Pointers

1
Pointers

• Chapter 14

2
What is a pointer?

• A pointer is a variable that holds a memory
  address.
• Pointers are an indirect way to access an object
  and thus the use of pointers is sometimes called
  indirection.

3
Defining a Pointer

• A pointer variable is defined by placing a in
  front of the variable name.
• long temp NULL
• char c NULL
• short a,b
• the is considered part of the variable name
  during declaration. The above definition is not
  the same as short a,b In this declaration, a
  would be a pointer to a short, and b would be a
  short.
• Even though youre technically declaring youll
  be storing a memory address, you still need to
  specify what type of data the pointer points to.
• You should always set your variables equal to
  NULL at the time of declaration. By default,
  theyre given a random address and use of the
  pointer could cause data corruption and will most
  likely cause a program and/or OS crash.

4
Using Pointers

• To set a pointer to point to an already existing
  variable, you need to use the address operator,
• short a77short tempNULLtemp a
• temp now holds the address to variable a.

5
Using Pointers

• To modify or retrieve the data that a pointer
  points to, you need to use the dereference
  operator,
• short a77short tempNULLtemp a //set
  temp to point to atemp 2 //set the stuff
  being pointed at by temp to 2printf(ad
  \n,temp) //prints the value of temp
  printf(ad \n,a) //prints the value of a
• Essentially, in the above example, using temp is
  equivalent to using a directly. Using variables
  directly is much faster than using them
  indirectly through pointers, so you should use
  them directly whenever possible.

6
What if .

• What if in the last example, I had forgotten to
  use the dereference operator when using temp?
• temp 2
• temp2 would have caused temp to point to memory
  address 2. Any use of temp after this would
  cause either a crash or data corruption.
• printf(ad \n,temp)
• printf(ad \n,temp) would have printed the
  memory address of temp rather than the data being
  pointed at by temp.

7
Pointers Functions

• Consider this code
• int main( )short x 1, y2swap(x,y)printf(x
  d, yd,x,y)return 0int swap( short a,
  short b)short temp aa bb tempreturn
  0
• What values of x and y is printf going to print?
  why?

8

• Now consider this
• int main( )short x 1, y2swap(x,y)printf(
  xd, yd,x,y)return 0int swap( short
  a, short b)short temp aa bb
  tempreturn 0
• What values of x and y are printed here? why?

9
Passing by Reference

• Using pointers to pass data to a function is
  called passing by reference. It allows you to
  manipulate the data of the calling function from
  within the called function which normally would
  be unavailable. It effectively allows you to
  return multiple values back to the calling
  function.