HPAVC/HPAVC CD-ROM.iso/CPTUTT22.ZIP/CHAP12.TXT

Text File | 1992-01-20 | 26KB | 589 lines

                                                       Chapter 12
                                           FLYAWAY ADVENTURE GAME

Now that you have learned lots of things about C++, and know how
to write and use a single isolated class, you have the problem of
how to build a program with several classes that work together to
accomplish some task.  After some amount of thought, it seems that
an adventure game would be a good candidate for a relatively large
example program.  It has lots of input and output and requires a
good deal of flexibility while running since there are so many
things that can be included in the game as obstacles, mazes, items
to find, and puzzles to solve.

The adventure game presented in this chapter is unique as far as
I know, since I have never heard of another adventure game
featuring an airport.  The location is not nearly as important as
the code used to get through the airport.  You are advised to play
the game to get familiar with what the code does, then study the
code to see how it works.  Finally, you are given an assignment to
extend the code which will be the real test of whether you
understand its operation.



PLAYING THE GAME
_________________________________________________________________

Prior to studying the source code for this game,  ===============
it would be to your advantage to spend some time    FLYAWAY.EXE
playing the game to get familiar with what the    ===============
game does.  Load the file FLYAWAY.EXE and begin
the adventure through the airport.  The
executable file is precompiled for you so you can begin executing
the program before you have to compile and link the whole thing.
The entire program is composed of 15 files and will take a little
effort on your part to properly compile and link it, but that will
come later.

If you have played adventure games before, sometimes called
interactive fiction, you should begin trying various commands to
find your way through the airport to your proper plane.  If you
have not played before, a few hints are in order concerning how to
play the game.

The object of the game is to get to your proper plane on time so
you can fly away to your vacation.  Of course there a few obstacles
and problems along the way and they will be brought up at the
appropriate time.  It will be up to you to solve the puzzles
associated with each problem.  To add a little excitement, you only
have about twenty-five minutes to get to your plane, with each move

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                              Chapter 12 - Flyaway Adventure Game

taking a minute, so you must hurry.  Of course, just getting to the
plane on time is not enough, there are a few additional
requirements.  You will find what they are as you progress through
the game.  You will probably find it necessary to restart the game
many times before you arrive at your destination unscathed and on
time.



THE METHOD OF PLAY
_________________________________________________________________

The method of play is extremely simple.  You simply wander around
the airport looking for things to do and places to go.  You move
around the airport by giving the system commands to go in a
direction with four choices available, north, south, east, or west.
You can abbreviate any of these four direction commands to the
first letter only, and you can use either upper or lower case.  The
system may move you to another area of the airport, or it may tell
you that you can't go that way.  Try loading the game now and
typing the four directions once each to see what happens.  If this
is not clear, enter the word help to get you started.

In addition to moving around, you can pick up items or ask for more
information in any of the rooms.  Try telling the system to look
around the room and see what additional information it gives you
for each room, some of the clues for solving the puzzle are given
in the clues issued in response to a look command.  Another
important command is inventory which will give you a list of the
items you possess at any given point in time.  Type the word
inventory at this time to see what items you possess.

The remainder of the commands consist of two words, a verb and a
noun.  These can be given in either order, since the system is
smart enough to know the difference, and additional words may be
given following the legal words.  If you give the system a command
that is not in its limited vocabulary, it will tell you it doesn't
understand that word.  Try telling the system to drop an item you
possess, or get an item that is located in the room you are
currently in.

Several friends have played this game with no more knowledge than
you have been given.  One solved it in 40 minutes, but most took
about an hour to complete the game.  After you play the game for
awhile, return to the text and we will study the source code for
the game.  The entire source code for the game is on your
distribution disk.  The game was purposely kept small so the code
could be easily grasped by a programming student.  There is no
reason the game could not have been made much larger by the
addition of more rooms, items, and traps.  You may choose to do
just that to gain experience in working with C++.




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                              Chapter 12 - Flyaway Adventure Game


A FEW SPECIAL CONSTANTS
_________________________________________________________________

The file named FLYAWAY.H contains the             ===============
definitions for TRUE and FALSE as well as the        FLYAWAY.H
enumerated type defining the legal dictionary of  ===============
words for use in playing the game.  The list was
started at a value of 1 so the value of zero can
be used to indicate that the word in question was not in the
library and hence not a legal word for use with the game.

