Pointers

1. Pointers

2. The structure of memory • Computer memory is a linear sequence of addressable locations • Addresses are numbered starting at zero • In personal computers, the smallest addressable unit is a byte (8 bits) • In large scientific computers, the unit is a "word" • A word is typically 32 to 128 bits, or even more

3. Pointers • A pointer is an address of a storage location • By convention, zero represents the "null" pointer • A pointer is a number, and must itself be stored • The size of a pointer depends on the amount of memory to be addressed • A 16-bit pointer can address 64K locations • A 32-bit pointer can address 4 trillion locations

4. Pointers in higher-level languages • FORTRAN and Algol had no (user-level) pointers • C makes pointers an arithmetic type • Pascal provides pointers, but restricts their use • Java has pointers but tries to hide them from the user • ...but it throws a NullPointerException!

5. char pointers in C • For any data type, whether built-in or user-defined, C allows a pointer to that type • char *pch makes pch a pointer to a character • You can use *pch as a character • You can use pch as a pointer to a character • You can do arithmetic on pointers • pch++ increments pch by the size of a character

6. Other pointers in C • If T is a type, T *p declares p a pointer to that type • You can use p as a pointer to a T • You can use *p as a T • p++ increments p by the size of a T • Important because of the way arrays are treated • You can make a pointer to any variable • If x is any variable, then &x is its address

7. Storage allocation in C • You can allocate storage (from the "heap") • To allocate storage, use malloc(num_bytes) • This gives a pointer of unknown type: (void *) • You then cast the pointer to the desired type • Example: • int *myStorage;myStorage = (int *) malloc(100); • This gives you space for 25 integers (100 bytes)

8. Arrays in C • Array indexing is syntactic sugar for pointers • a[i] is treated as *(a+i) • To zero out an array: • for (i = 0; i < size; i++) a[i] = 0; • for (i = 0; i < size; i++) *(a+i) = 0; • for (p = a; P < a+size; p++) *p = 0; • Because a[i] means *(a+i), i[a] is equally legal!

9. Strings in C • A character in C is a single (ASCII-encoded) byte • A string is a pointer to a character (a char*) • The first zero character ends the string • You must allocate space for a string • str = (char *) malloc(101); • To process all the characters of a string: • for (p = str; *p != 0; p++) process (*p);

10. Parameter transmission in C • All parameters are passed by value • To fake pass by reference, pass a pointer • Example: to add 1 to x, call addOne(&x) • Where:void addOne(int *n) { *n = *n + 1; } • "Real" call by reference does exactly this, but automatically (so you don't need the *n syntax)

11. Problems with pointers in C • No bounds checking--pointers can point outside the array,or even outside the program • No type checking--you can cast a pointer to anything • Memory leaks--you can forget to deallocate storage when you're done with it • Dangling references--you can deallocate storage before you're done with it • C enthusiasts say you just have to be careful

12. Pointers in Pascal • p: ^T; makes p a pointer to type T • To follow the pointer (and get the T), say p^ • To allocate space for a T, say new(p) • You can assign pointers to pointer variables • You can test pointers for equality and for null • You can NOT do pointer arithmetic • ...and that's about all

13. Pascal needs pointers less than C • Pascal has true arrays • A string is an array of characters • Strings are clumsy, but they don't use pointers • Pascal has both call by value and call by reference • No clumsy syntax needed in called routine • Pointers are still adequate for building data structures such as linked lists, trees, etc.

14. Pointers in Java • Java calls pointers "references" • You cannot directly manipulate references in Java • Any object reference is effectively a pointer • You can allocate memory (with new), use references, compare references, and do pointer-like things • References in Java are practically the same as pointers in Pascal, but with a simpler syntax

15. The End