encapsulation

WHY BOTHER WITH ENCAPSULATION?
—————————————————————–
Encapsulation
protects data from accidental corruption, and constructors
guarantee proper initialization.  Both prevent errors that we are
very prone to make since we are thinking only about the internals
of the class when we are writing it.  Later, when we are actually using the class, we have no need to concern ourselves with the internal structure or operation, but can spend our energies using the class to solve the overall problem we are working on.  As you may guess, there is a lot more to learn about the use and
benefits of classes so we will dive right into some new topics.

The purpose of this chapter is to illustrate how to use some of the traditional aspects of C or C++ with classes and objects.  Pointers to an object as well as pointers within an object will be illustrated.  Arrays embedded within an object, and an array of objects will be illustrated.  Since objects are simply another C++ data construct, all of these things are possible and can be used if needed.

In order to have a systematic study, we will use the program
named BOXES1.CPP from the last chapter as a starting point and we will add a few new constructs to it for each example program.  You will recall that it was a very simple program with the class definition, the class implementation, and the main program all in
one file.  This was selected as a starting point because we will eventually make changes to all parts of the program and it will be convenient to have it all in a single file for illustrative purposes.  It must be kept in mind however that the proper way to use these constructs is to separate them into the three files as
was illustrated in BOX.H, BOX.CPP, and BOXES2.CPP in the last
chapter.  This allows the implementor of box to supply the user with only the interface, namely  BOX.H.  Not giving him the implementation file named BOX.CPP, is practicing the technique of information hiding.  As we have said many times, it seems silly to break up such a small program into three separate files, and it is sort of silly.  The last chapter of this tutorial will illustrate a program large enough to require dividing the program up into many separate files.

AN ARRAY OF OBJECTS
—————————————————————–
Examine the file named OBJARRAY.CPP for our    ==================
first example of an array of objects.  This       OBJARRAY.CPP
file is nearly identical to the file named     ==================
BOX1.CPP until we come to line 44 where an

array of 4 boxes are declared.  Recalling the operation
of the constructor you will remember that each of the four box
objects will be initialized to the values defined within the
constructor since each box will go through the constructor as
they are declared.  In order to declare an array of objects, a
constructor for that object must not require any parameters.  (We have not yet illustrated a constructor with initializing parameters, but we will in the next program.)  This is an efficiency consideration since it would probably be an error to initialize all elements of an array of objects to the same value.  We will see the results of executing the constructor when we compile and execute the file later.

Line 49 defines a for loop that begins with 1 instead of the
normal starting index for an array leaving the first object,
named group[0], to use the default values stored when the
constructor was called.  You will observe that sending a message to one of the objects uses the same construct as is used for any object.  The name of the array followed by its index in square brackets is used to send a message to one of the objects in the array.  This is illustrated in line 50 and the operation of that code should be clear to you.  The other method is called in the
output statement in lines 57 and 58 where the area of the four
boxes in the group array are listed on the monitor.

Another fine point should be mentioned.  The integer variable named index is declared in line 49 and is still available for use in line 56 since we have not yet left the enclosing block which begins in line 43 and extends to line 65.

DECLARATION AND DEFINITION OF A VARIABLE
—————————————————————–
An extra variable was included for illustration, the one named
extra_data in line seven.  Since the keyword static is used to modify this variable in line 7, it is an external variable and only one copy of this variable will ever exist.  All seven
objects of this class share a single copy of this variable which
is global to the objects defined in line 44.

The variable is actually only declared here which says it will
exist somewhere, but it is not defined.  A declaration says the variable will exist and gives it a name, but the definition
actually defines a place to store it somewhere in the computers
memory space.  By definition, a static variable can be declared in a class header but it cannot be defined there, so it is usually defined in the implementation file.  In this case it is defined in line 16 and can then be used throughout the class.

Figure 6-1 is a graphical representation of some of the
variables.  Note that the objects named large, group[0],
group[1], and group[2] are not shown but they also share the

variable named extra_data.  They are not shown in order to
simplify the picture and enhance the clarity.

