Class 24: Pointers and Functions

Held: Friday, 28 February 2003

Summary: Today we continue our exploration of functions by considering some cases in which pointers and functions interact.

Related Pages:

• EBoard.
• Lab.

Notes:

• I've put some more notes on Big O notation online. You can find them in the outline.
• Are there questions about the postconditions of permute?
• Question: Why do you need to predeclare functions in C?

Overview:

• Detour: #include and #define preprocessor directives.
• Returning Pointers from Functions.
• Pointers to Functions.
• Lab.

Preprocessor Directives

• #include filename
  • Shove the source code of the filename into this program at this point.
• #define name value
  • Replace all instances of name with value in subsequent code.
• #define name(formal1...formaln) body
  • Replace each instance of name(actual1...actualn) with a copy of body in which each formal has been replaced by the corresponding actual.

Returning Pointers

• Since C procedures can return any type, they can certainly return pointers.
• For example, int *alloc_int(void); says that alloc_int is a procedure that returns a pointer to an integer.
• We might define that procedrue as

  int *alloc_int(void)
  {
    return (int *) malloc(sizeof(int));
  } /* alloc_int */
  

• Why bother? Sometimes pointers are the most natural thing to return.

Pointers to Functions

• At times, it may be useful to write a variable that can hold a function.
  • Why?
• In C, you can use the pointer to function type.
• You can declare a pointer to a function with

  int (*foo)();
  

• Note that this must clearly differ from the declaration of a function that returns a pointer.
• Can we do something similar in Java? How does it differ?
• Can we do something similar in Scheme? How does Scheme differ?

Lab

• Do the lab.
• Be prepared to reflect.