if you need the base-class object itself? For example, the containment version of the Student class implements the Name() method by having the method return the name member string object. But with private inheritance, the string object has no name. How, then, can Student code access the inner string object?
The answer is to use a type cast. Because Student is derived from string, it’s possible to type cast a Student object to a string object; the result is the inherited string object. Recall that the this pointer points to the invoking object, so *this is the invoking object—in this case, a type Student object. To avoid invoking constructors to create new objects, you use the type cast to create a reference:
const string & Student::Name() const
{
return (const string &) *this;
}
This code returns a reference to the inherited string object residing in the invoking Student object.
Accessing Base-Class Friends
The technique of explicitly qualifying a function name with its class name doesn’t work for friend functions because a friend function doesn’t belong to a class. However, you can use an explicit type cast to the base class to invoke the correct functions. This is basically the same technique used to access a base-class object in a class method, but with friends you have a name for the Student object, so the code uses the name instead of *this. For example, consider the following friend function definition:
ostream & operator<<(ostream & os, const Student & stu)
{
os << "Scores for " << (const String &) stu << ":
";
...
}
If plato is a Student object, then the following statement invokes that function, with stu being a reference to plato and os being a reference to cout:
cout << plato;
Consider the following line of code:
os << "Scores for " << (const String &) stu << ": ";
The typecast explicitly converts stu to a reference to a type string object; that type, in turn, invokes the operator<<(ostream &, const String &) function.
The reference stu doesn’t get converted automatically to a string reference. The fundamental reason is that with private inheritance, a reference or pointer to a base class cannot be assigned a reference or pointer to a derived class without an explicit type cast.
However, even if the example used public inheritance, it would have to use explicit type casts. One reason is that without a type cast, code like the following matches the friend function prototype, leading to a recursive call:
os << stu;
A second reason is that because the class uses MI, the compiler can’t tell which base class to convert to if both base classes happen to provide an operator<<() function. Listing 14.5 shows all the Student class methods, other than those defined inline in the class declaration.
// studenti.cpp -- Student class using private inheritance
#include "studenti.h"
using std::ostream;
using std::endl;
using std::istream;
using std::string;
// public methods
double Student::Average() const
{
if (ArrayDb::size() > 0)
return ArrayDb::sum()/ArrayDb::size();
else
return 0;
}
const string & Student::Name() const
{
return (const string &) *this;
}
double & Student::operator[](int i)
{
return ArrayDb::operator[](i); // use ArrayDb::operator[]()
}
double Student::operator[](int i) const
{
return ArrayDb::operator[](i);
}
// private method
ostream & Student::arr_out(ostream & os) const
{
int i;
int lim = ArrayDb::size();
if (lim > 0)
{
for (i = 0; i < lim; i++)
{
os << ArrayDb::operator[](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 >> (string &)stu;
return is;
}
// use string friend getline(ostream &, const string &)
istream & getline(istream & is, Student & stu)
{
getline(is, (string &)stu);
return is;
}
// use string version of operator<<()
ostream & operator<<(ostream & os, const Student & stu)
{
os << "Scores for " << (const string &) stu << ":
";
stu.arr_out(os); // use private method for scores
return os;
}
Again, because the example reuses the string and valarray code, relatively little new code is needed, aside from the private helper method.
Using the Revised Student Class
Once again, it’s time to test a new class. Note that the two versions of the Student class have exactly the same public interface, so you can test the two versions with exactly the same program. The only difference is that you have to include studenti.h instead of studentc.h, and you have to link the program with studenti.cpp instead of with studentc.cpp. Listing 14.6 shows the program. Be sure to compile it along with studenti.cpp.
// use_stui.cpp -- using a class with private inheritance
// compile with studenti.cpp
#include <iostream>
#include "studenti.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 << "
Student List:
";
for (i = 0; i < pupils; ++i)
cout << ada[i].Name() << endl;
cout << "
Results:";
for (i = 0; i < pupils; i++)
{
cout << endl << ada[i];
cout << "average: " << ada[i].Average() << endl;
}
cout << "Done.
";
return 0;
}
void set(Student & sa, int n)
{
cout << "Please enter the student's name: ";
getline(cin, sa);
cout << "Please enter " << n << " quiz scores:
";
for (int i = 0; i < n; i++)
cin >> sa[i];
while (cin.get() != '
')
continue;
}
Here is a sample run of the program in Listing 14.6:
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.
The same input as before leads to the same output that the containment version produces.