![6
Recall that . . .
char str [ 8 ];
• str is the base address of the array.
• We say str is a pointer because its value is an address.
• It is a pointer constant because the value of str itself
cannot be changed by assignment. It “points” to the
memory location of a char.
str [0] [1] [2] [3] [4] [5] [6] [7]
‘H’ ‘e’ ‘l’ ‘l’ ‘o’ ‘0’
6000](https://image.slidesharecdn.com/lecture2-221024085342-6b2cd5a2/85/Lecture2-ppt-6-320.jpg)
![14
ptr
Using a Pointer to Access the
Elements of a String
‘H’ ‘e’ ‘l’ ‘l’ ‘o’ ‘0’
char msg[ ] =“Hello”;
char* ptr;
ptr = msg;
*ptr = ‘M’ ;
ptr++;
*ptr = ‘a’;
msg
3000
3000
‘M’ ‘a’
3001](https://image.slidesharecdn.com/lecture2-221024085342-6b2cd5a2/85/Lecture2-ppt-14-320.jpg)
![Operator new Syntax
new DataType
new DataType [IntExpression]
• If memory is available, in an area called the heap (or free
store) new allocates the requested object or array, and
returns a pointer to (address of ) the memory allocated.
• Otherwise, program terminates with error message.
• The dynamically allocated object exists until the delete
operator destroys it.
22](https://image.slidesharecdn.com/lecture2-221024085342-6b2cd5a2/85/Lecture2-ppt-22-320.jpg)
![Operator delete Syntax
delete Pointer
delete [ ] Pointer
• The object or array currently pointed to by Pointer is
deallocated, and the value of Pointer is undefined. The
memory is returned to the free store.
• Good idea to set the pointer to the released
memory to NULL
• Square brackets are used with delete to deallocate a
dynamically allocated array.
25](https://image.slidesharecdn.com/lecture2-221024085342-6b2cd5a2/85/Lecture2-ppt-25-320.jpg)
![27
Example
char *ptr ;
ptr = new char[ 5 ];
strcpy( ptr, “Bye” );
ptr[ 0 ] = ‘u’;
delete [] ptr;
ptr = NULL;
‘B’ ‘y’ ‘e’ ‘0’
‘u’
ptr
3000
???
6000
6000
???
NULL
// deallocates the array pointed to by ptr
// ptr itself is not deallocated
// the value of ptr becomes undefined](https://image.slidesharecdn.com/lecture2-221024085342-6b2cd5a2/85/Lecture2-ppt-27-320.jpg)
![28
Pointers and Constants
char* p;
p = new char[20];
char c[] = “Hello”;
const char* pc = c; //pointer to a constant
pc[2] = ‘a’; // error
pc = p;
char *const cp = c; //constant pointer
cp[2] = ‘a’;
cp = p; // error
const char *const cpc = c; //constant pointer to a const
cpc[2] = ‘a’; //error
cpc = p; //error](https://image.slidesharecdn.com/lecture2-221024085342-6b2cd5a2/85/Lecture2-ppt-28-320.jpg)
Lecture2.ppt
- 1. 1 Procedural Concept • The main program coordinates calls to procedures and hands over appropriate data as parameters.
- 2. 2 Object-Oriented Concept • Objects of the program interact by sending messages to each other
- 3. 3 C++ • Supports Data Abstraction • Supports OOP – Encapsulation – Inheritance – Polymorphism • Supports Generic Programming – Containers • Stack of char, int, double etc – Generic Algorithms • sort(), copy(), search() any container Stack/Vector/List
- 4. 4 Pointers, Dynamic Data, and Reference Types • Review on Pointers • Reference Variables • Dynamic Memory Allocation – The new operator – The delete operator – Dynamic Memory Allocation for Arrays
- 5. 5 C++ Data Types structured array struct union class address pointer reference simple integral enum char short int long bool floating float double long double
- 6. 6 Recall that . . . char str [ 8 ]; • str is the base address of the array. • We say str is a pointer because its value is an address. • It is a pointer constant because the value of str itself cannot be changed by assignment. It “points” to the memory location of a char. str [0] [1] [2] [3] [4] [5] [6] [7] ‘H’ ‘e’ ‘l’ ‘l’ ‘o’ ‘0’ 6000
- 7. 7 Addresses in Memory • When a variable is declared, enough memory to hold a value of that type is allocated for it at an unused memory location. This is the address of the variable int x; float number; char ch; 2000 2002 2006 x number ch
- 8. 8 Obtaining Memory Addresses • The address of a non-array variable can be obtained by using the address-of operator & int x; float number; char ch; cout << “Address of x is “ << &x << endl; cout << “Address of number is “ << &number << endl; cout << “Address of ch is “ << &ch << endl; x number ch 2000 2002 2006
- 9. 9 What is a pointer variable? • A pointer variable is a variable whose value is the address of a location in memory. • To declare a pointer variable, you must specify the type of value that the pointer will point to, for example, int* ptr; // ptr will hold the address of an int char* q; // q will hold the address of a char
- 10. 10 Using a Pointer Variable int x; x = 12; int* ptr; ptr = &x; NOTE: Because ptr holds the address of x, we say that ptr “points to” x 2000 12 x 3000 2000 ptr
- 11. 11 int x; x = 12; int* ptr; ptr = &x; cout << *ptr; NOTE: The value pointed to by ptr is denoted by *ptr *: dereference operator 2000 12 x 3000 2000 ptr
- 12. 12 int x; x = 12; int* ptr; ptr = &x; *ptr = 5; Using the Dereference Operator 2000 12 x 3000 2000 ptr 5 // changes the value at the address ptr points to 5
