


                                                        Chapter 2
                                                   COMPOUND TYPES

ENUMERATED TYPES
-----------------------------------------------------------------
Examine the file named ENUM.CPP for an         ==================
example that uses an enumerated type                ENUM.CPP
variable.  The enumerated type is used in C++  ==================
in a very similar way that it was used in 
ANSI-C.  The keyword enum is not required to be used again when 
defining a variable of that type, but it can be used if desired.  
The name game_result is defined as an enumerated type making the 
use of the keyword enum optional.  However, it may be clearer for 
you to use the keyword when defining a variable in the same 
manner that it is required to be used in C, and you may choose to 
do so.

The example program uses the keyword enum in line 9, but omits it 
in line 8 to illustrate to you that it is indeed optional, but 
that is a trivial difference.  There is a bigger difference in 
the way an enumerated type is used in C++.  In C, the enumerated 
type is simply an int type variable, but in C++ it is not an int, 
but its own type.  Mathematical operations can not be performed 
on it, nor can an integer be assigned to it.  It cannot be 
incremented or decremented as it can be in C.  In the example 
program, an integer is used for the for loop because it can be 
incremented, then the value of the loop index is assigned to the 
enumerated variable by using a cast.  The cast is required or a 
compile error is reported.  The mathematical operations and the 
increment and decrement operators can be defined for the 
enumerated type, but they are not automatically available.  
Operator overloading will be studied later, and the last sentence 
will make much more sense at that time.

The remainder of this program should be no problem for you to 
understand.  After studying it, be sure to compile and execute it 
and examine the output.


A SIMPLE STRUCTURE
-----------------------------------------------------------------
Examine the example program named              ==================
STRUCTUR.CPP for an illustration using a very     STRUCTUR.CPP
simple structure.  This structure is no        ==================
different from that used in ANSI-C except for 
the fact that the keyword struct is not required to be used again 
when defining a variable of that type.  Lines 11 and 12 
illustrate the declaration of variables without the keyword, and 
line 13 indicates that the keyword struct can be included if 
desired.  It is up to you to choose which style you prefer to use 
in your C++ programs.


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                                       Chapter 2 - Compound Types

You should take note of the fact that this is a valid ANSI-C 
program except for the fact that it uses the stream library, the 
C++ comments, and the lack of use of the keyword struct in two of 
the lines.

Once again, be sure to compile and execute this program after 
studying it carefully, because the next example program is very 
similar but it introduces a brand new construct not available in 
standard C, the class.


A VERY SIMPLE CLASS
-----------------------------------------------------------------
Examine the example program named CLASS1.CPP   ==================
for our first example of a class in C++.           CLASS1.CPP
This is the first class example, but it will   ==================
not be the last, since the class is the major 
reason for using C++ over ANSI-C or some other programming 
language.  You will notice the keyword class used in line 4, in 
exactly the same way that the keyword struct was used in the last 
program, and they are in fact very similar constructs.  There are 
definite differences, as we will see, but for the present time we 
will be concerned more with their similarities.

The word animal in line 4 is the name of the class, and when we 
declare variables of this type in lines 12 through 14, we can 
either omit the keyword class or include it if desired as 
illustrated in line 14.  In the last program, we declared 5 
variables of a structure type, but in this program we declare 5 
objects.  They are called objects because they are of a class 
type.  The differences are subtle, and in this case the 
differences are negligible, but as we proceed through this 
tutorial, we will see that the class construct is indeed very 
important and valuable.  The class was introduced here only to 
give you a glimpse of what is to come later in this tutorial.

The class is a type which can be used to declare objects in much 
the same way that a structure is a type that can be used to 
declare variables.  Your dog named King is a specific instance 
of the general class of dogs, and in a similar manner, an object 
is a specific instance of a class.  It would be well to take note 
of the fact that the class is such a generalized concept that 
there are libraries of prewritten classes available in the 
marketplace.  You can purchase classes which perform some 
generalized operations such as managing stacks, queues, or lists, 
sorting data, managing windows, etc.  This is because of the 
generality and flexibility of the class construct.

