Pointers

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Pointers
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Variables and Pointers are like People

• Each person has a name and an address and
  belongings!
• A friend that depends upon you, uses your
  address, and has access to your belongings, is a
  pointer!

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Declaring and Initializing Pointers

• int num 45 base integer variable
• declared and initialized
• int num_ptr pointer to an integer
• declared ( still pointing to garbage)
• num_ptr num pointer to an integr
• initialized to a valid address

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What do Pointers Point To?

• A newly declared pointer points to garbage.
• A pointer MUST be initialized by assigning the
  address of a valid variable to it. Otherwise, it
  is dangerous to use it, it could be pointing
  anywhere, we dont know where!

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How are Pointers Different from Other Variables?

• Each variable has a name (num1, var1)
• Each variable has a location (address)
• Each variable has contents
• A pointer has a name (num1_ptr,var1_ptr)
• A pointers contents is the address of another
  variable!

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Pointer to integer int

• int num 45
• int num_ptr num
• variable
  pointer
• Name num num_ptr
• Contents 45 B1021205
• Address B1021205

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Pointer to char char

• char letter A variable
• char ltr_ptr letter pointer
• variable pointer
• Name letter ltr_ptr
  
• Contents A CF30A015
• Address CF30A015

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Pointer to an array datatype

• char my_string10 My String
• char string_ptr my_string
• NOTE the name of an array without the subscript
  holds the address to the first position of that
  array. It is a pointer, too.

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Pointer to struct struct

• Struct my_tagS
• int x, int y
• struct my_tagS struct1 4, 5
• struct my_tagS struct_ptr struct1

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Pointer to struct (arrow notation)

• struct_ptr-gtx 10
• (arrow notation replaces dot notation when using
  a POINTER to a struct)

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Accessing the Pointer Address (Pointer Arithmetic)

• adds the sizeof an int to the address in the
  pointer (the pointer subsequently points to the
  next integer space in memory).
• subtracts the sizeof an int, moves the pointer to
  the previous location in memory.

• num_ptr
• num_ptr 1
• num_ptr--
• num_ptr - 1

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Accessing Variable Values with Pointers
(Dereferencing)

• num_ptr holds an address
• num_ptr accesses the contents
• (asterisk) Dereferences a pointer
• num_ptr 3 Wrong - this tries to assign an
  address location of 3 to the pointer.
• num_ptr 3 Correct - changes the contents
  pointed to by num_ptr to 3.

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Passing Pointers as Parameters

• int my_int intvar pointer to be passed
• my_func( my_int ) function call
• void my_func ( int input_int ) new function
• declare a pointer using the (asterisk)
• pass the pointer without the (asterisk)
• receive the pointer into a new variable in the
  function header using the (asterisk)

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• Break
• Pointer Lab