Pointers and Dynamic Memory

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Chapter 5

• Pointers and Dynamic Memory

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Outline
The Pointer this dynamicClass Copy
Constructor The C Index Operator Matrices
Pointer Illustration Data Addresses in
Memory Declaring Pointer Variables Assigning
Values to Pointers Accessing Data with
Pointers Arrays and Pointers Operator
new Operator delete Illustrating the
Destructor Copy Constructor / Overloaded
Assignment Operator Declaration of dynamicClass
Objects
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- Pointers contain the address of data in
memory - Data is accessed by applying the
dereference operator
- Operators such as , , and apply to
pointers. - With such operators, pointers
can be used for algorithms involving
array traversal, but their primary
application is in the allocation and
maintenance of dynamic memory.
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Pointer Illustration
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Vertical and Horizontal View of Memory
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Data Addresses in Memory
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Declaring Pointer Variables

• Declare a pointer by stating the type followed by
  the variable name, but with a "" added
  immediately before the name.
• The pointer ptr is a variable whose value is the
  address of a data item of the designated type.

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Assigning Values to Pointers

• m is the address of the integer in memory. The
  assignment statement
• sets intPtr to point at an actual data item.
• The Figure illustrates the status of the two
  variables from the declaration statement and the
  resulting status after the assignment of a value
  to intPtr.

int m 50, intPtr
intPtr m
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Assigning Values to Pointers
int x 50, y 100, px x, py y
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Accessing Data with Pointers

• The operator is called the deference operator.
  It combines with pointer to allow access the
  contents referenced by the pointer.

int x 50, y 100, px x, py y px
py 2 //assign to x the value y 2 102 py
2 //double value of y to 200 (px)
//increase x from 102 to 103
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The Relationship Between Arrays and Pointers

• An Array name is a constant pointer to the first
  element in an array. It holds an address.
• int vals 4, 7, 11
• cout ltlt vals // displays 0x4a00
• cout ltlt vals0 // displays 4

starting address of vals 0x4a00
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Arrays and Pointers
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The Relationship Between Arrays and Pointers

• an Array name can be used as a pointer constant
• int vals 4, 7, 11
• cout ltlt vals // displays 4
• A Pointer can be used as an array name
• int valPtr vals
• cout ltlt valPtr1 //displays 7
• cout ltlt (valPtr1) //displays 7
• cout ltlt valPtr //displays address of vals

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Pointers in Expressions

• Given
• int vals 4,7,11, valPtr
• valPtr vals
• What is valPtr 1 ?
• It means (address in valptr) (1 size of an
  int)
• cout ltlt (valPtr1) //displays 7
• cout ltlt (valPtr2) //displays 11
• Must use ( ) in expression

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Array Access

• Array elements can be accessed in many ways
• Use of subscript and offset
  notation

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Array Access

• Conversion
• valsi is equivalent to (vals i)
• No bounds checking performed on array access,
  whether using array name or a pointer

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Pointer Arithmetic
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Pointers and Class Types

• The dereference and select operator, -gt, is used
  only with pointers to objects
• It accesses an object member by using the pointer
  name and member name as operands, rather than the
  deferenced object in parentheses and the dot
  operator

Rectangle box (2, 5), rectPtr // a 2 by 5
rectangle and a pointer rectPtr box //set
rectPtr to point at box cout ltlt(rectPtr).area()
//area of the box is 10 cout ltltrectPtr-gtarea() co
ut ltlt(rectPtr).setSides(7,10) //update
dimension of box cout ltltrectPtr-gtsetSides(7,10)
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Dynamic Memory Allocation

• Static memory allocation the compilation process
  creates an executable file in which the memory
  requirements for each variable and object are
  defined.
• Dynamic memory allocation A program can allocate
  storage from additional memory resource, heap,
  for a variable while it is running
• Use the new operator to allocate memory
• double dblPtr
• dblPtr new double
• new returns the address of a memory location if
  it is successful or 0 ( NULL ) if not.

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Dynamic Memory Allocation

• You can use new to dynamically allocate an
  array
• double arrayPtr
• cout ltlt "How many real numbers? "
• cin gtgt count
• arrayPtr new doublecount //count is a
  variable!
• You can use subscript or offset notation to
  access the array elements.
• for (int i 0 i lt count i)
• arrayptri i i
• or
• for (int i 0 i lt count i)
• (arrayptr i) i i

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Operator new
p new time24 // p is 0000
(midnight) q new time24(8, 15) // q is
815 AM
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Releasing Dynamic Memory

• Use delete to free dynamic memory
• delete fPtr //single element
• Use to free a dynamic array
• delete arrayPtr //array of elements
• Only use delete with dynamically allocated
  memory!

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Classes using dynamic memeory

• Memory management dynamically allocate
  deallocate memory from heap
• Memory leak allocate without corresponding
  deallocate
• Dynamic allocation done by constructor
• Dynamic deallocation done by destructor
• Example Page 235, dynamicClass
• Constructor
• Example 5-8

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Illustrating the Destructor

• Example page 238-239, Program 5-1

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Copy Constructor / Overloaded Assignment Operator

• Assignment and initialization for objects that
  allocate dynamic data, the class must provide
  special member functions, called copy constructor
  and the overloaded assignment operator, that
  enable the run time system to execute the
  operations.

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Declaration of dynamicClass Objects
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Action of using the default assignment operator
for the statement objBobjA
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Design of the overloaded assignment operator,
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The this Pointer

• this predefined pointer available to a classs
  member functions
• Always points to the instance (object) of the
  class whose function is being called
• Is passed as a hidden argument to all non-static
  member functions
• Can be used to access members that may be hidden
  by parameters with same name

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The Pointer this
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this Pointer Example

• class SomeClass
• private
• int num
• public
• void setNum(int num)
• this-gtnum num
• ...

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dynamicClass Copy Constructor Algorithm
PP 247 Program 5-2
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Overloaded Operator

• Can create classes that behave like arrays,
  provide bounds-checking on subscripts
• Must consider constructor, destructor
• Overloaded returns a reference to object, not
  an object itself

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The C Index Operator
arri 30 // arri is the //address into
which 30 is copied t arri 4 // add 4 to
the //value of the element at arri
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- vector implementation - The
miniVector class illustrates the key
points. 1) It allocates dynamic memory using
destructor copy
constructor overloaded
assignment operator 2) It implements
push_back() Therefore it must
control vector capacity in order to
minimize dynamic memory reallocation. 3) It
allows access to elements by using an
index Therefore
the class implements an overloaded index
operator
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- Two dimensional arrays in C - have the
same problems as one-dimensional
arrays 1) fixed size 2) no size
attribute 3) If it is a function argument, it
is necessary to specify the number of
columns as a constant.
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Matrices

• A Matrix is a two-dimensional array that
  corresponds to a row-column table of entries of a
  specified data type.
• Matrices are referenced using a pair of indices
  that specify the row and column location in the
  table.

Example The element mat03 is 2 The element
mat12 is 4.