Line 23 of the constructor sets the single global variable to 1
each time an object is declared.  Only one assignment is
necessary so the other six are actually wasted code.  To
illustrate that there is only one variable shared by all objects
of this class, the method to read its value also increments it.
Each time it is read in lines 60 through 64, it is incremented
and the result of the execution proves that there is only a
single variable shared by all objects of this class.  You will
also note that the method named get_extra() is defined within
the class declaration so it will be assembled into the final
program as inline code.

You will recall the 2 static variables we declared in lines 16
and 17 of DATE.H in chapter 5 of this tutorial.  We defined them
in lines 9 and 10 of DATE.CPP and overlooked a complete
explanation of what they did at that time.  The declaration and
definition of these variables should be considered a good example
of the proper place to put these constructs in your classes.

Be sure you understand this program and especially the static
variable, then compile and execute it to see if you get the same result as listed at the end of the program.

A STRING WITHIN AN OBJECT
—————————————————————–
Examine the program named OBJSTRNG.CPP for    ===================
our first example of an object with an           OBJSTRING.CPP
embedded string.  Actually, the object does   ===================
not have an embedded string, it has an
embedded pointer, but the two work so closely together that we
can study one and understand both.  You will notice that line 7
contains a pointer to a string named line_of_text.  The
constructor contains an input parameter which is a pointer to a
string which will be copied to the string named line_of_text
within the constructor.  We could have defined the variable
line_of_text as an actual array in the class, then used strcpy()
to copy the string into the object and everything would have
worked the same, but we will leave that as an exercise for you at
the end of this chapter.  It should be pointed out that we are
not limited to passing a single parameter to a constructor.  Any number of parameters can be passed, as will be illustrated later.

You will notice that when the three boxes are declared this time,
we supply a string constant as an actual parameter with each
declaration which is used by the constructor to assign the string
pointer some data to point to.  When we call get_area() in lines
48 through 53, we get the message displayed and the area
returned.  It would be prudent to put these operations in
separate methods since there is no apparent connection between

printing the message and calculating the area, but it was written
this way to illustrate that it can be done.  What this really
says is that it is possible to have a method that has a side
effect, the message output to the monitor, and a return value,
the area of the box.  However, as we discussed in chapter 4 when
we studied DEFAULT.CPP, the order of evaluation is sort of funny,
so we broke each line into two lines.

After you understand this program, compile and execute it.

AN OBJECT WITH AN INTERNAL POINTER
—————————————————————–
The program named OBJINTPT.CPP is our first    ==================
example program with an embedded pointer          OBJINTPT.CPP
which will be used for dynamic allocation of   ==================
data.  In line 7 we declare a pointer to an
integer variable, but it is only a pointer, there is no storage
associated with it.  The constructor therefore allocates an
integer type variable on the heap for use with this pointer in
line 21.  It should be clear to you that the three objects
created in line 45 each contain a pointer which points into the
heap to three different locations.  Each object has its own
dynamically allocated variable for its own private use.  Moreover
each has a value of 112 stored in its dynamically allocated data
because line 22 stores that value in each of the three locations,
once for each call to the constructor.

In such a small program, there is no chance that we will exhaust
the heap, so no test is made for unavailable memory.  In a real
production program, it would be expedient to test that the value
of the returned pointer is not NULL to assure that the data
actually did get allocated.

The method named set() has three parameters associated with it
and the third parameter is used to set the value of the new
dynamically allocated variable.  There are two messages passed,
one to the small box and one to the large box.  As before, the
medium box is left with its default values.

