Megaware 1/Megaware Volume 1.iso/programg/c-tutor/chap03.txt

Text File | 1990-07-20 | 14KB | 294 lines

                                                        Chapter 3
                                                         POINTERS

Because pointers are so important in C and C++, this chapter will
review some of the more important topics concerning pointers.  Even
if you are extremely conversant in the use of pointers, you should
not completely ignore this chapter because some new material is
presented here.


POINTER REVIEW
_________________________________________________________________

Examine the program named POINTERS.CPP for a     ================
simple example of the use of pointers.  This is    POINTERS.CPP
a pointer review and if you are comfortable with ================
the use of pointers, you can skip this example
program completely.
A pointer in either ANSI-C or C++ is declared with an asterisk
preceding the variable name.  The pointer is then a pointer to a
variable of that one specific type and should not be used with
variables of other types.  Thus pt_int is a pointer to an integer
type variable and should not be used with any other type.  Of
course, an experienced C programmer knows that it is simple to
coerce the pointer to be used with some other type by using a cast,
but he must assume the responsibility for its correct usage.

In line 12 the pointer named pt_int is assigned the address of the
variable named pig and line 13 uses the pointer named pt_int to add
the value of dog to the value of pig because the asterisk
dereferences the pointer in exactly the same manner as standard C.
The address is used to print out the value of the variable pig in
line 14 illustrating the use of a pointer with the stream output
operator cout.  Likewise, the pointer to float named pt_float is
assigned the address of x, then used in a trivial calculation in
line 18.

If you are not completely comfortable with this trivial program
using pointers, you should review the use of pointers in any good
C programming book or Coronado Enterprises C tutorial before
proceeding on because we will assume that you have a thorough
knowledge of pointers throughout the remainder of this tutorial.
It is not possible to write a C program of any significant size or
complexity without the use of pointers.


CONSTANT POINTERS AND POINTERS TO CONSTANTS
_________________________________________________________________

The definition of C++ allows a pointer to a constant to be defined
such that the value to which the pointer points cannot be changed
but the pointer itself can be moved to another variable or
constant.  The method of defining a pointer to a constant is

                                                         Page 3-1

                                             Chapter 3 - Pointers

illustrated in line 22.  In addition to a pointer to a constant,
you can also declare a constant pointer, one that cannot be
changed.  Line 23 illustrates this.  Note that neither of these
pointers are used in illustrative code.

Either of these constructs can be used to provide additional
compile time checking and improve the quality of your code.  If you
know a pointer will never be moved due to its nature, you should
define it as a constant pointer.  If you know that a value will not
be changed, it can be defined as a constant and the compiler will
tell you if you ever inadvertently attempt to change it.


A POINTER TO VOID
_________________________________________________________________

The pointer to void is actually a part of the ANSI-C standard but
is relatively new so it is commented upon here.  A pointer to void
can be assigned the value of any other pointer type.  You will
notice that the pointer to void named general is assigned an
address of an int type in line 15 and the address of a float type
in line 20 with no cast and no complaints from the compiler.  This
is a relatively new concept in C and C++.  It allows a programmer
to define a pointer that can be used to point to many different
kinds of things to transfer information around within a program.
A good example is the malloc() function which returns a pointer to
void.  This pointer can be assigned to point to any entity, thus
transferring the returned pointer to the correct type.

A pointer to void is aligned in memory in such a way that it can
be used with any of the simple predefined types available in C++,
or in ANSI-C for that matter.  They will also align with any
compound types the user can define since compound types are
composed of the simpler types.

Be sure to compile and execute this program.


DYNAMIC ALLOCATION AND DEALLOCATION
_________________________________________________________________

Examine the program named NEWDEL.CPP for our     ================
first example of the new and delete operators.      NEWDEL.CPP
The new and delete operators do dynamic          ================
allocation and deallocation in much the same
manner that malloc() and free() do in your old
favorite C implementation.

During the design of C++, it was felt that since dynamic allocation
and deallocation are such a heavily used part of the C programming
language, it should be a part of the language, rather than a
library add-on.  The new and delete operators are actually a part
of the C++ language and are operators, much like the addition
operator or the assignment operator.  They are therefore very

                                                         Page 3-2

                                             Chapter 3 - Pointers

efficient, and are very easy to use as we will see in this example
program.

Lines 14 and 15 illustrate the use of pointers in the tradition of
C and line 16 illustrates the use of the new operator.  This
operator requires one modifier which must be a type as illustrated
here.  The pointer named point2 is now pointing at the new integer
variable which exists on the heap, and can be used in the same way
that any dynamically allocated variable is used in ANSI-C.  Line
18 illustrates displaying the value on the monitor which was
assigned in line 17.

Line 20 allocates another new variable and line 21 causes point2
to refer to the same dynamically allocated variable as point1 is
pointing to.  In this case, the reference to the variable that
point2 was previously pointing to has been lost and it can never
be used or deallocated.  It is lost on the heap until we return to
the operating system when it will be reclaimed for further use, so
this is obviously not good practice.  Note that point1 is
deallocated with the delete operator in line 25, and point2 can not
actually be deleted.  Since the pointer point1 itself is not
changed, it is actually still pointing to the original data on the
heap.  This data could probably be referred to again using point1,
but it would be terrible programming practice since you have no
guarantee what the system will do with the pointer or the data.
The data storage is returned to the free list to be allocated in
a subsequent call, and will soon be reused in any practical
program.

