unions problems!

i just read my book and it says union members share the same memory. like a long and a double will both occupy the same block of 8 bytes (the biggest member of the union). why would anyone want to do that? please explain in detail about unions.....and give some example because all my book gave me was:

Code:

union shareLD
{
   double dval;
   double lval;
};

shareLD myUnion;
myUnion.dval = 5.4;
myUnion.lval = 5; // dval has been overwritten

// or unions can be written like this

union shareDLF
{
   double dval;
   long lval;
   float fval;
} uinst = {1.5};

and it says unions can be use in classes...why wouldnt anyone
want to use unions when they could use "new" and "delete"?

also does that mean two or more members of the union cannot
have values at the same time?

Unions were used long ago when systems didn't quite have the memory to spare. They're really not used too much anymore. And since they all share the same memory, all the members will have the same value.

>also does that mean two or more members of the union cannot
>have values at the same time?

Members of a union share the same memory. So if member A has a certain value, then member B, if the same type of A, also has that value.

>Unions were used long ago when systems didn't quite have the
>memory to spare. They're really not used too much anymore.

Unions are still used a lot, not only to save memory, but also for uniform interfaces and other applications. A usefull application is for example making a union with a char and a bitfield in it, so that you can easily switch between using a char or access the individual bits.