The three areas are displayed followed by the three stored values
in the dynamically allocated variables, and we finally have a
program that requires a destructor in order to be completely
proper.  If we simply leave the scope of the objects as we do
when we leave the main program, we will leave the three
dynamically allocated variables on the heap with nothing pointing
to them.  They will be inaccessible and will therefore represent
wasted storage on the heap.  For that reason, the destructor is
used to delete the variable which the pointer named point is
referencing, as each object goes out of existence.  In this case,
lines 37 and 38 assign zero to variables that will be
automatically deleted.  Even though these lines of code really do
no good, they are legal statements.

Actually, in this particular case, the variables will be
automatically reclaimed when we return to the operating system
because all program cleanup is done for us at that time.  If this
were a function that was called by another function however, the
heap space would be wasted.  This is an illustration of good
programming practice, that of cleaning up after yourself when you
no longer need some dynamically allocated variables.

One other construct should be mentioned again, that of the inline
method implementations in line 11 and 12.  As we mentioned in
chapter 5 and repeated earlier in this chapter, inline functions
can be used where speed is of the utmost in importance since the
code is assembled inline rather than by actually making a method
call.  Since the code is defined as part of the declaration, the
system will assemble it inline, and a separate implementation for
these methods is not needed.  If the inline code is too involved,
the compiler is allowed to ignore the inline request and will
actually assemble it as a separate method, but it will do it
invisibly to you and will probably not even tell you about it.

Remember that we are interested in using information hiding and
inline code prevents hiding of the implementation, putting it out
in full view.  Many times you will be more interested in speeding
up a program than you are in hiding a trivial implementation.
Since most inline methods are trivial, you should feel free to
use the inline code construct wherever it is expedient.  Be sure
to compile and execute this program.

A DYNAMICALLY ALLOCATED OBJECT
—————————————————————–
Examine the file named OBJDYNAM.CPP for our    ==================
first look at a dynamically allocated object.     OBJDYNAM.CPP
This is not any different than any other       ==================
dynamically allocated object, but an example
is always helpful.  In line 39 we declare a pointer to an object
of type box and since it is only a pointer with nothing to point
to, we dynamically allocate an object for it in line 44, with the
object being created on the heap just like any other dynamically
allocated variable.  When the object is created in line 44, the
constructor is called automatically to assign values to the two
internal storage variables.  Note that the constructor is not
called when the pointer is declared since there is nothing to
initialize.  It is called when the object is allocated.

Reference to the components of the object are handled in much the
same way that structure references are made, through use of the
pointer operator as illustrated in lines 50 through 52.  Of
course you can use the pointer dereferencing method without the
arrow such as (*point).set(12, 12); as a replacement for line 51
but the arrow notation is much more universal and should be used.
Finally, the object is deleted in line 54 and the program
terminates.  If there were a destructor for this class, it would

be called as part of the delete statement to clean up the object
prior to deletion.

You have probably noticed by this time that the use of objects is
not much different from the use of structures.  Be sure to
compile and execute this program after you have studied it
thoroughly.

AN OBJECT WITH A POINTER TO ANOTHER OBJECT
—————————————————————–
The program named OBJLIST.CPP contains an       =================
object with an internal reference to another       OBJLIST.CPP
object of its own class.  This is the           =================
standard structure used for a singly linked
list and we will keep the use of it very simple in this program.

The constructor contains the statement in line 21 which assigns
the pointer the value of NULL to initialize the pointer.  This is
a good idea for all of your programming, don’t allow any pointer
to point off into space, but initialize all pointers to something.
By assigning the pointer within the constructor, you guarantee
that every object of this class will automatically have its
pointer initialized.  It will be impossible to overlook the
assignment of one of these pointers.

Two additional methods are declared in lines 12 and 13 with the
one in line 13 having a construct we have not yet mentioned in
this tutorial.  This method returns a pointer to an object of the
box class.  As you are aware, you can return a pointer to a
struct in standard C, and this is a parallel construct in C++.
The implementation in lines 48 through 51 returns the pointer
stored within the object.  We will see how this is used when we
get to the actual program.