- 13. 13 char ch; ch = ‘A’; char* q; q = &ch; *q = ‘Z’; char* p; p = q; Self –Test on Pointers 4000 A ch 5000 4000 q Z 6000 p 4000 // the rhs has value 4000 // now p and q both point to ch
- 14. 14 ptr Using a Pointer to Access the Elements of a String ‘H’ ‘e’ ‘l’ ‘l’ ‘o’ ‘0’ char msg[ ] =“Hello”; char* ptr; ptr = msg; *ptr = ‘M’ ; ptr++; *ptr = ‘a’; msg 3000 3000 ‘M’ ‘a’ 3001
- 15. 15 Reference Variables Reference variable = alias for another variable - Contains the address of a variable (like a pointer) - No need to perform any dereferencing (unlike a pointer) - Must be initialized when it is declared int x = 5; int &z = x; // z is another name for x int &y ; //Error: reference must be initialized cout << x << endl; -> prints 5 cout << z << endl; -> prints 5 z = 9; // same as x = 9; cout << x << endl; -> prints 9 cout << z << endl; -> prints 9
- 16. 16 Why Reference Variables • Are primarily used as function parameters • Advantages of using references: – you don’t have to pass the address of a variable – you don’t have to dereference the variable inside the called function
- 17. 17 Reference Variables Example #include <iostream.h> // Function prototypes (required in C++) void p_swap(int *, int *); void r_swap(int&, int&); int main (void){ int v = 5, x = 10; cout << v << x << endl; p_swap(&v,&x); cout << v << x << endl; r_swap(v,x); cout << v << x << endl; return 0; } void r_swap(int &a, int &b) { int temp; temp = a; (2) a = b; (3) b = temp; } void p_swap(int *a, int *b) { int temp; temp = *a; (2) *a = *b; (3) *b = temp; }
- 18. 18 Dynamic Memory Allocation • Static memory - where global and static variables live • Heap memory - dynamically allocated at execution time - "managed" memory accessed using pointers • Stack memory - used by automatic variables In C and C++, three types of memory are used by programs:
- 19. 19 3 Kinds of Program Data • STATIC DATA: Allocated at compiler time • DYNAMIC DATA: explicitly allocated and deallocated during program execution by C++ instructions written by programmer using operators new and delete • AUTOMATIC DATA: automatically created at function entry, resides in activation frame of the function, and is destroyed when returning from function
- 20. 20 Dynamic Memory Allocation Diagram static data Stack Heap Run-time allocated memory Compile-time allocated memory Program code High-end Low-end
- 21. 21 Dynamic Memory Allocation • In C, functions such as malloc() are used to dynamically allocate memory from the Heap. • In C++, this is accomplished using the new and delete operators • new is used to allocate memory during execution time – returns a pointer to the address where the object is to be stored – always returns a pointer to the type that follows the new
- 22. Operator new Syntax new DataType new DataType [IntExpression] • If memory is available, in an area called the heap (or free store) new allocates the requested object or array, and returns a pointer to (address of ) the memory allocated. • Otherwise, program terminates with error message. • The dynamically allocated object exists until the delete operator destroys it. 22
- 23. 23 Operator new char* ptr; ptr = new char; *ptr = ‘B’; cout << *ptr; NOTE: Dynamic data has no variable name 2000 ??? ptr 5000 5000 ‘B’
- 24. 24 The NULL Pointer • There is a pointer constant called the “null pointer” denoted by NULL • But NULL is not memory address 0. • NOTE: It is an error to dereference a pointer whose value is NULL. Such an error may cause your program to crash, or behave erratically. It is the programmer’s job to check for this. while (ptr != NULL) { . . . // ok to use *ptr here }
- 25. Operator delete Syntax delete Pointer delete [ ] Pointer • The object or array currently pointed to by Pointer is deallocated, and the value of Pointer is undefined. The memory is returned to the free store. • Good idea to set the pointer to the released memory to NULL • Square brackets are used with delete to deallocate a dynamically allocated array. 25
- 26. 26 Operator delete char* ptr; ptr = new char; *ptr = ‘B’; cout << *ptr; delete ptr; 5000 5000 ‘B’ 2000 ptr ??? NOTE: delete deallocates the memory pointed to by ptr
- 27. 27 Example char *ptr ; ptr = new char[ 5 ]; strcpy( ptr, “Bye” ); ptr[ 0 ] = ‘u’; delete [] ptr; ptr = NULL; ‘B’ ‘y’ ‘e’ ‘0’ ‘u’ ptr 3000 ??? 6000 6000 ??? NULL // deallocates the array pointed to by ptr // ptr itself is not deallocated // the value of ptr becomes undefined
- 28. 28 Pointers and Constants char* p; p = new char[20]; char c[] = “Hello”; const char* pc = c; //pointer to a constant pc[2] = ‘a’; // error pc = p; char *const cp = c; //constant pointer cp[2] = ‘a’; cp = p; // error const char *const cpc = c; //constant pointer to a const cpc[2] = ‘a’; //error cpc = p; //error
- 29. 29 Take Home Message • Be aware of where a pointer points to, and what is the size of that space. • Have the same information in mind when you use reference variables. • Always check if a pointer points to NULL before accessing it.