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[Introduction] [Chapter 1] [Chapter
2] [Chapter 3] [Chapter
4] [Chapter 5] [ Chapter
6 ] [Chapter 7] [Chapter
8] [Chapter 9] [Chapter
10] [Chapter 11] [Chapter
12]
Chapter 6: More Encapsulation
WHY BOTHER WITH ENCAPSULATION?
We asked this question earlier, but now that we have a little experience,
we can provide a much better answer. 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
Example program > OBJARRAY.CPP
Examine the file named OBJARRAY.CPP for our first example of an array
of objects. This file is nearly identical to the file named BOX1.CPP
until we come to line 45 where an array of 4 boxes are defined.
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 the constructor will be executed for
each box as they are defined. In order to define an array of objects,
a constructor for that object with no parameters must be available.
(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 50 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 51
and the operation of that code should be clear to you. The other method
is called in the output statement in lines 58 and 59 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 defined in line 50 and is still available for use in line
57 since we have not yet left the enclosing block which begins in
line 44 and extends to line 68. But this is true only if your compiler
is aging slightly. If you have a new compiler, you may find that index
is undefined in line 57. See the discussion in Chapter 1 if this is
not clear.
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 8, 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 yet 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 17 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.
Each object has its own personal length and width because they are
not declared static.
Line 24 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. It is generally not a good idea
to assign a value to a static member
in a constructor, but in this case, it illustrates how the static
variable works. 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 61 through 65, 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 18 and
19 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
Example program > OBJSTRNG.CPP
Examine the program named OBJSTRNG.CPP for our first example of an
object with an 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 8 contains a pointer to a char
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 defined 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 50 through 54, 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
Example programs > OBJINTPT.CPP
The program named OBJINTPT.CPP is our first example program with an
embedded pointer which will be used for dynamic allocation of data.
In line 8 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 22. It should be clear to you that the three
objects defined in line 46 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 23 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 mandatory 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 38 and 39 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. 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 12 and 13. As we mentioned in chapter
5, 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
Example program > OBJDYNAM.CPP
Examine the file named OBJDYNAM.CPP for our first look at a dynamically
allocated object. This is not any different than any other dynamically
allocated object, but an example is always helpful.
In line 40 we define 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 45, with the object
being created on the heap just like any other dynamically allocated
variable. When the object is created in line 45, 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 defined 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 51 through 53. Of course you can
use the pointer dereferencing method without the arrow such as (*point).set(12,
12); as a replacement for line 52 but the arrow notation is much more
universal and should be used. Finally, the object is deleted in line
55 and the program terminates. If there were a destructor for this
class, it would be called automatically 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
Example program > OBJLIST.CPP
The program named OBJLIST.CPP contains an object with an internal
reference to another 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 22 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 13 and 14 with the one
in line 14 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 49 through 52 returns the pointer stored as
a member variable within the object. We will see how this is used
when we get to the actual program.
An extra pointer named box_pointer
is defined in the main program for use later and in line 67 we make
the embedded pointer within the small
box point to the medium box.
Line 68 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 70 we make the extra pointer point to the small
box. Continuing in line 71 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 71
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 70. 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 defined 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 writing this->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
Example program > OBJLINK.CPP
The next example program in this chapter is named OBJLINK.CPP and
is a complete example 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 42 through 52 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 57 through 59 we define 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 62 through
75 to generate the list where line 64 dynamically allocates a new
object of the box class and
line 65 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 78, 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 82 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 87
through 92.
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
Example program > NESTING.CPP
Examine the program named NESTING.CPP for an example of nesting classes
which results in 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 17, 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 22 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
Example program > OPOVERLD.CPP
The example file named OPOVERLD.CPP contains examples of overloading
operators. This 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 11 through 13, and the definitions in lines
17 through 41. 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 written 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 17 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 19 through 22. 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 17. 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 57 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 59 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 26 to perform this operation. Also in line 61 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 35 through 41. 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
then 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.
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
Example program > FUNCOVER.CPP
Examine the program named FUNCOVER.CPP for an example of function
name overloading within a 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 78 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 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 << operator which is part of the cout
class, 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
Example program > DEFMETHS.CPP
Even if you include no constructors or operator overloadings you get
a few defined 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.
- 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.
- If
the class author includes any constructor in the class, the default
constructor will not be supplied by the constructor.
- 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.
- 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 12 of the present program declares
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 79 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 15 and implemented it in lines 35 to 40. A careful
study of the implementation will reveal that the new class will indeed
be 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
51 through 54 of the present example program. The copy constructor
is used in line 85 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 in the same problem we had with copy constructor. The
assignment operator is declared in line 18 and defined in lines 42
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 92.
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
Example program > PHRASE.H
Using the inline keyword with
a class member can cause a bit of difficulty unless you 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.
Example program > PHRASE.INL
The example file named PHRASE.INL contains all of the inline code
for this class.
Example program > PHRASE.CPP
Note that the only reason for this file to exist is to define the
static string variable full_phrase.
Since this is a definition, and therefore some memory is defined,
it cannot be placed in the header file. If it were placed there, it
would seem to work all right in this program because the header file
is only used once, but using a bad technique like that would lead
to problems later. For illustrative purposes, all of the methods were
declared inline, so there are no member definitions in this class.
Example program > USEPHRAS.CPP
The file named USEPHRAS.CPP uses the phrase
class defined in the last two example files. 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.
Example program > TIME.H
In the last chapter we studied the date
class and in this chapter we will study a 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.
Example program > TIME.CPP
The implementation for the time class
is given in the file named TIME.CPP. Once 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.
As we have mentioned before, this code contains calls that are specific
to DOS and are therefore not portable to other platforms. If you are
using some other platform, you will need to change the code to make
valid calls to your operating system, or simply assign default values
to the member variables.
Example program > USETIME.CPP
The example program named USETIME.CPP is a very simple program that
uses the time class 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 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
- Modify
OBJDYNAM.CPP to make the objects named small and medium pointers,
then dynamically allocate them prior to using them.
- Modify
the loop in line 62 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 81 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.
- 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.
<< back next >>
[Introduction] [Chapter 1] [Chapter
2] [Chapter 3] [Chapter
4] [Chapter 5] [ Chapter 6 ] [Chapter 7] [Chapter
8] [Chapter 9] [Chapter
10] [Chapter 11] [Chapter
12] |
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