An extra pointer named box_pointer is declared in the main
program for use later and in line 66 we make the embedded pointer
within the small box point to the medium box.  Line 67 makes the
embedded pointer within the medium box point to the large box.
We have effectively generated a linked list with three elements.
In line 69 we make the extra pointer point to the small box.
Continuing in line 70 we use it to refer to the small box and
update it to the value contained in the small box which is the
address of the medium box.  We have therefore traversed from one
element of the list to another by sending a message to one of the objects.  If line 70 were repeated exactly as shown, it would cause the extra pointer to refer to the large box, and we would have traversed the entire linked list which is only composed of three elements.  Figure 6-2 is a graphical representation of the data space following execution of  line 69.  Note that only a portion of each object is actually depicted here to keep it simple.

ANOTHER NEW KEYWORD this
—————————————————————–
Another new keyword is available in C++, the keyword this.  The
word this is defined within any object as being a pointer to the
object in which it is contained.  It is implicitly declared as
class_name *this; and is initialized to point to the object for
which the member function is invoked.  This pointer is most
useful when working with pointers and especially with a linked
list when you need to reference a pointer to the object you are
inserting into the list.  The keyword this is available for this
purpose and can be used in any object.  Actually the proper way to refer to any variable within a list is through use of the predefined pointer this, by writingthis->variable_name, but the compiler assumes the pointer is used, and we can simplify every reference by omitting the pointer.  Use of the keyword this is not illustrated in a program at this point, but will be used in one of the larger example programs later in this tutorial.

You should study this program until you understand it completely then compile and execute it in preparation for our next example program.

A LINKED LIST OF OBJECTS
—————————————————————–
The next example program in this chapter is     =================
named OBJLINK.CPP and is a complete example        OBJLINK.CPP
of a linked list written in object oriented     =================
notation.  This program is very similar to
the last one.  In fact it is identical until we get to the main
program.  You will recall that in the last program the only way
we had to set or use the embedded pointer was through use of the
two methods named point_at_next() and get_next() which are listed
in lines 40 through 51 of the present program.  We will use these
to build up our linked list then traverse and print the list.
Finally, we will delete the entire list to free the space on the
heap.

In lines 56 through 58 we declare three pointers for use in the
program.  The pointer named start will always point to the
beginning of the list, but temp will move down through the list
as we create it.  The pointer named box_pointer will be used for
the creation of each object.  We execute the loop in lines 61
through 69 to generate the list where line 62 dynamically
allocates a new object of the box class and line 63 fills it
with nonsense data for illustration.  If this is the first
element in the list, the start pointer is set to point to this
element, but if elements already exist, the last element in the
list is assigned to point to the new element.  In either case,
the temp pointer is assigned to point to the last element of the
list, in preparation for adding another element if there is
another element to be added.

In line 72, the pointer named temp is caused to point to the
first element and it is used to increment its way through the
list by updating itself in line 75 during each pass through the
loop.  When temp has the value of NULL, which it gets from the
last element of the list, we are finished traversing the list.

Finally, we delete the entire list by starting at the beginning
and deleting one element each time we pass through the loop in
lines 79 through 84.

A careful study of the program will reveal that it does indeed
generate a linked list of ten elements, each element being an
object of class box.  The length of this list is limited by the
practicality of how large a list we desire to print out, but it
could be lengthened to many thousands of these simple elements
provided you have enough memory available to store them all.

Once again, the success of the dynamic allocation is not checked
as it should be in a correctly written program.  Be sure to
compile and execute this example program.

NESTING OBJECTS
—————————————————————–
Examine the program named NESTING.CPP for an    =================
example of nesting classes which results in        NESTING.CPP
nested objects.  A nested object could be       =================
illustrated with your computer in a rather
simple manner.  The computer itself is composed of many items
which work together but work entirely differently, such as a
keyboard, a disk drive, and a power supply.  The computer is
composed of these very dissimilar items and it is desirable to
discuss the keyboard separately from the disk drive because they
are so different.  A computer class could be composed of several
objects that are dissimilar by nesting the dissimilar classes
within the computer class.

