Lecture – Pointers1 C++ Pointers Joseph Spring/Bob Dickerson School of Computer Science Operating Systems and Computer Networks Based on notes by Bob Dickerson and Introduction to C++ Programming by Roberge and Smith

Lecture – Pointers2 Areas for Discussion Pointers: Introduction Declaring Pointers Dereferencing a Pointer Pointer Arithmetic The Address Operator Pointers and Arrays Dynamic Memory Allocation De-allocating Memory Allocation

Lecture – Pointers3 Pointers: Introduction

Pointers –Are variables that holds the memory address of a data value rather than the data value itself –point to the data value –Are useful for: Manipulating array elements Processing strings Dynamically allocating system memory –Are declared by preceding the identifier with the indirection operator * Lecture – Pointers4

5 Declaring Pointers

The declaration: int *iptr; creates a pointer iptr that can store the address of an integer We note: –At this stage iptr does not contain an address –We have to assign an address to iptr –We can assign an address by using the address- of operator & Lecture – Pointers6

Declaring Pointers Example int *iptr; //Declaring iptr as a pointer to an integer value int num = 5;// num contains the integer value 5 iptr = &num;// iptr now points to num To access the value pointed to by a pointer we can use the indirection operator * cout << *iptr << “\n”; Lecture – Pointers7

8 Dereferencing Pointers

Dereferencing a Pointer cout << *iptr << “\n”; Here the indirection operator is used to: –Output the integer value (5) –Stored in the memory location (num) –Pointed to by iptr Accessing a value pointed to by a pointer is called dereferencing a pointer The * operator is referred to as the dereferencing operator Lecture – Pointers9

10 Assigning Pointers

Assigning a Pointers contents to another Pointer This maybe achieved by using the assignment operator = int *iptrv2; //iptrv2 is a pointer to an integer value iptrv2 = iptr // iptrv2 points to what iptr points to (num) The above creates a second pointer iptrv2 to the integer variable num Lecture – Pointers11

Lecture – Pointers12 Pointers and Arrays

Recall an array variable is a pointer to the first element in an array char section[11] = “A Title”, // sample string *strptr;// pointer to a char in the string strptr = section; Here both section and strptr point to the first character in the array Lecture – Pointers13

Pointers and Arrays ‘A’ ‘ ’‘ ’ ‘T’ ‘i’ ‘t’ ‘l’ ‘e’ ‘\0’ Lecture – Pointers14 sectionstrptr

Pointer Arithmetic #include Using namespace std; Void main() { char section[11] = “A Title”, //sample string *strptr; //pointer to character in string //set strptr to point to the first character in string strptr = section; //display each character in the string. Stop when strptr points to the null character at the end of the string while(*strptr != ‘\0’) { cout << *strptr; //output character that pointer strptr points to strptr++; // Advance to the next character } cout << “\n” } Lecture – Pointers15

Pointers and Arrays ‘A’ ‘ ’‘ ’ ‘T’ ‘i’ ‘t’ ‘l’ ‘e’ ‘\0’ Lecture – Pointers16 section strptr points to this memory location after third execution of statement strptr++

Pointers and Arrays ‘A’ ‘ ’‘ ’ ‘T’ ‘i’ ‘t’ ‘l’ ‘e’ ‘\0’ Lecture – Pointers17 section Strptr points to this memory location after seventh execution of statement strptr++

Lecture – Pointers18 Dynamically Allocating Memory Storage and De-allocating Memory Storage

Dynamically Allocating Storage So far we allocate space for an array at compile time. –For example: const int MaxLength = 11; int num1; double list[500]; char inputstring[MaxLength]; Signals to the compiler to reserve enough memory for: A pair of integers An array of 500 double prcision floating type numbers A string containing 11 characters Lecture – Pointers19

Dynamically Allocating Storage This approach to memory allocation is inefficient, particularly for arrays and strings It is often the case that –the size of an array is not known until run-time –The size varies and although its maximum size may be, say, 500 it could regularly be much smaller Rather than –specifying as much memory as one might need at compile time, thereby regularly wasting lots of memory –better to specify the array size at run-time and dynamically allocate memory Lecture – Pointers20

Dynamically Allocating Storage We can dynamically allocate memory space for an array at run-time by using the new operator list = new double [listsize] –Allocates an array containing listsize double precision numbers –Assigns list to point to the array where list is of type double* Lecture – Pointers21

dynamic.cpp – code fragment.... int listsize;//input list size double *list;//pointer to dynamically allocated array cout << “Enter the array size: ”; //get size of array needed cin >> listsize; list = new double[listsize]; //allocate an arrray of specified size cout << “Enter the array elements: ”; //read in array elements For(int i=0; i < listsize; i++) cin >> list[i];.... Lecture – Pointers22

De-allocating Storage When we no longer require dynamically allocated memory we have to de-allocate it This is achieved using the delete operator delete [ ] list; –Note use of [ ] to indicate that list points to an array not a single double precision number Lecture – Pointers23

Lecture – Pointers24 Summary Pointers: Introduction Declaring Pointers Dereferencing a Pointer Pointer Arithmetic The Address Operator Pointers and Arrays Dynamic Memory Allocation De-allocating Memory Allocation