Re: Ada2012 Invariants and obaque types

["Vinzent Hoefler" <0439279208b62c95f1880bf0f8776eeb@t-domaingrabbing.de>]

Dmitry A. Kazakov wrote:

> On Tue, 21 Jun 2011 21:34:31 +0200, Vinzent Hoefler wrote:
>
>> Dmitry A. Kazakov wrote:
>>
>>> 'Valid is a hack around missing value initialization enforcement or some
>>> kludges to support Unchecked_Conversion. It cannot justify anything because
>>> 'Valid itself lacks credibility.
>>
>> How do you properly initialise and/or validate values coming from an untrusted
>> external source (i.e. "bus")? Always using the full bit pattern and write the
>> conversion routine yourself?
>
> Yes, I always do exactly this, at least in order to make my program
> portable.

Hmm. Reading bus values in a particular endianes and being portable... ;)

Example:

     --  type used for clock source and retrigger mode selections
     --  bits [4:2]
     --  RTG  PSC_SEL EXT_CLK   clock mode
     --    0        0       0   Internal, cpu clock, gated
     --    0        0       1   External
     --    0        1       0   Internal, prescaled, gated
     --    0        1       1   N/A (same as 001)
     --    1        0       0   Internal, cpu clock, retriggered
     --    1        0       1   N/A (same as 001)
     --    1        1       0   internal, prescaled, retriggered
     --    1        1       1   N/A (same as 001)
     type Clock_Source is (Internal_Gated,
                           External,
                           Prescaled_Gated,
                           Internal_Retriggered,
                           Prescaled_Retriggered);

     for Clock_Source use (Internal_Gated        => 2#000#,
                           External              => 2#001#,
                           Prescaled_Gated       => 2#010#,
                           Internal_Retriggered  => 2#100#,
                           Prescaled_Retriggered => 2#110#);

     for Clock_Source'Size use 3;

Would you actually do without the type representation here and start masking off the
bits by hand again instead of using 'Valid, just in case of of those "N/A" values
sneak in?


Vinzent.

-- 
f u cn rd ths, u cn gt a gd jb n cmptr prgrmmng.