Newbie Question - memory overlays

[Sean Kelly]

Jacques Collin wrote:
> I union would not fit the job.
> 
> In Delphi, I can have, for example,
> --  fdata1    :  file of infonode
> --  datastore : array[1..n] of infonode;
> --  ctrlpoint : array[1..100] of infonode absolute datastore[1]
> where
> the first 100 "infonodes" can be used as an array. And the rest of
> the "datastore" can be used as a list or even a tree.
> 
> at some point the "datstore" will be written out to disk.
> I would prefer to avoid a cumbersome work-around like a union
> (a.k.a variant records) in favour of the elegant Delphi solution
> of simply using the "absolute" attribute.
> 
> Any suggestions?

I don't think D has support for this in the general sense, but you can 
fake quite a lot with slices:

     void main()
     {
         byte[8] buf;
         char[]  chars = cast(char[]) buf[4 .. $];
         int[]   ints  = cast(int[]) buf[0 .. 4];
         int*    aint  = cast(int*) &buf[0];

         void print()
         {
             foreach( b; buf )
                 printf( "%d: %c\n", b, b );
             printf( "\n" );
         }

         buf[] = cast(byte[8]) "abcdefgh";
         print();
         chars[] = "abcd";
         print();
         ints[0] = 0;
         print();
         *aint = uint.max;
         print();
     }

prints:

     97: a
     98: b
     99: c
     100: d
     101: e
     102: f
     103: g
     104: h

     97: a
     98: b
     99: c
     100: d
     97: a
     98: b
     99: c
     100: d

     0:
     0:
     0:
     0:
     97: a
     98: b
     99: c
     100: d

     -1:  
     -1:  
     -1:  
     -1:  
     97: a
     98: b
     99: c
     100: d