CS 2130

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CS 2130

• Lecture 12a
• Void Pointers
• Function Pointers

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Void Pointers

• We talked about void pointers in an earlier
  lecture
• We stressed some things not to do
• We also mentioned that they had useful purposes
• For example
• malloc returns a void
• It is just returning a pointer to some newly
  allocated space
• The space has no associated type
• The programmer must make that association by
  assigning the returned value into the appropriate
  type pointer
• Since this type isn't known to the write of
  malloc it returns the generic void

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Another Example

• One of the problems with the standard swap( )
  function is that it is "hard-wired" to a
  particular type (i.e. we must know ahead of time
  that we are swapping two ints, or two floats,
  etc).
• Write a swap( ) function that will swap two items
  of arbitrary size. Your function will need to
  accept a third parameter, which is the sizeof the
  two items to be swapped. Do not use any library
  calls in your solution.

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• void swap(void a, void b, size_t size)
• char ap a
• char bp b
• int i
• for(i0 iltsize i)
• char t api
• api bpi
• bpi t

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Questions?
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Function Pointers
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What is...

• A variable?
• A symbol representing an address where we store
  something
• An array variable?
• A symbol representing an address where a block of
  memory has been reserved
• A function name?
• A symbol representing the address of a piece of
  code which we can jump to (and we normally expect
  it will contain code that will return control to
  the caller.)
• Assembly language for int fact(int n)
• fact save sp, -112, sp Beginning of
• fact function

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If we...

• Want to store the address of a variable we use a
  pointer (e.g. int ip)
• Same is true for holding the address of a
  function
• int fi(void) / Function that returns an int /
• int pfi(void) / Function that returns a
  pointer to
• an int, should be fpi /
• (int )fpi(void) / What compiler sees /
• int (pfi)(void) / Declaring pfi to be a
  pointer to
• a function! /

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Using it...

• int fi(void) / Function that returns an int
  /
• int fpi(void) / Function that returns a
  pointer /
• / to an int
  /
• int (pfi)(void) / Declaring pfi to be a
  pointer /
• to a function!
  /
• pfi fi
• pfi fi() / NO NO NO NO NO NO NO NO NO
  NO /
• Notice similarity to
• int ia10
• int ip
• ip ia

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Using it (Part Deux)...

• i fi()
• i pfi() ALL THE SAME!!!
• i pfi()
• Notice similarity to
• int ia10
• int ip
• ip ia

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But what good is a function pointer?

• Say you are writing a general purpose sorting
  function.
• You want it to be able to sort anything
• Numbers
• Strings
• ???
• Obviously comparing numbers and strings calls for
  two different techniques
• What if we write functions that do the comparison
  we need
• A function to compare numbers
• A function to compare strings
• A function to compare ???

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But what good is a function pointer?

• Now when we call the function to do the sorting
  we pass in a pointer to the appropriate function
  for the type of data we have!
• Or
• Imagine we are writing a routine to traverse a
  data structure
• What exactly do we do at each node?
• print it?
• modify it?
• test it?

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But what good is a function pointer?

• What if we passed into our traversal routine a
  pointer to a function which did what needed to be
  done???
• Advantage No need to continually modify our
  traversal routine. It's now generic!

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Basic Treenode with Root Pointer

• struct treenode
• char itemName
• int itemCount
• struct treenode left, right
• struct treenode root NULL
• int totalItems

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Post Order Traversal

• void
• postOrder( struct treenode theNode,
• void ( doThis )( struct treenode ) )
• if( theNode NULL )
• return
• postOrder( theNode-gtleft, doThis )
• postOrder( theNode-gtright, doThis )
• doThis( theNode )

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Closer Look

• void
• postOrder( struct treenode theNode,
• void ( doThis )( struct treenode ) )
• Note This function "postOrder" take two
  parameters
• A pointer to a treenode
• A pointer to a function

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Closer Look

• void
• postOrder( struct treenode theNode,
• void ( doThis )( struct treenode ) )

Function Pointer
Parameter list for function
Return value of function
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Counting Function

• void
• countItems( struct treenode theNode )
• totalItems theNode-gtitemCount

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Printing Function

• void
• printItems( struct treenode theNode )
• PRINTF(( "s d\n", theNode-gtitemName,
  theNode-gtitemCount ))

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Less than 5 items Function

• void
• checkItems( struct treenode theNode )
• if( theNode-gtitemCount lt 5 )
• PRINTF(( "s\n", theNode-gtitemName ))

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Clear Items Function

• void
• clearItems( struct treenode theNode )
• theNode-gtitemCount 0

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Free Nodes Function

• void
• freeNodes( struct treenode theNode )
• free( theNode-gtleft )
• free( theNode-gtright )
• free( theNode-gtitemName )

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Main Function

• int
• main( void )
• root buildTree( )
• totalItems 0 / Global /
• postOrder( root, countItems )
• PRINTF(( "There are d items.\n",
• totalItems ))

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Main Function

• PRINTF(( "List of all the items and"
• " how many there are\n" ))
• postOrder( root, printItems )
• PRINTF(( "The following items are in"
• " short supply\n" ))
• postOrder( root, checkItems )
• / clear itemCount for all items /
• postOrder( root, clearItems )

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Main Function

• / free all nodes /
• postOrder( root, freeNodes )

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NOTE

• Function Header
• void
• postOrder( struct treenode theNode,
• void ( doThis )( struct treenode ) )
• Function Call
• doThis( theNode )
• Function Header
• void
• countItems( struct treenode )
• Function Call
• postOrder( root, countItems )

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Questions?
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