mahimuddin.delaxo

Get the Basics of pointer and there syntax well

Pointers — Before and After
There's a lot of nice, tidy code you can write without knowing about pointers. But onceyou learn to use the power of pointers, you can never go back. There are too many things
that can only be done with pointers. But with increased power comes increased
responsibility. Pointers allow new and more ugly types of bugs, and pointer bugs can
crash in random ways which makes them more difficult to debug. Nonetheless, even with
their problems, pointers are an irresistibly powerful programming construct. (The
following explanation uses the C language syntax where a syntax is required; there is a
discussion of Java at the section.)

Why Have Pointers?
Pointers solve two common software problems. First, pointers allow different sections of
code to share information easily. You can get the same effect by copying information
back and forth, but pointers solve the problem better. Second, pointers enable complex
"linked" data structures like linked lists and binary trees.

What Is A Pointer?
Simple int and float variables operate pretty intuitively. An int variable is like a
box which can store a single int value such as 42. In a drawing, a simple variable is a
box with its current value drawn inside



A pointer works a little differently— it does not store a simple value directly. Instead, a
pointer stores a reference to another value. The variable the pointer refers to is
sometimes known as its "pointee". In a drawing, a pointer is a box which contains the
beginning of an arrow which leads to its pointee. (There is no single, official, word for
the concept of a pointee — pointee is just the word used in these explanations.)
The following drawing shows two variables: num and numPtr. The simple variable num
contains the value 42 in the usual way. The variable numPtr is a pointer which contains
a reference to the variable num. The numPtr variable is the pointer and num is its
pointee. What is stored inside of numPtr? Its value is not an int. Its value is a
reference to an int.



Pointer Dereference
The "dereference" operation follows a pointer's reference to get the value of its pointee.
The value of the dereference of numPtr above is 42. When the dereference operation is
used correctly, it's simple. It just accesses the value of the pointee. The only restriction is
that the pointer must have a pointee for the dereference to access. Almost all bugs in
pointer code involve violating that one restriction. A pointer must be assigned a pointee
before dereference operations will work.
The NULL Pointer
The constant NULL is a special pointer value which encodes the idea of "points to
nothing." It turns out to be convenient to have a well defined pointer value which
represents the idea that a pointer does not have a pointee. It is a runtime error to
dereference a NULL pointer. In drawings, the value NULL is usually drawn as a diagonal
line between the corners of the pointer variable's box...



The C language uses the symbol NULL for this purpose. NULL is equal to the integer
constant 0, so NULL can play the role of a boolean false. Official C++ no longer uses the
NULL symbolic constant — use the integer constant 0 directly. Java uses the symbol
null.
Pointer Assignment
The assignment operation (=) between two pointers makes them point to the same
pointee. It's a simple rule for a potentially complex situation, so it is worth repeating:
assigning one pointer to another makes them point to the same thing. The example below
adds a second pointer, second, assigned with the statement second = numPtr;.
The result is that second points to the same pointee as numPtr. In the drawing, this
means that the second and numPtr boxes both contain arrows pointing to num.
Assignment between pointers does not change or even touch the pointees. It just changes
which pointee a pointer refers to.



After assignment, the == test comparing the two pointers will return true. For example
(second==numPtr) above is true. The assignment operation also works with the
NULL value. An assignment operation with a NULL pointer copies the NULL value
from one pointer to another.
Make A Drawing
Memory drawings are the key to thinking about pointer code. When you are looking at
code, thinking about how it will use memory at run time....make a quick drawing to work
out your ideas. This article certainly uses drawings to show how pointers work. That's the
way to do it.

Sharing
Two pointers which both refer to a single pointee are said to be "sharing". That two or
more entities can cooperatively share a single memory structure is a key advantage of
pointers in all computer languages. Pointer manipulation is just technique — sharing is
often the real goal. In Section 3 we will see how sharing can be used to provide efficient
communication between parts of a program.
Shallow and Deep Copying
In particular, sharing can enable communication between two functions. One function
passes a pointer to the value of interest to another function. Both functions can access the
value of interest, but the value of interest itself is not copied. This communication is
called "shallow" since instead of making and sending a (large) copy of the value of
interest, a (small) pointer is sent and the value of interest is shared. The recipient needs to
understand that they have a shallow copy, so they know not to change or delete it since it
is shared. The alternative where a complete copy is made and sent is known as a "deep"
copy. Deep copies are simpler in a way, since each function can change their copy
without interfering with the other copy, but deep copies run slower because of all the
copying.
The drawing below shows shallow and deep copying between two functions, A() and B().
In the shallow case, the smiley face is shared by passing a pointer between the two. In the
deep case, the smiley face is copied, and each function gets their own...



