Power-Programmierung/CD2.mdf/c/tutorial/cpptutor/text/chap03.txt

Text File | 1994-05-15 | 14.1 KB | 290 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 unique to C++ is presented here.


POINTER REVIEW
-----------------------------------------------------------------
Examine the program named POINTERS.CPP for a   ==================
simple example of the use of pointers.  This      POINTERS.CPP
is 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 then 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.  Figure 3-1 is a graphical representation
of the data space following execution of line 13.  Note that a
box containing a dot represents a pointer.

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

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                                             Chapter 3 - Pointers

or constant.  The method of defining a pointer to a constant is
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 and would also be heavily used in C++, it
should be a part of the language, rather than a library add-on.

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                                             Chapter 3 - Pointers

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 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 dynamically allocated 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 be deleted since it is now pointing to nothing.  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 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.


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                                             Chapter 3 - Pointers

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 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 familiar with.  The brackets are required
in lines 48 and 50 to tell the compiler that it is deallocating
an array.

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 briefly discussed here as a refresher.  If
you are comfortable with the use of pointers to functions, you
can skip this discussion entirely.

There is nothing unusual about this program except for the
pointer to a function declared in line 7.  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 4 through 6 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 14 we call the function print_stuff() with
the parameter pi and in line 15 we assign the function pointer

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                                             Chapter 3 - Pointers

named function_pointer the value of print_stuff() and use the
function pointer to call the same function again in line 16.
Lines 14 and 16 are therefore identical in what is accomplished
because of the pointer assignment in line 15.  In lines 17
through 22, 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 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.

The use and operation 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.



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