If however, we wished to discuss disk drives, we may wish to
examine the characteristics of disk drives in general, then
examine the details of a hard disk, and the differences of floppy
disks.  This would involve inheritance because much of the data
about both drives could be characterized and applied to the
generic disk drive then used to aid in the discussion of the
other three.  We will study inheritance in the next three
chapters, but for now we will look at the embedded or nested
class.

This example program contains a class named box which contains an
object of another class embedded within it in line 16, the
mail_info class.  It is depicted graphically in figure 6-3.  This
object is available for use only within the class implementation
of box because that is where it is defined.  The main program has
objects of class box defined but no objects of class mail_info,

so the mail_info class cannot be referred to in the main program.
In this case, the mail_info class object is meant to be used
internally to the box class and one example is given in line 21
where a message is sent to the label.set() method to initialize
the variables.  Additional methods could be used as needed, but
these are given as an illustration of how they can be called.

Of prime importance is the fact that there are never any objects
of the mail_info class declared directly in the main program,
they are inherently declared when the enclosing objects of class
box are declared.  Of course objects of the mail_info class could
be declared and used in the main program if needed, but they are
not in this example program.  In order to be complete, the box
class should have one or more methods to use the information
stored in the object of the mail_info class.  Study this program
until you understand the new construct, then compile and
execute it.

If the class and the nested classes require parameter lists for
their respective constructors an initialization list can be
given.  This will be discussed and illustrated later in this
tutorial.

OPERATOR OVERLOADING
—————————————————————–
The example file named OPOVERLD.CPP contains   ==================
examples of overloading operators.  This          OPOVERLD.CPP
allows you to define a class of objects and    ==================
redefine the use of the normal operators.
The end result is that objects of the new class can be used in as
natural a manner as the predefined types.  In fact, they seem to
be a part of the language rather than your own add-on.

In this case we overload the + operator and the * operator, with
the declarations in lines 10 through 12, and the definitions in
lines 16 through 40.  The methods are declared as friend
functions so we can use the double parameter functions as listed.
If we did not use the friend construct, the function would be a
part of one of the objects and that object would be the object to
which the message was sent.  Including the friend construct
allows us to separate this method from the object and call the
method with infix notation.  Using this technique, it can be writ-
ten as object1 + object2 rather than object1.operator+(object2).
Also, without the friend construct we could not use an
overloading with an int type variable for the first parameter
because we can not send a message to an integer type variable
such as int.operator+(object).  Two of the three operator
overloadings use an int for the first parameter so it is
necessary to declare them as friend functions.

There is no upper limit to the number of overloadings for any given operator.  Any number of overloadings can be used provided the parameters are different for each particular overloading.

The header in line 16 illustrates the first overloading where the
+ operator is overloaded by giving the return type followed by the keyword operator and the operator we wish to overload.  The two formal parameters and their types are then listed in the parentheses and the normal function operations are given in the implementation of the function in lines 18 through 21.  The observant student will notice that the implementation of the friend functions are not actually a part of the class because the class name is not prepended onto the method name in line 16.
There is nothing unusual about this implementation, it should be
easily understood by you at this point.  For purposes of
illustration, some silly mathematics are performed in the method implementation, but any desired operations can be done.

The biggest difference occurs in line 56 where this method is
called by using the infix notation instead of the usual message sending format.  Since the variables small and medium are objects of the box class, the system will search for a way to use the + operator on two objects of class box and will find it in the overloaded operator+ method we have just discussed.  The operations within the method implementation can be anything we
need them to be, and they are usually much more meaningful than the silly math included here.

In line 58 we ask the system to add an int type constant to an
object of class box, so the system finds the other overloading of
the + operator beginning in line 25 to perform this operation.
Also in line 60 we ask the system to use the * operator to do
something to an int constant and an object of class box, which it
satisfies by finding the method in lines 34 through 40.  Note
that it would be illegal to attempt to use the * operator the
other way around, namely large * 4 since we did not define a
method to use the two types in that order.  Another overloading
could be given with reversed types, and we could use the reverse
order in a program.

