Review on pointers and dynamic objects

1

• Review on pointers and dynamic objects

2
Memory Management

• Static Memory Allocation
• Memory is allocated at compiling time
• Dynamic Memory
• Memory is allocated at running time

int a200
int n cin gtgt n an???
3
Static vs. Dynamic Objects

• Static object
• Memory is acquired automatically
• Memory is returned automatically when object goes
  out of scope

• Dynamic object
• Memory is acquired by program with an allocation
  request
• new operation
• Dynamic objects can exist beyond the function in
  which they were allocated
• Object memory is returned by a deallocation
  request
• delete operation

4
Why pointers?

• Dynamic objects are implemented or realized
  by pointers which are parts of low-level
  physical memory
• We dont like it, but can not avoid it.
• Low level languages directly manipulate them
• High level languages want to hide the pointers
  (conceptually remove them)

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Pointers

• A pointer is a variable used for storing the
  address of a memory cell.
• We can use the pointer to reference this memory
  cell

Memory address
1024
1020
10032


1024


100
a
int a int p
Integer a
Pointer p
6
Getting an address address operator
int a100 a ? the address of a
Memory address
1024
1020
100





a
int a 100 cout ltlt a ? 100 Cout ltlt a ?
1024
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Dereferencing Operator

• We can access to the value stored in the variable
  pointed to by preceding the pointer with the
  star operator (),

Memory address
1024
10032
1020
100
88

1024


a
p
int a 100 int p a cout ltlt a ltlt endl cout
ltlt a ltlt endl cout ltlt p ltlt " " ltlt p ltlt
endl cout ltlt p ltlt endl
p gives 100
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Pointer to pointer
int a int p int q
a 58 p a q p
a, p, and q are the same object whose value is
58! But q a is illegal!
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• An asterisk () has two usages
• In a definition, an asterisk indicates that the
  object is a pointer.
• char s // s is of type pointer to char
• (char s is possible)
• In expressions, an asterisk before a pointer
  indicates the object the pointer pointed to,
  called dereferencing
• int i 1, j
• int ptr // ptr is an int pointer
• ptr i // ptr points to i
• j ptr 1 // j is assigned 2
• cout ltlt ptr ltlt j ltlt endl // display "12"

10
Writing pointer type properly in C
int a int b
?
int a, b a, b are both integer pointers
int a, b a is integer pointer, b is just
integer!
typedefine int MyInt MyInt k
int k
I dont like this!
Recommended!!!
typedefine int IntPt IntPt a, b
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Summary
has two usages - pointer type
definition int a
int p -
dereferencing p is an integer variable
if p a has two usages
- getting address p a -
reference int b a
b is an alternative name for a First
application in passing parameters (swap example)
int a10 int b100 int p int q P a Q
b
p q p q
?
?
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Pointers and References
Reference (implemented as a (const) pointer) is
an abstraction, Not available in C, only in C.
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Pointer vs. Reference

• A pointer can be assigned a new value to point at
  a different object, but a reference variable
  always refers to the same object. Assigning a
  reference variable with a new value actually
  changes the value of the referred object.
• int p
• int m 10
• int j m //valid
• p m //p now points at m
• int n 12
• j n //the value of m is set to 12. But j still
  refers to m, not to n.
• cout ltlt value of m ltlt m ltltendl //value of m
  printed is 12
• n 36
• Cout ltlt value of j ltlt j ltlt endl //value of
  j printed is 12
• p n

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• A reference variable is different from a pointer

int x10 int ref Ref x
int x10 int ref x
int ref
x
10
10
x
ref
ref
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Traditional Pointer Usage

• void swap(char ptr1, char ptr2)
• char temp ptr1
• ptr1 ptr2
• ptr2 temp
• int main()
• char a 'y'
• char b 'n'
• swap(a, b)
• cout ltlt a ltlt b ltlt endl
• return 0

Uese pass-by-value of pointers to change
variable values C language does not have call by
reference!
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Pass by Reference (better than pointers)

• void swap(char y, char z)
• char temp y
• y z
• z temp
• int main()
• char a 'y'
• char b 'n'
• swap(a, b)
• cout ltlt a ltlt b ltlt endl
• return 0

y, z are references, only names, not like ptr1,
ptr2 that are variables
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Pointers and Arrays

• Double faces of an array int a10
• a is the name of an array,
• a is also is a constant pointer to its first
  element

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Pointers and Arrays
The name of an array points only to the first
element not the whole array.
2
a0
a
4
a1
6
a2
8
a3
22
a4
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Dereference of an array name
include ltiostreamgt Using namespace std void
main() int a5 2,4,6,8,22 cout ltlt a ltlt
" " ltlt a0 ltlt " " ltlt
(a0) ..." //main
2
a0
Result is 2 2 2
4
a1
6
a2
8
a3
22
a4
This element is called a0 or a
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Array name as pointer

• To access an array, any pointer to the first
  element can be used instead of the name of the
  array.

We could replace p by a
include ltiostreamgt Using namespace std void
main() int a5 2,4,6,8,22 int p
a int i 0 cout ltlt ai ltlt " " ltlt
p ...
2
2
a
21
dynamic objects
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Summary
Static variables (objects)
Dynamic variables (objects)
A (direct) named memory location
A static part (pointer) (indirect) nameless
memory location (dynamic part)
int a a 20
int pa pa new int pa 20
20
20
a
pa
static
dynamic
static
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Dynamic array
Simple dynamic variable
int p new int p 10 delete p
int p new int100 for (i1ilt100i) pi
10 delete p
10
10
10
10
p
p

• delete two actions
• Return the object pointed to
• Point the pointer p to NULL

delete p is not sufficient for an array!!!