Re: how to get the offset of the current struct

[Jose E. Marchesi]

>> Hi,
>>
>> In some poke struct, for the _print function, I needed to get the offset of
>> the current struct:
>>
>> type ABXML_string_pool =
>>   struct
>>   {
>>     // String pool header:
>>     uint<16> string_pool_type;
>>     ...
>>
>>     method _print = void:
>>     {
>>       var my_offset = /*** HOW ***/;
>>       ...
>>     }
>>   }
>>
>> What did not work:
>>   this'offset                 because this is not known
>>   string_pool_type'offset     because string_pool_type is an integer
>>
>> What did work: Adding a zero-sized array the beginning:
>>
>> type ABXML_string_pool =
>>   struct
>>   {
>>     uint<8>[0] struct_start;
>>     // String pool header:
>>     uint<16> string_pool_type;
>>     ...
>>
>>     method _print = void:
>>     {
>>       var my_offset = struct_start'offset;
>>       ...
>>     }
>>   }
>>
>> But this feels like a bad workaround. There should be a better way in the 
>> Poke
>> language. Is there?
>>
>> Bruno
>
> method _print = void:
> {
>   var my_offset = SELF'offset;
>   var my_ios = SELF'ios;
>   var name_of_first_field = SELF'ename (1);
>
>   [...]
> }
>
> Any attribute that works for values of type `any' can be applied to
> SELF.

Some other useful attributes:

  SELF'mapped -> Tells whether the struct is mapped or not.
  SELF'length -> Gives the number of fields in the struct.
  SELF'eoffset(N) -> Gives the offset of the Nth field withing the struct.
  SELF'ename(N) -> Gives the name of the Nth field.
  SELF'elem(N) -> Gives the value of the Nth field as an `any' value.

The manual has this nice example of using these:

   For example, this is how we could print the name of the current
alternative in a pretty-printer for an union:

     type Foo =
       union int<32>
         {
           int<32> One == 1;
           int<32> Two == 2;
           method _print = void:
           {
             printf ("%s (%v)", SELF'ename (0), SELF'elem (0));
           }
         };