The new keyword public in line 5, followed by a colon, is 
necessary in this case because the variables in a class are 
defaulted to a private type and we could not access them at all 
without making them public.  Don't worry about this program yet, 
we will cover all of this in great detail later in this tutorial.

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                                       Chapter 2 - Compound Types

Be sure to compile and run this example program to see that it 
does what we say it does with your compiler.  Keep in mind that 
this is your first example of a class and it illustrates 
essentially nothing concerning the use of this powerful C++ 
construct.


THE FREE UNION OF C++
-----------------------------------------------------------------
Examine the program named UNIONEX.CPP for an    =================
example of a free union.  In ANSI-C, all           UNIONEX.CPP
unions must be named in order to be used, but   =================
this is not true in C++.  When using C++ we 
can use a free union, a union without a name.  The union is 
embedded within a simple structure and you will notice that there 
is not a variable name following the declaration of the union in 
line 11.  In ANSI-C, we would have to name the union and give a 
triple name (three names dotted together) to access the members.  
Since it is a free union, there is no union name, and the 
variables are accessed with only a doubly dotted name as 
illustrated in lines 18, 22, 26, and others.

You will recall that a union causes all the data contained within 
the union to be stored in the same physical memory locations, 
such that only one variable is actually available at a time.  
This is exactly what is happening here.  The variable named 
fuel_load, bomb_load, and pallets are stored in the same physical 
memory locations and it is up to the programmer to keep track of 
which variable is stored there at any given time.  You will 
notice that the transport is assigned a value for pallets in line 
26, then a value for fuel_load in line 28.  When the value for 
fuel_load is assigned, the value for pallets is corrupted and is 
no longer available since it was stored where fuel_load is 
currently stored.  The observant student will notice that this is 
exactly the way the union is used in ANSI-C except for the way 
components are named.

The remainder of the program should be easy for you to 
understand, so after you study and understand it, compile and 
execute it.


C++ TYPE CONVERSIONS
-----------------------------------------------------------------
Examine the program named TYPECONV.CPP for a   ==================
few examples of type conversions in C++.  The     TYPECONV.CPP
type conversions are done in C++ in exactly    ==================
the same manner as they are done in ANSI-C, 
but C++ gives you another form for doing the conversions.Lines 10 
through 17 of this program use the familiar "cast" form of type 
conversions used in ANSI-C, and there is nothing new to the 
experienced C programmer.  You will notice that lines 10 through 
13 are actually all identical to each other.  The only difference 

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                                       Chapter 2 - Compound Types

is that we are coercing the compiler to do the indicated type 
conversions prior to doing the addition and the assignment in 
some of the statements.  In line 13, the int type variable will 
be converted to type float prior to the addition, then the 
resulting float will be converted to type char prior to being 
assigned to the variable c.

Additional examples of type coercion are given in lines 15 
through 17 and all three of these lines are essentially the same.

The examples given in lines 19 through 26 are unique to C++ and 
are not valid in ANSI-C.  In these lines the type coercions are 
written as though they are function calls instead of the more 
familiar "cast" method as illustrated earlier.  Lines 19 through 
26 are identical to lines 10 through 17.

You may find this method of type coercion to be clearer and 
easier to understand than the "cast" method and in C++ you are 
free to use either, or to mix them if you so desire, but your 
code could be very difficult to read if you indescriminantly mix 
them.

Be sure to compile and execute this example program.


PROGRAMMING EXERCISES
-----------------------------------------------------------------
1.  Starting with the program ENUM.CPP, add the enumerated value 
    of FORFEIT to the enumerated type game_result, and add a 
    suitable message and logic to get the message printed in some 
    way.

2.  Add the variable height of type float to the class of 
    CLASS1.CPP and store some values in the new variable.  Print 
    some of the values out.  Move the new variable ahead of the 
    keyword public: and see what kind of error message results.  
    We will cover this error in chapter 5 of this tutorial.

















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