CSC195, Class 15: Structs and Other User-Defined Types

Overview:
* As little time as possible on structs, unions, and typedefs.
* Lab
* Reflection

Notes:
* Math research opportunities meeting after class.
* Read about strings in K&R and man pages.
* Questions about work this week?
* Friday: Fun with strings

----------------------------------------

C lets you combine multiple variables into one big "structure"
using the struct keyword

struct { int x, int y } fred;

You can now refer to fred.x and fred.y as variables

More generally

struct point { int x, int y };

defines the type named "struct point" with two fields, x and
y.

struct point wilma;

You can now refer to wilma.x and wilma.y

WHY BOTHER?

* "To fail to accomodate object-oriented programming"
* Easier to read, e.g., if you're doing 3D graphics programming
  for the guy with the tie
  + Group things together
  + Refer to the group by name rather than individually

----------------------------------------

C includes a fun variant called the union

union whatever { int i, char ch };

HOW DO THEY DIFFER?
* Structs assume you will use all their fields
* Unions assume only one at a time
* Good way to say "I need one of the following data types,
  but I'm not sure which one yet".
* Advantage: You *can* share storage (but it's up to the
  implementer).

----------------------------------------

C lets you name other types with

typedef TYPE NAME

typedef int four_byte_integer

WHY WOULD WE EVER BOTHER DOING THIS?

* Makes the code clearer.
* Permits some error checking.
* The particular example I chose permits more portable code.

----------------------------------------

LAB

* What's going on with
  typedef struct pair { int x; int y; } pair;
  "pair" is another name for "struct pair"
  One type, two names

  struct pair { int x; int y };

  typedef struct { int x; int y; } pair;

* Typedefs belong *outside* of your main().

* Why are wombats 8 bytes?

* Don't forget your "friend" splint.

* Nothing can precede declarations in a function body!

* C vs. Java
+ Java
class pair { int x; int y; }
pair alpha = new pair();
pair beta = new pair();
alpha.x = 5;
alpha.y = 6;
beta = alpha;
beta.x = 100;
PRINT alpha.x;
   prints 100
Assignment is "beta refers to the same object as alpha"
+ C
struct { int x; int y; } pair;
pair alpha;
pair beta;
alpha.x = 5;
alpha.y = 6;
beta = alpha;
beta.x = 100;
PRINT beta.x
  Prints 5
Assignment is "copy fields of alpha into beta"