Re: On packages hierarchy

[Shark8 <OneWingedShark@gmail.com>]

On 29-Jul-14 13:42, Simon Wright wrote:
> Given that Light_Number'Size is set to 3 (above), and is biased into the
> bargain, I don't see how this is going to work reliably? GNAT is
> certainly going to give you warnings about source & target having
> different sizes.

It should work as expected:

> testbed.adb:37:58: warning: size clause forces biased representation for "Light_Number"
> testbed.adb:60:09: warning: types for unchecked conversion have different sizes
> testbed.adb:60:09: warning: size of "Light_Number" is 3, size of "Gl_Enum" is 32
> testbed.adb:60:09: warning: source will be extended with 29 high order zero bits
>...

and ( from 
http://docs.adacore.com/gnat-unw-docs/html/gnat_rm_9.html#SEC485 )
>  For example, suppose we have the declaration:
>
>  	
>
>    type Small is range -7 .. -4;
>    for Small'Size use 2;
>
> Although the default size of type Small is 4, the Size clause is accepted by GNAT and
> results in the following representation scheme:
>
>  	
>
>   -7 is represented as 2#00#
>   -6 is represented as 2#01#
>   -5 is represented as 2#10#
>   -4 is represented as 2#11#

Which means that:
1 would be represented ad 2#000#
2 would be represented ad 2#001#
3 would be represented ad 2#010#
etc
That internal representation *is* the zero-based indexing that OpenGL 
expects, and Convert (as shown above) extends its size with zero-bits.

So there's no reason it shouldn't work.
Granted it's going off a lot of implementation-defined behavior and your 
method of an array or my suggestion of a proper conversion-function 
expression-function would handle this in an implementation independent 
manner.