• Ronald Weidner <xecronix yahoo.com> Dec 19 2008

Ronald Weidner <xecronix yahoo.com> writes:

#include <iostream>
using namespace std;

class Mother
{
        private:
        int iAge;

        public:
        Mother ()
        {
                cout << "Mother Constructor: no parameters\n";
                iAge = 0;
        }

        Mother (int a)
        {
                cout << "Mother Constructor: int parameter\n";
                iAge = a;
        }

        void PrintAge()
        {
                cout << "Age: " << iAge << "\n";
        }

        void SetAge(int a)
        {
                iAge = a;
        }

        int GetAge()
        {
                return iAge;
        }
};

// Here we will create a Daughter class that will inherit from Mother
class Daughter : public Mother
{
        public:
        Daughter (int a)
        {
                cout << "Daughter Constructor: int parameter\n";
                // The Lesson to be Learned Here...
                // We need to call SetAge() because in C++  constructors
                // are not inherited.  So... just because there is a
                // Mother(int a ) constructor doesn't mean that the
                // child class constructor will call it even though
                // calling parameters match.  See the "Son" class to see
                // how map constructors so that the desired one is called.

                SetAge(a);

                // This PrintAge is defined in this class and over rides
                // the PrintAge defined in Mother.
                PrintAge();
        }

        // Over ride PrintAge defined in Mother.
        void PrintAge()
        {
                // Here is the C++ equivalent to java's super()
                // The main distinction here is that this did not
                // have to be the first line of the method.  We can,
                // in C++ execute other commands before mimicking super()
                Mother::PrintAge();
                cout << "\n";
        }
};


// Here we will create a Son class that will also inherit from Mother
class Son : public Mother
{
        public:
        // Here we are "mapping" the Mother constructor to the Son
        // constructor.  What will happen here is Son(int a) constructor
        // will be called.  Immediately before execution of the Son(int a)
        // construtor, the Mother(int a) code will be executed,
        // Then the Son(int a) code will execute.
        Son (int a) : Mother(a)
        {
                cout << "Son Constructor: int parameter\n";
                PrintAge();
        }

        // Comment out this constructor to see that Son will not inherit from
        // Mother the constructor with the empty params ().  If a constructor
        // is defined, there will be no more "free" constructors. (Inherited or
        // otherwise.)
        ///*
        Son() : Mother()
        {
                cout << "Son Constructor: No Parameters\n";
        }
        //*/

        void PrintAge()
        {
                cout << "The Son's age is: " << GetAge() << "\n\n";
        }
};

int main ()
{
        Daughter katie(15);
        Son bobby(12);

        // If we comment out the Son() constructor in the Son class, we will
see that
        // we will not inherit the Mother().  That's because once we define a
constructor
        // in a class, (Son(int a) in this case.) we will no longer get a
default
        // constructor, or inherit parent constructors.

        // Additionally, we cannot instantiate this class as follows...
        //
        // Son ronnie();
        //
        // Well, technically we could but, this may lead to compile time
errors that could
        // potentially be hard to track down.  If Son() is not defined, a
compile time
        // error should immediately occur.  But it won't.  Instead the compile
error won't
        // occur until you try to use the object.  (Which could be many lines
of code later.)
        //
        // So, as a best practice let's not use Son ronnie() to represent the
empty param
        // constructor.  Instead we should use Son ronnie .  The error doesn't
seem to
        // manifest when using "new" to create the object under the same
conditions.  Instead,
        // a compile time exception is thrown as it should given the
rules/conditions stated
        // above.

        Son ronnie;
        ronnie.SetAge(18);
        ronnie.PrintAge();
        return 0;
}


// Program Output...
// **************
//
// Mother Constructor: no parameters
// Daughter Constructor: int parameter
// Age: 15
//
// Mother Constructor: int parameter
// Son Constructor: int parameter
// The Son's age is: 12
//
// Mother Constructor: no parameters
// Son Constructor: No Parameters
// The Son's age is: 18

// ****************

// --
// Ronald Weidner