[RESOLVED] How Unions will be used practically in C?

[raghunadhs]

Hi All,
I posted this thread once, but i could not get proper reply. if any one know, please resopnd to this thread.

I came to know that, Unions will be very much useful in Embeded programing. but i don't know how they will be used....if you know this please let me know it.

Thanks:
regards:
raghunadhs.v

[SpS]

A union is essentially a structure in which all of the fields overlay each other; you can only use one field at a time. (You can also cheat by writing to one field and reading from another, to inspect a type's bit patterns or interpret them differently, but that's obviously pretty machine-dependent.) The size of a union is the maximum of the sizes of its individual members.

The primary usefulness of a union is to conserve space, since it provides a way of letting many different types be stored in the same space. Unions also provide crude polymorphism. However, there is no checking of types, so it is up to the programmer to be sure that the proper fields are accessed in different contexts. The relevant field of a union variable is typically determined by the state of other variables, possibly in an enclosing struct.

[raghunadhs]

Thanks SPS,
it really a good explenation. could you please provide me a sample example, regaring to the practical usage of union.

Actually I tried, but could not find any proper example.
Thanks:
regards:
raghunadhs.

[SpS]

Example

#include <iostream>
using namespace std;

union Packed
{
    char i;
    short j;
    int k;
    long l;
    float f;
    double d;
};

int main()
{
    cout << "sizeof(Packed) = " << sizeof(Packed) << endl;
    Packed x;
    x.i = 'c';
    cout << x.i << endl;
    x.d = 3.14159;
    cout << x.d << endl;
}

[raghunadhs]

Hi SPs,
i know that unions will save memory, and instead of taking few variable declerations(say 5), if we place those 5 variables in a union, then the memory will be allocated, based on one of the fields whose memory is higher than others. it will be useful if we access only one filed at a time(and will be failed if try to access all the fileds).
The example what you provided is defnetly a good one, and is very much useful for beginners.but i need an example regarding to unions, where it will be very much useful(mandetory)? for example....
there is a oil tank monitor, the operator can do one operation at a time. the operaitons may be (1) finding temperature (consider it as a floting point)
(2)volume of oil (consider it as integer..(eventhough some times float))
the operator can press (1) or (2), if he presses (1)
union TankMoniter
{
float Temperature;
int Volume;
}T;

T.temperature=readtemperature();
....
....
So,here memory will be saved....I know that this is not a proper one, if u say like this one, it will be more useful to me.

[chemicalNova]

There is no real situation where unions are mandatory these days. As I said in your other thread, the only real use for unions these days is to save space on architectures like the PS2/3, Xbox, etc.

It should also be noted that, only 1 member of a union can be assigned at any given time, and that is the only member that can be accessed.

chem

[sunburnt]

A nifty trick with unions is to create a bit field where each bit can be accessed by name but the whole field can be read or written to as well.

typedef union bitfield
{
   unsigned char value;
   // each member is one bit in 'value'
   struct
   {
      int A : 1;
      int B : 1;
      int C : 1;
      int D : 1;
      int E : 1;
      int F : 1;
      int G : 1;
      int H : 1;
   };
   
} bitfield_t;



int main()
{
   bitfield_t b;

   // read from file, network, etc...
   b.value = 0xAC;

   if (b.A) // instead of (if (b & (1 << A_FIELD))
   {
        //whatever
   }
   

   return 0;
   
}

[raghunadhs]

Thanks chemical nova, i am agreeing with you. i also could not find other exapmples.
Thanks sun burnt, my faculty memeber also told an example which resembles like your one. but at that time i could not concentrate on that. now i cam to an understand..

Thanks All,
regards:
raghunadhs.

[QUOTE=sunburnt]A nifty trick with unions is to create a bit field where each bit can be accessed by name but the whole field can be read or written to as well.

[wossname]

Unions also provide crude polymorphism.

If you are using it like that then your code is seriously flawed.

[chemicalNova]

He's not.. its a direct quote from Wiki

chem

[wossname]

Well I didn't see the quote tag... oh there isn't one. Funny that.

Written by someone that doesn't understand polymorphism and plagiarised by a kindred spirit