struct to byte[]

[Hasan Aljudy]

Derek Parnell wrote:
> On Wed, 13 Dec 2006 20:48:43 -0800, BCS wrote:
> 
>> Derek Parnell wrote:
>>> Yes, but unfortunately the actual function is faulty. Here is what I had to
>>> do to get it to work ...
>>>
>>> ubyte [] toByteArray (T) (inout T t)
>>> {
>>>     union ubyte_abi
>>>     {
>>>         ubyte[] x;
>>>         struct
>>>         {
>>>            uint  xl; // length
>>>            void* xp; // ptr
>>>         }
>>>     }
>>>     ubyte_abi res;
>>>     res.xp = cast(void*)&t;
>>>     res.xl = T.sizeof;
>>>     return res.x;
>>> }
>>>
>> What didn't work???
>> Unless arrays are broken, that should be the same.;
> 
> The original function was 
> 
>   ubyte [] toByteArray (T) (T t) 
>   {
>      return (cast(ubyte *)&t)[0..T.sizeof].dup
>   }
> 
> With this, the '&t' phrase takes the address of the data as
> passed-by-value. Which means that when passing a struct or basic type, you
> get the address of the copy of the data which of course is not in scope
> when you return from the function. The '.dup' takes another copy of the
> data (this time on the heap) and you return the ubyte[] reference to the
> second copy. So you end up with a ubyte[] array that references a copy of
> the struct/data you supplied to the function and doesn't reference the
> initial data at all. Of course, that might be what you are trying to do ;-)
> I just thought that what was being attempted was to have a ubyte[] to
> access the data in the original struct instance and not a copy of it.
> 
> 
> My function avoids taking copies of the data and returns a ubyte[]
> reference to the actual struct/data instance passed to the function.
> 
> Both are invoked with identical syntax thanks to IFTI and D's handling of
> inout parameters.
> 

But why did you need the union/struct trick?

And with this, isn't the compiler free to rearrangre struct fields??

     union ubyte_abi
     {
         ubyte[] x;
         struct	//isn't the compiler free to rearrangre struct fields??
         {
            uint  xl; // length
            void* xp; // ptr
         }
     }