Pointers

1. Pointers (Walls & Mirrors - Beginning of Chapter 4)

2. What’s a Pointer? • A pointer variable is a variable that can contain the location of another variable as its value. • The location of a variable is usually implemented by indicating its address in (RAM) memory. • The location (or address) of a variable is called a pointer. • Sometimes, for brevity, a pointer variable is simply called a pointer. You will need to be careful to understand whether pointer refers to a variable or the address of a variable.

3. Pointers -“Real Life” Examples • Suppose that your friend, Sam, borrows your copy of Walls & Mirrors. In its place, he leaves you the note Borrowed your Walls & Mirrors book. Thanks, Sam • This note is like a pointer, since it it not your book, but it tells you where to go to find it. (The paper on which the note is written is like a pointer variable.)

4. Pointer variable Other variable Pointers - Graphical Representation • A variable is often represented as a box. • The value of the variable is written inside the box. • If the variable is a pointer variable, containing a pointer, the box will contain the “tail” of an arrow that points to another variable.

5. Pointers - Suggestion • If you have a problem with pointers, draw the layout. • It may be difficult to understand what is going on without a graphical representation of the pointer relationships.

6. Pointer Declarations int *iptr; // iptr is a pointer to an int char *cptr; // cptr is a pointer to a char float *fptr; // fptr is a pointer to a float List *Lptr; // Lptr is a pointer to a List object Sphere *Sptr; // Sptr is a pointer to a Sphere object

7. Pointer Operations • Assignment: = • A pointer variable can be assigned only a pointer (i.e. the address of a variable) or NULL (which equals 0). • Comparison: = =, != • Pointers can be compared for equality. • Addition/Subtraction: +,  • Pointers can be incremented or decremented with an integer. • Dereferencing: * • *ptr returns the value of the object pointed to by ptr. • Address of: & • &ptr returns the address of ptr (i.e. pointer to ptr).

8. Pointer Operations - Address of • The Address of operator & returns the address of an object. float PI = 3.14159; float *PIptr; &PI returns the address of the variable PI,not 3.14159 (the value stored in PI). PIptr = &PI stores the address of variable PI in variable, PIptr. &PIptr returns the address of variable PIptr.

9. Pointer Operations - Dereferencing • The Dereferencing operator * returns the value of the object to which its operand points. float PI = 3.14159; float *PIptr; float X; PIptr = Π // PIptr contains the address of PI X = *PIptr; // Value stored in PI (3.14159) is // assigned to X *(*(&PIptr)) = *PIptr = *(&PI) = PI = 3.14159

10. Pointer Initialization int *ptr; // pointer to int declared, value undefined int x = 5; // int declared and initialized to 5 cout << x; // prints 5 cout << *ptr; // Error! Prints undefined value, since ptr not // initialized ptr = &x; // ptr now contains the address of x cout << *ptr; // prints 5

11. Pointer Initialization - Suggestion • When a pointer variable is declared it is (by default) uninitialized. Therefore, where it is pointing is undefined. • It’s a good practice to initialize newly declared pointer variables to the NULL pointer (= 0). • This will insure that the pointer variable is not pointing anywhere it shouldn’t. • This will help you determine if a valid pointer has been assigned to it. if( ptr = = NULL ) cout << “ptr has not been initialized” << endl;

12. ptr new int variable new Operator • The operator new creates a new object of a given type. • new returns a pointer to the newly created object. ptr = new int;

13. new Operator (Cont’d.) • An object created with new does not have a name and is not declared. • An object created with new can only be used by following (dereferencing) a pointer to it. • You need to be careful to not lose the pointer to an object created with new, since there is no other way to access it. • Memory that was allocated with new and has become inaccessible is called a memory leak. • For programs that run for long periods of time, memory leaks can be the reason for system failure.

14. new Operator - Example 1 int *ptr; // pointer to int declared, value undefined *ptr = 5; // Error! ptr contains invalid address and // space for int not allocated ptr = new int; // space for int allocated and pointer to it // assigned to ptr *ptr = 5; // 5 is stored in the int pointed to by ptr

15. new Operator - Example 2 int *p, *q; // declare two pointer to int variables p = new int; // allocate space for an int; make p point to it *p = 25; // store 25 in the int pointed to by p What is the effect of the following? q = p;

16. p q new int 25 new Operator - Example 2 (Cont’d.) Draw a picture!

17. new Operator - Example 3 int *p, *q; // declare two pointer to int variables p = new int; // allocate space for an int; make p point to it q = new int; // allocate space for an int; make q point to it *p = 35; // store 35 in the int pointed to by p What is the effect of the following? *q = *p;

18. p q new int 35 35 new int new Operator - Example 3 (Cont’d.) Draw a picture!

19. new Operator - Example 3 (Cont’d.) What would have happened if we had executed q = p; instead of *q = *p;

20. p q new int 35 ? new int new Operator - Example 3 (Cont’d.) The new int, previously pointed to by q is LOST and cannot be recovered. This is called a memory leak.

21. Arrays and Pointers int a[50]; int *aptr = a; a is equivalent to &a[0] aptr = a; is equivalent to aptr = &a[0]; aptr+5 is equivalent to &a[5] *(aptr+5) is equivalent to a[5]