CSci 152: Programming II Fall 2004

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CSci 152 Programming IIFall 2004

• Pointers and Dynamic Memory

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• THE POINTER DATA TYPE AND POINTER VARIABLES
• Pointer variable A variable whose content is an
  address (that is, a memory address).
• Declaring Pointer Variables
• The general syntax of declaring a pointer
  variable is
• dataType identifier
• int p
• char ch

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• The statement
• int p
• is equivalent to the statement
• int p
• which is equivalent to the statement
• int p
• The character can appear anywhere between the
  data type name and the variable name.
• int p, q
• Only p is the pointer variable, not q. Here q is
  an int variable.
• To avoid confusion, we prefer to attach the
  character to the variable name.

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• THE ADDRESS OF OPERATOR ()
• In C, the ampersand, , called the address of
  operator, is a unary operator that returns the
  address of its operand.
• Given the statements
• int x
• int p
• the statement
• p x
• assigns the address of x to p. That is, x and
  the value of p refers to the same memory
  location.

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• THE DEREFERENCING OPERATOR ()
• C also uses as a unary operator.
• When used as a unary operator, , commonly
  referred to as the dereferencing operator or
  indirection operator, refers to the object to
  which its operand (that is, a pointer) points
• int x 25
• int p
• p x //store the address of x in p
• The statement
• coutltltpltltendl
• prints the value of x. Also the statement
• p 55
• will store 55 in the memory location pointed to
  by p, that is, in x.

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• int p
• int num
• p is a pointer variable of the type int and num
  is a variable of type int .

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• num 78

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• p num

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• p 24

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• 1. p, p, and p all have different meanings.
• 2. p means the address of pthat is, 1200 (in
  Figure 15-4).
• 3. p means the content of p (1800 in Figure
  15-4).
• 4. p means the content (24 in Figure 15-4) of
  the memory location (1800 in Figure 15-4) pointed
  to by p (that is, pointed to by the content of
  memory location 1200).

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• Example 15-1
• int p
• int x

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• After the statement
• x 50

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• After the statement
• p x

• After the statement
• p 38

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• 1. A declaration such as
• int p
• allocates memory for p only, not for p.
• 2. Assume the following
• int p
• int x
• a. p is a pointer variable
• b. Content of p only points to a memory location
  of the type int
• c. Memory location x exists and is of the type
  int.
• The assignment
• p x
• is legal. After the execution of this statement
  p is valid and meaningful.

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• Example 15-2
• //Chapter 15 Example 15-2
• include ltiostreamgt
• using namespace std
• int main()
• int p
• int x 37
• coutltlt"Line 1 x "ltltxltltendl //Line 1
• p x //Line 2
• coutltlt"Line 3 p "ltltp
• ltlt", x "ltltxltltendl //Line 3

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• coutltlt"Line 8 Value of the memory location "
• ltlt"pointed to by p "ltltpltltendl //Line 8
• coutltlt"Line 9 Address of x
  "ltltxltltendl //Line 9
• coutltlt"Line 10 Value of x
  "ltltxltltendl //Line 10
• return 0
• Sample Run
• Line 1 x 37
• Line 3 p 37, x 37
• Line 5 p 58, x 58
• Line 6 Address of p 006BFDF4
• Line 7 Value of p 006BFDF0
• Line 8 Value of the memory location pointed to
  by p 58
• Line 9 Address of x 006BFDF0
• Line 10 Value of x 58

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• CLASSES, STRUCTS, AND POINTER VARIABLES
• You can also declare pointers to other data
  types, such as classes and structs.
• struct studentType
• char name26
• double gpa
• int ssn
• char grade
• studentType student
• studentType studentPtr
• studentPtr student

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• student.gpa 3.9
• (studentPtr).gpa 3.9
• store 3.9 in the component gpa of the object
  student.
• Since dot has a higher precedence than the
  dereferencing operator, parentheses are
  important.
• C provides another operator, called the member
  access operator arrow, -gt.
• The syntax for accessing a class (struct) member
  using the operator -gt is
• pointerVariableName-gtclassMemberName
• The following statements are equivalent.
• (studentPtr).gpa 3.9
• studentPtr-gtgpa 3.9

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• Example 15-3
• class classExample
• public
• void setX(int a)
• void print() const
• private
• int x
• void classExamplesetX(int a)
• x a
• void classExampleprint() const
• coutltlt"x "ltltxltltendl

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• int main()
• classExample cExpPtr //Line 1
• classExample cExpObject //Line 2
• cExpPtr cExpObject //Line 3
• cExpPtr-gtsetX(5) //Line 4
• cExpPtr-gtprint() //Line 5
• return 0
• Output
• x 5

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• INITIALIZING POINTER VARIABLES
• p NULL
• p 0

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• DYNAMIC VARIABLES
• Variables that are created during program
  execution are called dynamic variables.
• With the help of pointers, C creates dynamic
  variables.
• C provides two operators, new and delete, to
  create and destroy dynamic variables,
  respectively.
• When a program requires a new variable, the
  operator new is used when a program no longer
  needs a dynamic variable, the operator delete is
  used.
• In C, new and delete are reserved words.