You will notice that when using operator overloading, we are also
using function name overloading since some of the function names
are the same.

When we use operator overloading in this manner, we actually make
our programs look like the class is a natural part of the
language since it is integrated into the language so well.  C++
is therefore an extendible language and can be molded to fit the
mechanics of the problem at hand.

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Chapter 6 – More Encapsulation

OPERATOR OVERLOADING CAVEATS
—————————————————————–
Each new topic we study has its pitfalls which must be warned
against and the topic of operator overloading seems to have the
record for pitfalls since it is so prone to misuse and has
several problems.  The overloading of operators is only available
for classes, you cannot redefine the operators for the predefined
simple types.  This would probably be very silly anyway since the
code could be very difficult to read if you changed some of them
around.

The logical and “&&” and the logical or “||” operators can be
overloaded for the classes you define, but they will not operate
as short circuit operators.  All members of the logical
construction will be evaluated with no regard concerning the
outcome.  Of course the normal predefined logical operators will
continue to operate as short circuit operators as expected, but
not the overloaded ones.

If the increment “++” or decrement “–” operators are overloaded,
the system has no way of telling whether the operators are used
as preincrement or postincrement (or predecrement or
postdecrement) operators.  Which method is used is
implementation dependent, so you should use them in such a way
that it doesn’t matter which is used.

Be sure to compile and execute OPOVERLD.CPP before continuing on
to the next example program.

FUNCTION OVERLOADING IN A CLASS
—————————————————————–
Examine the program named FUNCOVER.CPP for an  ==================
example of function name overloading within a     FUNCOVER.CPP
class.  In this program the constructor is     ==================
overloaded as well as one of the methods to
illustrate what can be done.

This file illustrates some of the uses of overloaded names and a
few of the rules for their use.  You will recall that the
function selected is based on the number and types of the formal
parameters only.  The type of the return value is not significant
in overload resolution.

In this case there are three constructors.  The constructor which
is actually called is selected by the number and types of the
parameters in the definition.  In line 77 of the main program the
three objects are declared, each with a different number of
parameters and inspection of the results will indicate that the
correct constructor was called based on the number of parameters.

In the case of the other overloaded methods, the number and type
of parameters is clearly used to select the proper method.  You

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Chapter 6 – More Encapsulation

will notice that one method uses a single integer and another
uses a single float type variable, but the system is able to
select the correct one.  As many overloadings as desired can be
used provided that all of the parameter patterns are unique.

You may be thinking that this is a silly thing to do but it is,
in fact, a very important topic.  Throughout this tutorial we
have been using an overloaded operator and you haven’t been the
least confused over it.  It is the cout operator which operates
as an overloaded function since the way it outputs data is a
function of the type of its input variable or the field we ask
it to display.  Many programming languages have overloaded
output functions so you can output any data with the same
function name.

Be sure to compile and execute this program.

SEPARATE COMPILATION
—————————————————————–
Separate compilation is available with C++ and it follows the
identical rules as given for ANSI-C separate compilation.  As
expected, separately compiled files can be linked together.
However, since classes are used to define objects, the nature of
C++ separate compilation is considerably different from that used
for ANSI-C.  This is because the classes used to create the
objects are not considered as external variables, but as included
classes.  This makes the overall program look different from a
pure ANSI-C program.  Your programs will take on a different
appearance as you gain experience in C++.

YOU GET SOME METHODS BY DEFAULT
—————————————————————–
Even if you include no constructors or         ==================
operator overloadings you get a few defined       DEFMETHS.CPP
automatically by the compiler.  Examine the    ==================
file named DEFMETHS.CPP which will illustrate
those methods provided by the compiler, and why you sometimes
can’t use the defaults but need to write your own to do the job
the defaults were intended to do for you.

