Pointers

1
Pointers

• Day 8

2
What is a Pointer?

• A pointer is a type of variable that holds a
  memory address.
• In our compiler, memory addresses are 32 bits (4
  bytes).
• Anything can be at the address stored in a
  pointer, e.g. char, short, int, a user defined
  class, a string, an array, a function, etc.

3
Using Pointers and Addresses

• Declare a pointer using the indirection
  operator
• Find an address using the address of operator

int MyVar 5 // an integer int pMyVar // a
pointer to an integer
cout ltlt MyVar ltlt MyVar cout ltlt address of
MyVar ltlt MyVar pMyVar MyVar
4
The Value at the Address

• We retrieve the value of the data at the address
  in a pointer using the indirection operator

int MyVar 5 int pMyVar MyVar cout ltlt
value of MyVar ltlt MyVar cout ltlt value
at pMyVar ltlt pMyVar cout ltlt address of
MyVar ltlt MyVar cout ltlt address in pMyVar
ltlt pMyVar
5
Writing to the Address

• You can write to the address that the pointer
  points to

int MyVar 5 int pMyVar MyVar pMyVar
10 // MyVar is set to 10
6
Pointer Initialization

• Pointers should be initialized to an object, or
  to null (0)

int MyVar 5 int pMyVar MyVar int
pOther 0 pOther NULL
7
new and The "Free Store"

• You allocate memory on the free store in C by
  using the new keyword.
• new is followed by the type of the object that
  you want to allocate.
• The return value from new is a memory address, it
  must be assigned to a pointer.
• If new cannot allocate memory, it returns the
  null pointer (0).

8
delete and The "Free Store"

• When you finish with allocated memory, you must
  free it by calling delete.

int pNewVar pNewVar new int if (pNewVar !
NULL) pNewVar 5 cout ltlt "pNewVar " ltlt
pNewVar ltlt endl delete pNewVar pNewVar
NULL
9
The points-to operator -gt

• To access data members and functions of an object
  through a pointer, we may use the shorthand -gt

Cat Frisky Cat pFrisky Frisky Frisky.SetAg
e(5) cout ltlt "Frisky's age " ltlt
Frisky.GetAge() (pFrisky).SetAge(6) cout ltlt
"Frisky's age " ltlt (pFrisky).GetAge() pFrisky-gt
SetAge(7) cout ltlt "Frisky's age " ltlt
pFrisky-gtGetAge()
10
const Pointers, andPointers to const

• You can declare const pointers, which cant be
  reassigned to point to other objects
• You can declare pointers to const objects, which
  cant be used to change the objects to which they
  point

int iOne, iTwo, MyVar int const pTwo
iOne pTwo iTwo // cant reassign const
pointer const int pThree MyVar pThree 5
// cant change const variable MyVar 5 //
not an error