Since the delete operator is defined to do nothing if it is passed
a NULL value, it is legal to ask the system to delete the data
pointed to by a pointer with the value of NULL, but nothing will
actually happen.  It is considered a no-op and is actually wasted
code.  The delete operator can only be used to delete data
allocated by a new operator.  If the delete is used with any other
kind of data, the operation is undefined and anything can happen.
According to the ANSI standard, even a system crash is a legal
result of this illegal operation, and can be defined as such by the
compiler writer.

In line 27, we declare some floating point variables.  You will
remember that in C++ the variables do not have to be declared at
the beginning of a block.  A declaration is an executable statement
and can therefore appear anywhere in a list of executable
statements.  One of the float variables is allocated within the
declaration to illustrate that this can be done.  Some of the same
operations are performed on these float type variables as were done
on the int types earlier.

Some examples of the use of a structure are given in lines 35
through 41 and should be self explanatory.

Finally, since the new operator requires a type to determine the
size of the dynamically allocated block, you may wonder how you can

                                                         Page 3-3

                                             Chapter 3 - Pointers

allocate a block of arbitrary size.  This is possible by using the
construct illustrated in line 47 where a block of 37 char sized
entities, which will be 37 bytes, is allocated.  A block of 133
bytes greater than the size of the date structure is allocated in
line 49.  It is therefore clear that the new operator can be used
with all of the flexibility of the malloc() function which you are
used to using.

The standard functions which you have been using in C for dynamic
memory management, malloc(), calloc(), and free(), are also
available for use in C++ and can be used in the same manner they
were used in C.  The new and delete operators should not be
intermixed with the older function calls since the results may be
unpredictable.  If you are updating code with the older function
calls, continue to use them for any additions to the code.  If you
are designing and coding a new program you should use the newer
constructs because they are a built in part of the language rather
than an add on and are therefore more efficient.

Be sure to compile and execute this program.


POINTERS TO FUNCTIONS
_________________________________________________________________

Examine the program named FUNCPNT.CPP for an      ===============
example of using a pointer to a function.  It       FUNCPNT.CPP
must be pointed out that there is nothing new     ===============
here, the pointer to a function is available in
ANSI-C as well as in C++ and works in the manner
described here for both languages.  It is not regularly used by
most C programmers, so it is defined here for your information.

There is nothing unusual about this program except for the pointer
to a function declared in line 8.  This declares a pointer to a
function which returns nothing (void) and requires a single formal
parameter, a float type variable.  You will notice that all three
of the functions declared in lines 5 through 7 fit this profile and
are therefore candidates to be called with this pointer.  If you
have not used prototyping in C, these lines will look strange to
you.  Don't worry about them at this point since we will study
prototyping in the next chapter of this tutorial.

Observe that in line 15 we call the function print_stuff() with the
parameter pi and in line 16 we assign the function pointer named
function_pointer the value of print_stuff() and use the function
pointer to call the same function again in line 17.  Lines 15 and
17 are therefore identical in what is accomplished because of the
pointer assignment in line 16.  In lines 18 through 23, a few more
illustrations of the use of the function pointer are given.  You
will be left to study these on your own.

Since we assigned the name of a function to a function pointer, and
did not get an assignment error, the name of a function must be a

                                                         Page 3-4

                                             Chapter 3 - Pointers

pointer to that function.  This is exactly the case, a function
name is a pointer to that function, but it is a pointer constant
and cannot be changed.  This is exactly the case we found when we
studied arrays in ANSI-C at some point in our C programming
background.  An array name is a pointer constant to the first
element of the array.

Since the name of the function is a constant pointer to that
function, we can assign the name of the function to a function
pointer and use the function pointer to call the function.  The
only caveat is that the return value and the number and types of
parameters must be identical.  Most C and C++ compilers will not,
and in fact, can not warn you of type mismatches between the
parameter lists when the assignments are made.  This is because the
assignments are done at runtime when no type information is
available to the system, rather than at compile time when all type
information is available.

This use and operations of pointers must be thoroughly understood
when we get to the material on dynamic binding and polymorphism
later in this tutorial.  It will be discussed in detail at that
time.

Be sure to compile and execute this program.


PROGRAMMING EXERCISES
_________________________________________________________________


1.   When dynamically allocated data is deleted, it is still
     actually in memory, stored on the heap.  Repeat the output
     statement from line 23 of NEWDEL.CPP immediately following the
     delete in line 25 to see if the values are really still there.
     Repeat it once again just prior to the end of the program when
     the data spaces should have been written over to see if you
     get garbage out.  Even if your compiler reports the correct
     data, it is terrible practice to count on this data still
     being there because in a large dynamic program, the heap space
     will be used repeatedly.

2.   Add a function to FUNCPNT.CPP which uses a single integer for
     a parameter and attempt to call it by using the function
     pointer to see if you get the correct data into the function.










                                                         Page 3-5