The #ifndef in line 5 is required because this header file is
included in many of the other files and if it is included more than
once, there will be a multiple definition, and hence an error.  A
class only needs to be defined once, so after it is defined by one
of the includes, the name ITEMS_H will be defined and any other
defines will be ignored.  This is necessary because of the separate
compilation capability of C++.  This was described in more detail
near the end of chapter 7.


THE FIRST CLASS - clock
_________________________________________________________________

Examine the file named CLOCK.H for the            ===============
definition of the clock class.  This is the           CLOCK.H
class for the game clock, and only one instance   ===============
of this class will be used.  It will be used for
the object time_of_day defined in line 23 of
FLYAWAY.CPP.

The class is very simple, consisting of only two variables, the
hour and the minute, and four methods.  The first method is the
constructor used to initialize the clock to 8:51 as you can see if
you refer to the implementation of this class in the file named
CLOCK.CPP.  The next two methods are used to get the current values
of the two variables.  The final method is much more interesting
since it does more.  It updates the time of day clock and outputs
the user prompt to ask for the next command.  This may not be the
best place to output the user prompt since this class is devoted
to the time of day and associated operations, but this was chosen
as the place to do it since the time of day is part of the user
prompt.  You will notice that the clock was initialized to 8:51,
but the first time output was 8:52 when you played the game.  In
order to simplify the coding later, when we need to decide if we
made it to the plane on time, the time was incremented at the
beginning of each game move.  The time is therefore the same when
the command is entered and when it is executed, hence the time is
incremented prior to even the first output.

The clock class is by far the simplest class in the adventure game
and should be simple for you to understand.  After you are sure you
understand it, we will go on to the next class.

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                              Chapter 12 - Flyaway Adventure Game


INPUT COMMAND PARSING
_________________________________________________________________

The input command parsing routines are defined    ===============
within the words class and the code for the           WORDS.H
class is in WORDS.CPP.  The code is               ===============
straightforward and simple to understand if you
study it, so only a few comments will be made
about this class.

The method get_command() reads two words from the keyboard by
calling the function read_a_line() and stores the words in the
class members verb and noun.  It stores zero for either or both of
the words if it does not find a valid noun and a valid verb.

Two methods are included to provide the verb or noun which was
input as the last user input.  This allows any code that has
visibility of the object based on this class to find out what the
player would like to do.

There are four methods beginning with is_ in this class that are
used to determine if a word is a verb, a noun, a direction, or an
operation.  These are called upon from various places within the
program.  What they do should be easy for you to understand, but
it will take a little thought on your part to see why these are
needed in other parts of the code.

Finally the simple method named stop_game() is used to set the verb
to the value of quit so the game will be ended by the control logic
in the main program FLYAWAY.CPP.

All of the source code for the implementation is given in the file
named WORDS.CPP.  Since this code is fairly simple and well
commented, you will be left on your own to study it to whatever
depth you desire.


THE SECOND CLASS - items
_________________________________________________________________

If you examine the files named ITEMS.H and        ===============
ITEMS.CPP, you will find the complete                 ITEMS.H
definitions of the handling of the items that     ===============
you carried around the airport in the game.
There were exactly four transportable items that
could be located in each room or carried by yourself, the keys, the
candy, the ticket, and the money.  The keys and the money keep you
from getting through security and the ticket and candy are required
to get you safely on the plane and enroute to your destination.

The four items are stored in the class named items in the form of
TRUE or FALSE since that is the only required indication.  A TRUE
means the item is located here, and a FALSE means the item is not

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                              Chapter 12 - Flyaway Adventure Game

here.  The values of TRUE and FALSE are defined in FLYAWAY.H.
Finally, there are six methods to operate on these items.

The first method is a constructor to set all items to FALSE, and
the next two are used to either get a specific item, or drop one.
The fourth method is used to tell us if the item is located here
and the last two are used to tell us what items are on hand in this
location.  You will notice that the final two are different because
different text was desired depending on whether you are carrying
the items, or they are located in a room somewhere.

This file, like all other header files, is protected from multiple
inclusion by the #ifndef construct discussed earlier.


This class is used in line 24 of FLYAWAY.CPP to define an object
for the player named personal_items which stores the list of items
the player is carrying around.  It is also used in the class
location as an embedded or nested object to store the items that
are located in each of the 19 locations in the game.

Once again, the implementation for this class is so simple that you
will have no difficulty in understanding it.


