Date post: | 22-Dec-2015 |
Category: |
Documents |
View: | 232 times |
Download: | 0 times |
Why pointers? - low-level, but efficient manipulation of memory
- dynamic objects
Objects whose memory is allocated during program execution. (Dynamic objects can survive after the function ends in which they were allocated).
Dynamic objects allow flexible-sized arrays and lists
Computer Memory and Variables A variable is a symbolic name assigned to some memory
storage The size of this storage depends on the type of the variable:
one byte char, 4 bytes int The name of variable is in ‘symbol table’ by compiler: each
symbol is associated with its type, location, scope …
So, the compiler manages its location good for programmers!
100100…… ……Memory address: 1024
……1020
Symbol table
a: int, 1024, …
int a=100;
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, so to ‘manipulate’ it!
100100…… …… 10241024 ……Memory address: 1024 10032
……1020
Integer a Pointer p
Object whose value represents the location (memory address) of another object
int a;
int* p;
Define a pointer type variable
Examples of (uninitialized) pointers
int* ip; char* s;
int *ip; // ip is a pointer to an int
char *s; // s is a pointer to a char
TypeName *variablename; int *p;
Each type of object (variable) has a pointer type, therefore a pointer variable of that type.
TypeName* variablename; int* p;
equivalent
Getting an address: address operator &
int a=100; “&a” “the address of a”
100100…… …… …… ……Memory address: 1024
int a = 100;
cout << a; 100
Cout << &a; 1024
……1020
a
Store the address in a pointer variable
The value of pointer p is the address of variable a
1001008888 …… 10241024 ……Memory address: 1024 10032
……1020
a p
int a = 100;
int* p = &a;
cout << a << " " << &a <<endl;
cout << p << " " << &p <<endl;
Result is:
100 1024
1024 10032
p is pointing to a
A pointer is also a variable, so it has its own memory address
Dereferencing Operator * We can access to the value stored in the variable
pointed to by preceding the pointer with the “star” operator (*),
1001008888 …… 10241024 ……Memory address: 1024 10032
……1020
int a = 100;
int* p = &a;
cout << a << endl;
cout << &a << endl;
cout << p << " " << *p << endl;
cout << &p << endl;
a p
*p gives 100
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 << *ptr << j << endl; // display "12"
Summary on two operators * and &
* has two usages:
- pointer
- dereferencing
& has two usages:
- getting address
- reference (‘call by ref’, see later)
Null Address
0 is a pointer constant that represents the empty or null address Indicates that pointer is not pointing to storage of a
valid object Cannot dereference a pointer whose value is null
int* ptr;
ptr = 0;
cout << *ptr << endl; // invalid, ptr
// does not point to
// a valid int
Example#include <iostream> using namespace std;int main (){
int value1 = 5, value2 = 15; int* p1;int* p2; p1 = &value1; // p1 = address of value1p2 = &value2; // p2 = address of value2 *p1 = 10; // value pointed to by p1=10 *p2 = *p1; // value pointed to by p2= value // pointed to by p1 p1 = p2; // p1 = p2 (pointer value copied) *p1 = 20; // value pointed to by p1 = 20 cout << "value1==" << value1 << "/ value2==" << value2; return 0;
}
Result:
Value1 is 10
Value2 is 20
Another Pointer Exampleint a = 3;char s = ‘z’;double d = 1.03;int* pa = &a;char* ps = &s;double* pd = &d;cout << sizeof(pa) << sizeof(*pa) << sizeof(&pa) << endl;cout << sizeof(ps) << sizeof(*ps) << sizeof(&ps) << endl;cout << sizeof(pd) << sizeof(*pd)
<< sizeof(&pd) << endl;
Writing pointer type properly in C++ …
int *a, *b;
a, b are both integer pointers
int* a, b;
a is integer pointer, b is just integer!
typedef int* IntPt;
IntPt a, b;
typedef int MyInt;
MyInt k;int k;
int* a;
int* b;?
Recommended:
Traditional Pointer Usagevoid swap(char* ptr1, char* ptr2){
char temp = *ptr1;*ptr1 = *ptr2;*ptr2 = temp;
}int main() {
char a = 'y';char b = 'n';swap(&a, &b);cout << a << b << endl;return 0;
}
Use pass-by-value of pointers to ‘change’ variable values
C language does not have ‘call by reference’, only ‘call by value’!
Reference: X&
int& b a; b is an alternative name for a
void f(int& b) {};
int main() {
int a;
f(a);
}
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 << a << b << endl;return 0;
}
y, z are ‘references’, only names, not like ptr1, ptr2 that are variables
it is also possible (not recommended)…
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 << a << b << endl;return 0;
}
This is just an alternative way of writing ‘reference’.
Arrays are pointers!
The name of an array points only to the first element not the whole array.
1000
1012
1016
1004
1008
#include <iostream>Using namespace std;
void main (){// Demonstrate array name is a pointer constantint a[5];cout << "Address of a[0]: " << &a[0] << endl << "Name as pointer: " << a << endl;
}
/* result:Address of a[0]: 0x0065FDE4Name as pointer: 0x0065FDE4*/
Array name is a pointer constant
Result is:
2
2
2
#include <iostream>
Using namespace std;
void main(){
int a[5] = {2,4,6,8,22};
cout << *a << " "
<< a[0] << " "
<< *(&a[0]);
..."
} //main
2
4
8
6
22a[4]
a[0]
a[2]
a[1]
a[3]
Dereference of an array name
This element is called a[0] or *a
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 <iostream>
Using namespace std;
void main(){
int a[5] = {2,4,6,8,22};
int* p = a;
int i = 0;
cout << a[i] << " "
<< *p;
...
}
2
4
8
6
22a[4]
a[0]
a[2]a[1]
a[3]
a p
a
Array name as pointer
2
2
Multiple Array PointersBoth a and p are pointers to the same array.
2 2
4 4
#include <iostream>
Using namespace std;
void main(){
int a[5] = {2,4,6,8,22};
int* p = &a[1];
cout << a[0] << " "
<< p[-1];
cout << a[1] << " "
<< p[0];
...
}
2
4
8
6
22a[4]
a[0]
a[2]
a[1]
a[3]
p
P[0]
A[0]
Pointer ArithmeticGiven a pointer p, p+n refers to the element that is
offset from p by n positions.
2
4
8
6
22
a
a + 2
a + 4
a + 3
a + 1 p
p + 2
p + 3
p - 1
p + 1
*(a+n) is identical to a[n]
Dereferencing Array Pointers
a[3] or *(a + 3)
24
86
22
a
a + 2
a + 4a + 3
a + 1a[2] or *(a + 2)a[1] or *(a + 1)
a[0] or *(a + 0)
a[4] or *(a + 4)
Summary * two usages:
- pointer type definition: int a;
int* p;
- dereferencing: *p is an integer variable if
p = &a;
& 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 a=10;
int b=100;
int* p;
int* q;
p=&a;
q=&b;
p = q;
*p = *q;
?
?