Pointers

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Pointers

• ACS 169

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Pointers

• Pointer
• a data type
• stores a memory address
• points to whatever the memory location contains
• A pointer is a variable that can store a memory
  address.
• The type of item pointed to by a pointer variable
  is the target type.

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Examples

• Declaring Pointers type name
• Pointer Declarations
• int ptr
• char cptr
• double dptr

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Pointers

• int ptrint v 5ptr v
• The operator is called the address operator
• either of the following assignments will store
  the value of 10 in the variable v
• v 10
• ptr 10

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Pointers Dereferencing cont.

• So far the asterisk is used in two ways
• 1. int ptr
• 2. ptr 10
• the first declares the variable ptr as a pointer
  of type integer but it does not make it point to
  any memory address.
• the second assigns the value 10 to the memory
  location pointed at by ptr. dereferencing

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Question

• Explain each line in the following code
• int ptr
• int number 42
• ptr number
• ptr 0
• cout ltlt ptr ltltendl
• cout ltlt number ltlt endl

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Pointer Graph Representation
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Dereferencing
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Pointers cont.

• Explain the following code
• int p1
• int p2
• p1 count
• int p2
• p2 p1
• p2 p1

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Quiz 2
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Dynamic Variables

• Dynamic Variables (DVs) are different than normal
  variables in two aspects
• DVs are not declared (have no identifiers, no
  variable names)
• DVs are created during execution phase of a
  program and not during compilation, the keyword
  new is used for this purpose.

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Dynamic Variables cont.

• Example
• int ptr
• ptr new int
• this statement creates a DV of type integer and
  uses ptr to point to it. There is no identifier
  (name) for the variable pointed at by ptr

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Dynamic Variables cont.

• The creation of new DVs is called memory
  allocation and the allocated memory is called
  dynamic memory
• ptr new int
• Makes ptr point to a newly allocated integer
  variable from dynamic memory.

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Dynamic Variables cont.

• int ptr
• ptr new int
• ptr 33

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Dynamic Variables cont.

• int n
• n new int(17)
• MyType p
• p new MyType
• or
• p new MyType(32.0, 17)

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The new operator Objects

• throttle t_ptrt_ptr new throttle(50)
• calls the throttle constructor with an integer
  argument

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Dynamic Arrays.

• To declare a dynamic array use
• int ptr
• ptr new int10
• the new operator allocates a dynamic array of
  10 integers and makes ptr point to its first
  element.

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Dynamic Arrays cont.

• the statement
• ptr5 33
• will store the value 33 as the 6th element in
  the array pointed to by ptr

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Heap

• When new allocates a dynamic variable or dynamic
  array, the memory comes from a location called
  the programs heap (also called the free store).
• bad_alloc exception is thrown when new attempts
  to allocate memory and fails.

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Question

• Determine what the following code will do
• int array_size
• int numbers
• cout ltlt how many numbers do you have?
• cin gtgt array_size
• numbers new intarray_size

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Answer

• The operator new is used to dynamically allocate
  an array of size array_size that the user enters
  interactively.

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Question

• Who should initialize the components of a
  dynamically allocated array whose components are
  of a class data type ?
• Ans the default constructor will initialize all
  components of the array

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Delete Operator

• It is an efficient practice to release any heap
  memory that is no longer needed.
• The delete operator is used in C to release
  memory to the heap that is no longer needed.

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Delete operation

• Examples int ptr ptr new int delete
  ptr ptr NULL
• int p p new int30 delete p

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Pointers as value parameters

• int main_ptr
• main_ptr new int
• make_it_42(main_ptr)
• void make_it_42(int my_ptr)
• my_ptr 42

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Pointers as value parameters

• The following function prototype
• void make_it_42(int my_ptr)
• the int indicates that the parameter is of
  data type integer pointer.
• the parameter is a value parameter because of
  the absence of the operator.

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Array Parameters

• void make_it_all_42( double num, size_t n)
• .
• double numbers
• num new double10
• make_it_all_42(num, 10)
• .
• void make_it_all_42(double num, int n)
• for(int i0 iltn i) numi 42

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• void make_it_all_42(double num, int n)
• for(size_t i0 iltn i)
• numi 42
• void make_it_all_42(double num, int n)
• for(size_t i0 iltn i) numi 42

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Array parameters cont.

• A parameter that is a pointer or array may
  include the const keyword. No changes to the
  actual parameter or the array are possible in
  this case
• bool is_42(const int my_ptr)
• double average(const double data, )

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Pointers Reference Parameters

• Sometimes a function needs to change a pointer
  parameter so that the pointer points to a new
  location.
• Example
• void alloc_doubles(double ptr, int n)

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Pointer Reference Parameters

• void alloc_doubles(double ptr, int n)
• double numbers
• int array_size
• alloc_doubles(numbers, array_size)
• void alloc_doubles(double ptr, int n)
• cingtgt n
• ptr new doublen

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Pointer Arithmetic

• The only legal arithmetic operators on pointers
  are adding or subtracting an integer, or
  subtracting one pointer from another.

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Pointer Arithmetic

• In C, pointer arithmetic is automatically done
  in units of the pointer's underlying base type.
• Adding 1 to a pointer to an array element gives a
  pointer to the next element - regardless of
  whether we have an array of ints, an array of
  doubles, or an array of any other type.

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Pointer Arithmetic

• int ar10
• ar i is a pionter to the ith element beyond ar
•  
• ari is equivalent to ar i
•  
• ari is equivalent to (ar i)

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What is the output?

• int main()
• int array
• array new int10
• array 33
• (array 3) 14
• coutltltarray0ltltarray3
• return 0