THE FLIGHT AND GATE CLASS - schedule
_________________________________________________________________

Examine the files named SCHEDULE.H and           ================
SCHEDULE.CPP for our first example of a rather      SCHEDULE.H
large class, the one that does the flight and    ================
gate scheduling.  You will find a large number
of variables in this class, and eight methods to
handle the variables.  Instead of a detailed description of each
variable and method, we will only give a brief overview of the
class.

Only one object of this class is declared named flight_info in line
22 of the main program named FLYAWAY.CPP.  The constructor
initializes the flight possibilities, and the method named
shuffle_gates() shuffles all gates around if the player arrives at
his correct gate without reading the monitor in the waiting area.
Once the monitor in the waiting area is read, the flights_frozen
variable is made TRUE.  Likewise, the players destination is
changed during every move by the method named shuffle_flights()
until the player reads his ticket invoking the method named
list_actual_destination().

This class contains the methods to list the data seen on the
monitor, as well as the data seen when invoking the command look
at one of the gates.  Finally, this class contains the method named
check_flight() which searches through the list of requirements to
see if the player has completed all requirements to successfully
reach the final destination for his vacation.

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                              Chapter 12 - Flyaway Adventure Game


You will notice that several of the location objects were required
to be available within this code and are listed as extern in lines
12 through 21 of the implementation of the class.  The only other
thing to point out is the rest room requirement prior to boarding
the flight.  Line 28 is where the global variable is defined and
initialized, then in line 77 it is set TRUE if the current location
is the rest room, since this is called once during each player
move.  Finally, the state of this variable is checked in line 230
of this file and the appropriate action taken.  You will note that
the main program is not aware that the rest room variable exists
or that anything happens as a result of this variable.  In addition
to information hiding, we may coin a new term, something like
"Information Ignorance", since the main program did not even need
to be aware that there was a requirement to visit the rest room.

Even though this is a relatively large and complex class, it is
well commented so no further information will be given concerning
the implementation.



THE MOST USED CLASS - location
_________________________________________________________________

The file named LOCATION.H is the header file for ================
the class named location.  It is the class that     LOCATION.H
controls all of the moves from location to       ================
location.

This class is a bit unusual in that most of the stored data is in
the form of pointers to the various entities.  The first four are
the locations to which we will go if we move in one of the four
directions from the current location.  You will note that they are
pointers to those four locations.  Next we have pointers to two
different character strings associated with this room.  Finally in
line 22, we declare the object named list_of_items which is an
object of class items defined earlier.  Note that this is an
embedded class, a class embedded within the location class.  It is
not a parent class which we are inheriting something from.  In fact
we are instantiating an object of class items for use within the
room since the room is allowed to store any combination of the four
items contained in the class named items.

There is no constructor used with this class since we choose to
initialize the locations one by one.  The method named init() has
6 variable parameters, all of which are pointers, associated with
it which it uses to initialize the first six variables of this
object.  The last variable, an object of class items, is
initialized through use of the constructor associated with its
class.  Referring to lines 40 through 171 of the implementation for
the map class, you will find all of the initialization code for the
19 objects of class location.  If you drew a map when you played
the game, you will see the interconnections between the various

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                              Chapter 12 - Flyaway Adventure Game

locations embedded in the initialization statements.  Notice there
is no way to get back to the car from the passenger drop off area,
because presumably the car leaves when you get out of it.

The next method, named move(), returns a pointer to the new
location if a move was legal, otherwise it returns a NULL value.
The observant student will also notice that there are special cases
involved with getting out of the snack bar and getting through
security.  These are located here because they are part of the move
logic.  If you played the game to the complete conclusion, you
surely had trouble with at least one of these situations.

The rest of the methods in this class should be self explanatory
and will not be discussed any further.



THE LOCATION MESSAGES
_________________________________________________________________

Examine the file named MESSAGE.TXT for a          ===============
complete listing of the messages output to the      MESSAGE.TXT
monitor when each location was entered.  You      ===============
will also find the text for each of the messages
output in response to a look command in this
file.  These were put into a separate file only for the purpose of
reducing the size of the map class implementation file.  It does
not reduce the compile time since these messages are not separately
compiled.  They are included into the file and compiled each time
the map file MAP.CPP is compiled.  You will note that a few of the
messages have no text at all, only the empty quote marks, but are
included in order to have something for the initialization code to
work with.

Three other messages are stored here for convenience in lines 5
through 40.  Their use and meaning should be self-evident.



THE MAIN PROGRAM
_________________________________________________________________

We finally reach the main program, the one that   ===============
actually does the top level control.  Examine       FLYAWAY.CPP
the program named FLYAWAY.CPP and we will look    ===============
at some of its interesting characteristics.

