Class 30: Pointers

Held: Tuesday, 15 March 2011

Summary: We begin our study of pointers, one of the more important data types in C. Pointers support the concept of memory as a first-class value.

Related Pages:

• EBoard.
• Lab: Pointers.
• Reading: K&R 5.1-5.2.

Notes:

• I have responded to most of the questions on the first reading on pointers.
• Tonight's reading: K&R 5.3-5.4.
• Break reading (or skimming): Managing Projects with GNU Make, Chapters 1 and 2.
• HWs 5 and 6 returned.

Overview:

• Memory as a First-Class Value.
• Examples from K&R.
• Detour: Writing Type-Generic Code.
• Lab.

Memory and Pointers

• Everything (or at least every variable) has to be stored somewhere in memory.
• The program needs to know where it's stored.
• In C, you can access information about where things are stored.
  • In Scheme (at least standard Scheme), you cannot.
  • We say that memory locations are first-class values in C.
• The address-of operation (&) tells you where in memory that thing is stored.
• An address is also called a pointer.
• The type of a pointer is specified by the type of the thing pointed to, followed by a *. So int * means a pointer to an integer.
• You can find the contents of something pointed to by prefixing the pointer with *.
• One of the most common uses of pointers is to permit functions to modify their (non-array) parameters. If you pass a pointer, you can dereference the pointer (get its contents) and modify those contents.

K&R Examples

• Many can be found in Examples/KR5/.
• getint
• swap

Detour: Writing Type-Generic Code

• There are lots of possible versions of swap.
• Do we really want to use copy-paste-change? No!
• What's the alternative? Use the C preprocessor.
• We write generic code in which
  • The type is replaced by TYPE
  • The name is replaced by PREFIX(num)
• For a particular type, we define those two macors.

Lab

• Do The Lab.
• Be prepared to reflect.