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Old Jun 8th, 2006, 11:49 PM   #1
Harakim
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What are unions?
What are unions?

I want to make a union like...
Union u { char *cptr; short *sptr; int *iptr; }

Then I would like to use it scroll through a list with those pointers, so I don't have to keep casting. Is this possible and is there a better way to do it?
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Old Jun 9th, 2006, 6:08 AM   #2
volatile
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Why do you want to make a union when you don't know what its for?

A union is a data structure wich size is only as big as its biggest member, let me clarify:

(Toggle Plain Text) 
#include <iostream>

using namespace std;

int main()
{
	union { char a; int b; } ;
	
	a = 'a';
	cout << "a is " << a << endl;
	b = 0x99887762;
	
	cout << "a is " << a << endl;
	cout << "b is " << hex << b << endl;
	
	a = 'A';
	cout << "a is " << a << endl;
	cout << "b is " << hex << b << endl;
	
	cin.get();
	return EXIT_SUCCESS;
}
Output:
(Toggle Plain Text) 
a is a
a is b
b is 99887762
a is A
b is 99887741

In the above program I used an unnamed union, this way I dont have to access its members using a union object. An unnamed union is a union wich has not been defines as a type or a named object. blah.

char a; and int b; overlaps in memory. They both start at the same address in memory. This way together, they only take up as much space as the biggest one, wich is b.

If I write to a, b will change. However a will only change if I write to b's LO byte(first in memory, little endian).


Quote:
Then I would like to use it scroll through a list with those pointers, so I don't have to keep casting. Is this possible and is there a better way to do it?
I dont understand, do you have some code?
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Old Jun 9th, 2006, 7:17 AM   #3
grumpy
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A union is a type that has all its members occupying the same memory. So, in volatile's example &a == &b (if we ignore the types of a and b). The intent of a union is that it holds only one value at a time.
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Old Jun 9th, 2006, 7:40 AM   #4
jim mcnamara
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I've got what he wants -
In Windoze there is a variant datatype - which is represented in MS C by a big union.
This union allows you to reference a BSTR then use it as an int or a float.

Except: in vanilla C the void * does this for you. So you probably don't want a union.
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Old Jun 9th, 2006, 7:07 PM   #5
Harakim
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I am trying to help write a client application for a game server as I mentioned a while ago. Someone else sent me some of the code they were working on in the same project. Here is some of their code:
(Toggle Plain Text) 
...
	switch(*msgType) {
		// server load
		case SV_LOAD:
			msgType+=6;
			break;
		// server tick
		case SV_TICK:
			msgType+=2;
			break;
		// setting the name of our own char (in 3 parts)
		case SV_SETCHAR_NAME1:
			memcpy(cplayer.name,msgType+1,15);
			msgType+=16;
			break;
		case SV_SETCHAR_NAME2:
			memcpy(cplayer.name+15,msgType+1,15);
			msgType+=16;
			break;
		case SV_SETCHAR_NAME3:
			memcpy(cplayer.name+30,msgType+1,10);
			msgType+=16;
			break;

		// setting mode (speed)
		case SV_SETCHAR_MODE:
			cplayer.mode = *(msgType+1);
			msgType+=2;
			break;

		// setting attributes (str, int, will, brav)
		case SV_SETCHAR_ATTRIB:
			cplayer.attrib[*(msgType+1)][0]=*(msgType+2);
			cplayer.attrib[*(msgType+1)][1]=*(msgType+3);
			cplayer.attrib[*(msgType+1)][2]=*(msgType+4);
			cplayer.attrib[*(msgType+1)][3]=*(msgType+5);
			cplayer.attrib[*(msgType+1)][4]=*(msgType+6);
			cplayer.attrib[*(msgType+1)][5]=*(msgType+7);
			msgType+=8;
			break;

		// setting skills
		case SV_SETCHAR_SKILL:
			cplayer.skill[*(msgType+1)][0]=*(msgType+2);
			cplayer.skill[*(msgType+1)][1]=*(msgType+3);
			cplayer.skill[*(msgType+1)][2]=*(msgType+4);
			cplayer.skill[*(msgType+1)][3]=*(msgType+5);
			cplayer.skill[*(msgType+1)][4]=*(msgType+6);
			cplayer.skill[*(msgType+1)][5]=*(msgType+7);
			msgType+=8;
			break;

		// setting hp
		case SV_SETCHAR_HP:
			cplayer.hp[0]=*(unsigned short*)(msgType+1);
			cplayer.hp[1]=*(unsigned short*)(msgType+3);
			cplayer.hp[2]=*(unsigned short*)(msgType+5);
			cplayer.hp[3]=*(unsigned short*)(msgType+7);
			cplayer.hp[4]=*(unsigned short*)(msgType+9);
			cplayer.hp[5]=*(unsigned short*)(msgType+11);
			msgType+=13;
			break;

		// setting endurance
		case SV_SETCHAR_ENDUR:
			cplayer.end[0]=*(unsigned short*)(msgType+1);
			cplayer.end[1]=*(unsigned short*)(msgType+3);
			cplayer.end[2]=*(unsigned short*)(msgType+5);
			cplayer.end[3]=*(unsigned short*)(msgType+7);
			cplayer.end[4]=*(unsigned short*)(msgType+9);
			cplayer.end[5]=*(unsigned short*)(msgType+11);
			msgType+=13;
			break;

		// setting mana
		case SV_SETCHAR_MANA:
			cplayer.mana[0]=*(unsigned short*)(msgType+1);
			cplayer.mana[1]=*(unsigned short*)(msgType+3);
			cplayer.mana[2]=*(unsigned short*)(msgType+5);
			cplayer.mana[3]=*(unsigned short*)(msgType+7);
			cplayer.mana[4]=*(unsigned short*)(msgType+9);
			cplayer.mana[5]=*(unsigned short*)(msgType+11);
			msgType+=13;
			break;

		// setting items
		case SV_SETCHAR_ITEM:
			cplayer.item[*(unsigned long*)(msgType+1)]=*(short int*)(msgType+5);
			cplayer.item_p[*(unsigned long*)(msgType+1)]=*(short int*)(msgType+7);
			msgType+=9;
			break;

		// setting equipped items
		case SV_SETCHAR_WORN:
			cplayer.worn[*(unsigned long*)(msgType+1)]=*(short int*)(msgType+5);
			cplayer.worn_p[*(unsigned long*)(msgType+1)]=*(short int*)(msgType+7);
			msgType+=9;
			break;
...
This is a portion of the "message handling" code of the client. The server sends a packet full of messages and this code will decode them 1 by 1. As you can see, this involves a lot of casting. I would like to use a union of pointers to remove the casting.

Thank you very much for your clear responses, you have made it clear what a union is and that it will be a good tool for my work. : )
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Old Jun 13th, 2006, 1:41 PM   #6
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Honesetly, what's the point of unions.... What would be a practical use for them?
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Old Jun 16th, 2006, 11:16 AM   #7
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They're a lot less useful now, but think back to when 1 Mb of memory was regarded as way too much. Space-saving measures were implemented any way they could be.
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