Beginning with the main() entry point itself, we see that following
a call to airport.initialize(), we enter a single do while loop
which terminates when the player enters the word quit or when the
verb quit comes up some other way.  There are other ways to set the
verb to quit because it is generated internally in some cases such
as at end of game.


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                              Chapter 12 - Flyaway Adventure Game

The loop itself consists of 5 method calls.  First we call the
function named input_words.get_command() to get the players input
command in line 30.  Next we send two messages to the object named
flight_info to shuffle the flights and gates if the proper actions
have not been performed, then we call airport.perform_action()
which we will describe in a few paragraphs.  Finally, we send a
messages to the object named flight_info to check if the player has
reached one of the gates.  Remember that within most of the methods
we perform checks to see if we need to do the thing requested in
the message, then either perform the action or simply return to the
caller or message sender.


THE WORKING METHOD
_________________________________________________________________

The only function we have not mentioned yet is    ===============
the one that does most of the interesting work,        MAP.H
the function named perform_action() which begins  ===============
in line 183 of the MAP.CPP file.  This function
looks at the verb and noun, if there is one, and
causes the correct action to be performed.  Because of the way we
packaged all of the other routines, this function is a snap to
implement and to study.  If you go through each of the else if
clauses in this function, you will have no trouble understanding
what action is taken for each of the input commands.  You will
notice that many of the actions have conditional clauses before the
action is taken.  For example, it is illegal to buy candy unless
the player has money, the location has candy, and the location must
be the snack_bar according to the rules of the game.

Finally, at the end of this method in line 277, we have the default
case if nothing else was accomplished.  It is assumed that there
was something funny requested such as a request to get a monitor.
Both of these are legal words but they make no sense together.



FINAL COMMENTS ON FLYAWAY
_________________________________________________________________

Now that you have played the game for awhile and studied the game
in detail, you should have an appreciation for how this game can
be written.  Of course, it could be written in any of several
thousand different ways of packaging and definition.  This has been
only one of the ways.

Because the student may be left with the sinking feeling that this
method simply fell out of the sky or was arrived at in some other
esoteric way, it would only be fair to point out that several
earlier attempts at outlining this project were attempted and
rejected prior to this arrangement.  Also, when this tutorial was
being updated from version 2.0 to 2.2, the entire program was
restructured.  In version 2.0 and prior versions, about 50% of the

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                              Chapter 12 - Flyaway Adventure Game

code was in classes, but due to additional programing experience,
about 98% of the flyaway program is now encapsulated in classes.

Object oriented programming requires the same forethought as non-
object oriented programming, but the object oriented compiler will
help you in the coding and debugging phase since the compiler will
find and flag many of the oversight errors we are so good at
introducing into our code.  It was observed during the coding and
debugging phase of this project that in nearly every case, when the
program finally got through the compiler, the program would
actually run without bombing out the system.  This is not always
the case using any standard procedural programming language.




YOUR PROGRAMMING PROJECT
_________________________________________________________________

This programming assignment is intended to give you a little
experience in working with a relatively large project as opposed
to the very small programs we have been working with in this
tutorial.

Add a suitcase to the game, to be found in the car at arrival, and
which must be checked in at the ticket counter prior to attempting
to get through airport security.  This will not be trivial since
several classes will be affected.  Some of the operations you will
have to do are listed below.

1.   Add the noun "suitcase" and the verb "check" to the word list.
     Of course, they must be entered at the right place in the
     list.

2.   Add the suitcase to the items class, including additional code
     to each of its methods.

3.   Initialize the items at location your_car to include the
     suitcase.

4.   Add an additional check when passing through security to check
     that the player is not carrying the suitcase.  You can add any
     sort of penalty desired, including death by firing squad for
     attempting such an obviously crooked deed.

5.   You will need to add a check when the player finally gets on
     his correct airplane to see that he checked his suitcase.  If
     he did not, you could output any desired text indicating
     stupidity or forgetfulness.

Since I have not actually added the suitcase to the game and tested
it, I am not sure that this is all that will be required, but it
should be the majority of effort required.  The bottom line of this
effort is that if you understand this program enough to perform

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                              Chapter 12 - Flyaway Adventure Game

this modification, you have a good understanding of how the program
works and how objects work together to perform a task.

Once you understand this program, you should define a programming
project for yourself that will use object oriented programming
techniques and begin designing and programming it.  The best way
to learn to use OOP is to actually use it.

Good luck in your OOP endeavors.














































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