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• The syntax to use the operator new is
• new dataType //to allocate a single
  variable
• new dataTypeintExp //to allocate an array of
  variables
• where intExp is any expression evaluating to a
  positive integer.
• The operator new allocates memory (a variable) of
  the designated type and returns a pointer to
  itthat is, the address of this allocated memory.
  
• The allocated memory is uninitialized.

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• int p
• char q
• int x
• The statement
• p x
• stores the address of x in p. No new memory is
  allocated.
• The statement
• p new int
• creates a variable during execution somewhere in
  the memory, and stores the address of the
  allocated memory in p.
• The allocated memory is accessed via pointer
  dereferencing, p.
• The statement
• q new char16

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• int p //p is a pointer of the type int
• char name //name is a pointer of the type char
• string str //str is a pointer of the type
  string
• p new int
• p 28 //stores 28 in the allocated memory
• name new char5
• strcpy(name, "John") //stores John in name
• str new string //allocates memory of the type
  string
• //and stores the address of the
• //allocated memory in str
• str "Sunny Day" //stores the string "Sunny
  Day" in
• //memory pointed to by str

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• The C operator delete is used to destroy
  dynamic variables. The syntax to use the operator
  delete has two forms
• delete pointer //to destroy a single dynamic
  variable
• delete pointer //to destroy a dynamically
  created
• //array
• The statements
• delete p
• delete name
• deallocate memory referenced by the pointers p
  and name.

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• OPERATIONS ON POINTER VARIABLES
• Assignment
• Value of one pointer variable can be assigned to
  another pointer variable of the same type.
• Relational operations
• Two pointer variables of the same type can be
  compared for equality, and so on.
• Some limited arithmetic operations.
• Integer values can be added and subtracted from a
  pointer variable.
• Value of one pointer variable can be subtracted
  from another pointer variable.

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• int p, q
• The statement
• p q
• copies the value of q into p.
• The expression
• p q
• evaluates to true if both p and q have the same
  value.
• The expression
• p ! q
• evaluates to true, if both p and q point to
  different memory locations.

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• Arithmetic operations, that are allowed, differ
  from the arithmetic operations on numbers.
• int p
• double q
• char chPtr
• studentType stdPtr //studentType data type is
• //as defined before
• The memory allocated for an int variable is 4
  bytes, a double variable is 8 bytes, and a char
  variable is 1 byte. The memory allocated for a
  variable of the type studentType is 39 bytes.
• The statement
• p or p p1
• increments the value of p by 4 bytes.

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• The statements
• q
• chPtr
• increment the value of q by 4 bytes and the
  value of chPtr by 1 byte
• The statement
• stdPtr
• increments the value of stdPtr by 39 bytes.
• The statement
• p p 2
• increments the value of p by 8 bytes.

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• When an integer is added to a pointer variable,
  the value of the pointer variable is incremented
  by the integer times the size of the memory that
  the pointer is pointing to.
• When an integer is subtracted from a pointer
  variable, the value of the pointer variable is
  decremented by the integer times the size of the
  memory to which the pointer is pointing.

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• DYNAMIC ARRAYS
• An array created during the execution of a
  program is called a dynamic array.
• To create a dynamic array, we use the second form
  of the new operator.
• The statement
• int p
• declares p to be a pointer variable of the type
  int.
• The statement
• p new int10
• allocates 10 contiguous memory locations, each
  of the type int, and stores the address of the
  first memory location into p.

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• The statement
• p 25
• stores 25 into first memory location.
• p
• p 35
• stores 35 into the second memory location.
• C allows us to use array notation to access
  these memory locations.
• p0 25
• p1 35

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• int list10
• list is a pointer variable.
• We always want list to point the first array
  component.
• Any attempt to use increment or decrement
  operation on list will result in compile time
  error.
• If p is a pointer variable of the type int, then
  the statement
• p list
• copies the value of list into p. We are allowed
  to perform increment and decrement operations on
  p.
• An array name is a constant pointer.

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• Example 15-4
• int intList //Line 1
• int arraySize //Line 2
• coutltlt"Enter array size " //Line 3
• cingtgtarraySize //Line 4
• coutltltendl //Line 5
• intList new intarraySize //Line 6
• The statement at Line 1 declares intList to be a
  pointer of the type int
• The statement at Line 2 declares arraySize to be
  an int variable.
• The statement at Line 3 prompts the user to enter
  the size of the array.
• The statement at Line 4 inputs the array size
  into the variable arraySize.
• The statement at Line 6 creates an array of the
  size specified by arraySize, and the base address
  of the array is stored in intList.
• From this point on, you can treat intList just
  like any other array. For example, you can use
  the array notation to process the elements of
  intList and pass intList as a parameter to the
  function.

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• Functions and Pointers
• A pointer variable can be passed as a parameter
  to a function either by value or by reference.
• In C, to make a pointer a reference parameter
  in a function heading, appears before the
  between the data type name and the identifier.
• void example(int p, double q)
• .
• .
• .
• Both p and q are pointers. The parameter p is a
  reference parameter the parameter q is a value
  parameter.

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• Pointers and Function Return Value
• In C, a function can return a value of the type
  pointer.
• The return type of the function
• int testExp(...)
• .
• .
• .
• is a pointer of the type int.