Re: Static assertions

[Jeffrey Carter <jeffrey.carter@boeing.com>]

Robert A Duff wrote:
> 
> Jeffrey Carter <jeffrey.carter@boeing.com> writes:
> 
> > However, is there any reason to write
> >
> >    Assert_1 : constant := Boolean'Pos (Assert'(Integer'Size = 73) );
> >
> > rather than
> >
> >    Assert_1 : constant Assert := Integer'Size = 73;
> >
> > ? Are there compilers that won't check this during compilation?
> 
> I think the former is required to get a compile time error,
> whereas the latter is merely allowed to produce a warning.
> The subtle difference is that the former has a static expression
> raising an exception, whereas the latter has a perfectly good
> static expression, but the assignment will raise an exception.
> I'm too lazy to look it up in the RM, so I could be mistaken...
> 
> Furthermore, if the second one doesn't get detected at compile time, it
> might not get detected at all.  I believe 11.6 allows the compiler to
> "optimize away" the constraint check, so it never happens at run time.
> (I'm assuming Assert_1 is a dead variable.)

I see. Assert_1 cannot be a dead variable, since it is a constant, but
it will probably never be referenced. I guess it's a "dead constant".

It would appear that

   Assert_1 : constant Assert := Assert'(Integer'Size = 73);

must cause a compilation error, since Assert is a static subtype, so the
qualified expression is static, and a static qualified expression must
be evaluated at compile time.

I'm just trying to come up with something a little shorter and less
obscure than using Boolean'Pos.

-- 
Jeffrey Carter