Pointers

1
Pointers

• (Walls Mirrors - Beginning of Chapter 4)

2
Whats a Pointer?

• A pointer variable is a variable that can contain
  the location of another variable as its value.
• The location of a variable is usually implemented
  by indicating its address in (RAM) memory.
• The location (or address) of a variable is called
  a pointer.
• Sometimes, for brevity, a pointer variable is
  simply called a pointer. You will need to be
  careful to understand whether pointer refers to a
  variable or the address of a variable.

3
Pointers -Real Life Examples

• Suppose that your friend, Sam, borrows your copy
  of Walls Mirrors. In its place, he leaves you
  the note
• Borrowed your Walls Mirrors book.
• Thanks, Sam
• This note is like a pointer, since it it not your
  book, but it tells you where to go to find it.
• (The paper on which the note is written is like
  a pointer variable.)

4
Pointers - Graphical Representation

• A variable is often represented as a box.
• The value of the variable is written inside the
  box.
• If the variable is a pointer variable, containing
  a pointer, the box will contain the tail of an
  arrow that points to another variable.

5
Pointers - Suggestion

• If you have a problem with pointers, draw the
  layout.
• It may be difficult to understand what is going
  on without a graphical representation of the
  pointer relationships.

6
Pointer Declarations

• int iptr // iptr is a pointer to an int
• char cptr // cptr is a pointer to a char
• float fptr // fptr is a pointer to a float
• List Lptr // Lptr is a pointer to a List
  object
• Sphere Sptr // Sptr is a pointer to a Sphere
  object

7
Pointer Operations

• Assignment
• A pointer variable can be assigned only a pointer
  (i.e. the address of a variable) or NULL (which
  equals 0).
• Comparison , !
• Pointers can be compared for equality.
• Addition/Subtraction , ?
• Pointers can be incremented or decremented with
  an integer.
• Dereferencing
• ptr returns the value of the object pointed to
  by ptr.
• Address of
• ptr returns the address of ptr (i.e. pointer to
  ptr).

8
Pointer Operations - Address of

• The Address of operator returns the address of
  an object.
• float PI 3.14159 float PIptr
• PI returns the address of the variable PI, not
  3.14159 (the value stored in PI).
• PIptr PI stores the address of variable PI in
  variable, PIptr.
• PIptr returns the address of variable PIptr.

9
Pointer Operations - Dereferencing

• The Dereferencing operator returns the value of
  the object to which its operand points.
• float PI 3.14159 float PIptr float
  X
• PIptr PI // PIptr contains the address of PI
• X PIptr // Value stored in PI (3.14159) is
• // assigned to X
• ((PIptr)) PIptr (PI) PI 3.14159

10
Pointer Initialization

• int ptr // pointer to int declared, value
  undefined
• int x 5 // int declared and initialized to 5
• cout ltlt x // prints 5
• cout ltlt ptr // Error! Prints undefined value,
  since ptr not
• // initialized
• ptr x // ptr now contains the address of x
• cout ltlt ptr // prints 5

11
Pointer Initialization - Suggestion

• When a pointer variable is declared it is (by
  default) uninitialized. Therefore, where it is
  pointing is undefined.
• Its a good practice to initialize newly declared
  pointer variables to the NULL pointer ( 0).
• This will insure that the pointer variable is not
  pointing anywhere it shouldnt.
• This will help you determine if a valid pointer
  has been assigned to it.
• if( ptr NULL )
• cout ltlt ptr has not been initialized ltlt endl

12
new Operator

• The operator new creates a new object of a given
  type.
• new returns a pointer to the newly created
  object.
• ptr new int

13
new Operator (Contd.)

• An object created with new does not have a name
  and is not declared.
• An object created with new can only be used by
  following (dereferencing) a pointer to it.
• You need to be careful to not lose the pointer to
  an object created with new, since there is no
  other way to access it.
• Memory that was allocated with new and has become
  inaccessible is called a memory leak.
• For programs that run for long periods of time,
  memory leaks can be the reason for system failure.

14
new Operator - Example 1

• int ptr // pointer to int declared, value
  undefined
• ptr 5 // Error! ptr contains invalid address
  and
• // space for int not allocated
• ptr new int // space for int allocated and
  pointer to it
• // assigned to ptr
• ptr 5 // 5 is stored in the int pointed to by
  ptr

15
new Operator - Example 2

• int p, q // declare two pointer to int
  variables
• p new int // allocate space for an int make
  p point to it
• p 25 // store 25 in the int pointed to by p
• What is the effect of the following?
• q p

16
new Operator - Example 2 (Contd.)

• Draw a picture!

17
new Operator - Example 3

• int p, q // declare two pointer to int
  variables
• p new int // allocate space for an int make
  p point to it
• q new int // allocate space for an int make
  q point to it
• p 35 // store 35 in the int pointed to by p
• What is the effect of the following?
• q p

18
new Operator - Example 3 (Contd.)

• Draw a picture!

19
new Operator - Example 3 (Contd.)

• What would have happened if we had executed
• q p
• instead of
• q p

20
new Operator - Example 3 (Contd.)

• The new int, previously pointed to by q is LOST
  and cannot be recovered. This is called a memory
  leak.

21
Arrays and Pointers

• int a50
• int aptr a
• a is equivalent to a0
• aptr a is equivalent to aptr a0
• aptr5 is equivalent to a5
• (aptr5) is equivalent to a5