Before we actually look at the program, we will list a few rules
that all compiler writers must follow in order to deliver a
useful implementation of C++.  First we will state the rules,
then take a closer look at them and the reason for their
existence.

1.  If no constructors are defined by the writer of a class, the
compiler will automatically generate a default constructor
and a copy constructor.  Both of these constructors will be
defined for you shortly.

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Chapter 6 – More Encapsulation

2.  If the class author includes any constructor in the class,
the default constructor will not be supplied by the
constructor.

3.  If the class author does not include a copy constructor, the
compiler will generate one, but if the writer includes a copy
constructor, the compiler will not generate one
automatically.

4.  If the class author includes an assignment operator, the
compiler will not include one automatically, otherwise it
will generate a default assignment operator.

Any class declared and used in a C++ program must have some way
to construct an object because the compiler, by definition, must
call a constructor when we define an object.  If we don’t provide
a constructor, the compiler itself will generate one that it can
call during construction of the object.  This is the default
constructor and we have used it unknowingly in a lot of our
example programs.  The default constructor does not initialize
any of the member variables, but it sets up all of the internal
class references it needs, and calls the base constructor or
constructors if they exist.  We haven’t studied inheritance yet,
but we will in the next chapter of this tutorial so we will know
then what base classes are all about.  Line 11 of the present
program contains a default constructor which is called when you
define an object with no parameters.  In this case, the
constructor is necessary because we have an embedded string in
the class that requires a dynamic allocation and an
initialization of the string to the null string.  It will take
little thought to see that our constructor is much better than
the default constructor which would leave us with an
uninitialized pointer.

The default constructor is used in line 78 of this example
program.

THE COPY CONSTRUCTOR
—————————————————————–
The copy constructor is generated automatically for you by the
compiler if you fail to define one yourself.  It is used to copy
the contents of an object to a new object during construction of
that new object.  If the compiler generates it for you, it will
simply copy the contents of the original into the new object as a
byte by byte copy, which may not be what you want.  For simple
classes with no pointers, that is usually sufficient, but in the
present example program, we have a pointer as a class member so a
byte by byte copy would copy the pointer from one to the other
and they would both be pointing to the same allocated member.
For this program, we declared our own copy constructor in line 14
and implemented it in lines 34 to 39.  A careful study of the
implementation will reveal that the new class will indeed be

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Chapter 6 – More Encapsulation

identical to the original, but the new class has its own string
to work with.  Since both constructors contain dynamic
allocation, we must assure that the allocated data is destroyed
when we are finished with the objects, so a destructor is
mandatory as implemented in lines 50 through 53 of the present
example program.  The copy constructor is used in line 84 of the
current example program.

THE ASSIGNMENT OPERATOR
—————————————————————–
It is not too obvious, but an assignment operator is required for
this program also, because the default assignment operator simply
copies the source object to the destination object byte by byte.
This would result the same problem we had with copy constructor.
The assignment operator is declared in line 17 and defined in
lines 41 through 49 where we deallocate the old string in the
existing object prior to allocating room for the new text and
copying the text from the source object into the new object.  The
assignment operator is used in line 91.

It should be fairly obvious to the student that when a class is
defined which includes any sort of dynamic allocation, the above
three methods should be included in addition to the proper
destructor.  If any of the four entities are omitted, the program
may have terribly erratic behavior.  Be sure to compile and
execute this example program.

A PRACTICAL EXAMPLE
—————————————————————–
Using the inline keyword with a class member   ==================
can cause a bit of difficulty unless you            PHRASE.H
understand how the compiler uses the inline    ==================
code definition to perform the inline code
insertion.  Examine the header file named PHRASE.H which includes
some inline methods.  These are included as an illustration of
one means of defining the inline methods in a clean way that the
compiler can use efficiently.  When any implementation uses this
class, it must have access to the inline implementation in order
to insert the proper inline code for the member functions.  One
way to do this is to put all of the inline methods in a separate
file named with the INL extension, then including that file into
the end of the .H file as shown here.  This makes all of the
inline code available for the compiler while compiling files
that use this class.