Bad Pointers
When a pointer is first allocated, it does not have a pointee. The pointer is "uninitialized"
or simply "bad". A dereference operation on a bad pointer is a serious runtime error. If
you are lucky, the dereference operation will crash or halt immediately (Java behaves this
way). If you are unlucky, the bad pointer dereference will corrupt a random area of
memory, slightly altering the operation of the program so that it goes wrong some
indefinite time later. Each pointer must be assigned a pointee before it can support
dereference operations. Before that, the pointer is bad and must not be used. In our
memory drawings, the bad pointer value is shown with an XXX value...



Bad pointers are very common. In fact, every pointer starts out with a bad value.
Correct code overwrites the bad value with a correct reference to a pointee, and thereafter
the pointer works fine. There is nothing automatic that gives a pointer a valid pointee.

Quite the opposite — most languages make it easy to omit this important step. You just
have to program carefully. If your code is crashing, a bad pointer should be your first
suspicion.
Pointers in dynamic languages such as Perl, LISP, and Java work a little differently. The
run-time system sets each pointer to NULL when it is allocated and checks it each time it
is dereferenced. So code can still exhibit pointer bugs, but they will halt politely on the
offending line instead of crashing haphazardly like C. As a result, it is much easier to
locate and fix pointer bugs in dynamic languages. The run-time checks are also a reason
why such languages always run at least a little slower than a compiled language like C or
C++.
Two Levels
One way to think about pointer code is that operates at two levels — pointer level and
pointee level. The trick is that both levels need to be initialized and connected for things
to work. (1) the pointer must be allocated, (1) the pointee must be allocated, and (3) the
pointer must be assigned to point to the pointee. It's rare to forget step (1). But forget (2)
or (3), and the whole thing will blow up at the first dereference. Remember to account for
both levels — make a memory drawing during your design to make sure it's right.
Syntax
The above basic features of pointers, pointees, dereferencing, and assigning are the only
concepts you need to build pointer code. However, in order to talk about pointer code, we
need to use a known syntax which is about as interesting as....a syntax. We will use the C
language syntax which has the advantage that it has influenced the syntaxes of several
languages.
Pointer Type Syntax
A pointer type in C is just the pointee type followed by a asterisk (*)...
int* type: pointer to int
float* type: pointer to float
struct fraction* type: pointer to struct fraction
struct fraction** type: pointer to struct fraction*
Pointer Variables
Pointer variables are declared just like any other variable. The declaration gives the type
and name of the new variable and reserves memory to hold its value. The declaration
does not assign a pointee for the pointer — the pointer starts out with a bad value.
int* numPtr; // Declare the int* (pointer to int) variable "numPtr".
// This allocates space for the pointer, but not the pointee.
// The pointer starts out "bad".

The & Operator — Reference To
There are several ways to compute a reference to a pointee suitable for storing in a
pointer. The simplest way is the & operator. The & operator can go to the left of any
variable, and it computes a reference to that variable. The code below uses a pointer and
an & to produce the earlier num/numPtr example.




void NumPtrExample() {
int num;
int* numPtr;
num = 42;
numPtr = # // Compute a reference to "num", and store it in numPtr
// At this point, memory looks like drawing above
}
It is possible to use & in a way which compiles fine but which creates problems at run
time — the full discussion of how to correctly use & is in Section 2. For now we will just
use & in a simple way.
The * Operator — Dereference
The star operator (*) dereferences a pointer. The * is a unary operator which goes to the
left of the pointer it dereferences. The pointer must have a pointee, or it's a runtime error.
Example Pointer Code
With the syntax defined, we can now write some pointer code that demonstrates all the
pointer rules...
void PointerTest() {
// allocate three integers and two pointers
int a = 1;
int b = 2;
int c = 3;
int* p;
int* q;
// Here is the state of memory at this point.
// T1 -- Notice that the pointers start out bad...



p = &a; // set p to refer to a
q = &b; // set q to refer to b
// T2 -- The pointers now have pointees




// Now we mix things up a bit...
c = *p; // retrieve p's pointee value (1) and put it in c
p = q; // change p to share with q (p's pointee is now b)
*p = 13; // dereference p to set its pointee (b) to 13 (*q is now 13)
// T3 -- Dereferences and assignments mix things up




Bad Pointer Example
Code with the most common sort of pointer bug will look like the above correct code, but
without the middle step where the pointers are assigned pointees. The bad code will
compile fine, but at run-time, each dereference with a bad pointer will corrupt memory in
some way. The program will crash sooner or later. It is up to the programmer to ensure
that each pointer is assigned a pointee before it is used. The following example shows a
simple example of the bad code and a drawing of how memory is likely to react...
void BadPointer() {
int* p; // allocate the pointer, but not the pointee
*p = 42; // this dereference is a serious runtime error
}
// What happens at runtime when the bad pointer is dereferenced...