Please explain the difference between a struct and a union in a demo program.

Thank you!<br>Gamemania

Planet of Wonders ( http://tech-war.virtualave.net/ )

: Please explain the difference between a struct and a union in a demo program.
:
: Thank you!Gamemania
:
: Planet of Wonders ( http://tech-war.virtualave.net/ )
:
Unions all share the same space in memory. Writing to one of its members could overwrite one or all of the other members. A union is as big as its biggest member. For example (I assume you're using a compiler/whatever that has the sizes I'm used to for its variables):

union {
  double D; // 64 bits
  float F; // 32 bits
  short S; // 16 bits
  unsigned char C; // 8 bites
} MyUnion;

int Size = sizeof(MyUnion); // Size == 64

If you set MyUnion.D equal to 50.2 then it would overwrite all of the other variables.

A struct has each variable packeted into a single block of memory (not always consecutively though). So, if we did this:

struct {
  double D; // 64 bits
  float F; // 32 bits
  short S; // 16 bits
  unsigned char C; // 8 bites
} MyStruct;

int Size = sizeof(MyStruct); // Size == 120
// I assume no byte alignment is done to the struct

Obviously writing to one member of the struct doesn't overwrite the others (under normal circumstances). So they all have their own memory and don't share it.

http://druidgames.cjb.net

Do you guys use unions often in your programs?


: : Please explain the difference between a struct and a union in a demo program.
: :
: : Thank you!Gamemania
: :
: : Planet of Wonders ( http://tech-war.virtualave.net/ )
: :
: Unions all share the same space in memory. Writing to one of its members could overwrite one or all of the other members. A union is as big as its biggest member. For example (I assume you're using a compiler/whatever that has the sizes I'm used to for its variables):
:

: union {
:   double D; // 64 bits
:   float F; // 32 bits
:   short S; // 16 bits
:   unsigned char C; // 8 bites
: } MyUnion;
: 
: int Size = sizeof(MyUnion); // Size == 64
: 

:
: If you set MyUnion.D equal to 50.2 then it would overwrite all of the other variables.
:
: A struct has each variable packeted into a single block of memory (not always consecutively though). So, if we did this:
:

: struct {
:   double D; // 64 bits
:   float F; // 32 bits
:   short S; // 16 bits
:   unsigned char C; // 8 bites
: } MyStruct;
: 
: int Size = sizeof(MyStruct); // Size == 120
: // I assume no byte alignment is done to the struct
: 

: Obviously writing to one member of the struct doesn't overwrite the others (under normal circumstances). So they all have their own memory and don't share it.
:
: http://druidgames.cjb.net
:
:

Gamemania

Planet of Wonders ( http://tech-war.virtualave.net/ )

No, not often, but I have before, and other libraries (DirectX, WinSock2, et cetera) use them, so they're good to know.
http://druidgames.cjb.net