The example file named PHRASE.INL contains     ==================
all of the inline code for this class.  If         PHRASE.INL
this class had methods that were not           ==================
inlined, they could be packaged into a file
named PHRASE.CPP in the usual manner.  Note that for illustrative

purposes, all of the methods were declared inline, so there is no
implementation file for this class.

The file named USEPHRAS.CPP uses the phrase    ==================
class defined in the last two example files.      USEPHRAS.CPP
It is plain to see that this class is no       ==================
different than any others we have studied.
It simply illustrates a way to package inline code in a simple
and very efficient manner.

ANOTHER PRACTICAL EXAMPLE
—————————————————————–
We come again to the practical part of this lesson where we study
a practical class that can actually be used in a program but is
still simple enough for the student to completely understand.

In the last chapter we studied the date          ================
class and in this chapter we will study a             TIME.H
simple time class.  You should begin by          ================
studying the file named TIME.H which will
look very similar to the date class header.  The only major
difference in this class from the date class is the overloaded
constructors and methods.  The program is a very practical
example that illustrates very graphically that many constructor
overloadings are possible.

The implementation for the time class is       ==================
given in the file named TIME.CPP.  Once             TIME.CPP
again, the code is very simple and you         ==================
should have no problem understanding this
example in its entirety.  It should be pointed out that three of
the four overloadings actually call the fourth so that the code
did not have to be repeated four times.  This is a perfectly good
coding practice and illustrates that other member functions can
be called from within the implementation.

The example program named USETIME.CPP is a      =================
very simple program that uses the time class       USETIME.CPP
in a very rudimentary way as an illustration    =================
for you.  You should be able to understand
this program in a very short time.  It will be to your advantage
to completely understand the practical example programs given at
the end of the last chapter and the end of this chapter.  As
mentioned above, we will use the time class and the date class
as the basis for both single and multiple inheritance in the
next three chapters.

WHAT SHOULD BE THE NEXT STEP?
—————————————————————–
At this point you have learned enough C++ to write meaningful
programs and it would be to your advantage to stop studying and

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Chapter 6 – More Encapsulation

begin using the knowledge you have gained.  Because C++ is an
extension to ANSI-C, it can be learned in smaller pieces than
would be required if you are learning a completely new language.
You have learned enough to study and completely understand the
example program given in chapter 12, the Flyaway adventure game.
You should begin studying this program now.

One of your biggest problems is learning to think in terms of
object oriented programming.  It is not a trivial problem if you
have been programming in procedural languages for any significant
length of time.  However, it can be learned by experience, so you
should begin trying to think in terms of classes and objects
immediately.  Your first project should use only a small number
of objects and the remainder of code can be completed in standard
procedural programming techniques.  As you gain experience, you
will write more of the code for any given project using classes
and objects but every project will eventually be completed in
procedural code.

After you have programmed for a while using the techniques
covered up to this point in the tutorial, you can continue on to
the next few chapters which will discuss inheritance and virtual
functions.

PROGRAMMING EXERCISES
—————————————————————–
1.  Modify OBJDYNAM.CPP to make the objects named small and
medium pointers, then dynamically allocate them prior to
using them.

2.  Modify the loop in line 61 of OBJLINK.CPP so that the loop
will store 1000 elements in the list before stopping.  You
will probably wish to remove the printout from line 74 so the
program will stop in a reasonable time.  You may also get an
integer overflow indicated by wrong answers if you send a
message to get_area() with such large numbers.  That will
depend upon your compiler.  You should also add a test to
assure that the memory did not become exhausted after each
dynamic allocation.

3.  Write a program that uses both the date and time classes in a
meaningful manner.  No answer will be given in the ANSWERS
directory for this exercise since it is so straight forward.
These classes can be used in all of your future C++ programs
to time stamp the time and date of execution.