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From: Robert A Duff <bobduff@world.std.com>
Subject: Re: Static assertions
Sender: bobduff@world.std.com (Robert A Duff)
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Date: Fri, 18 May 2001 21:27:24 GMT
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Date: 2001-05-18T21:27:24+00:00
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Jeffrey Carter <jeffrey.carter@boeing.com> writes:

> Isn't that "Danger, Will Robinson"?

Probably.

> I certainly agree with this advice. One thing that always stopped me
> short in my tracks was code with constructs such
> 
>    ... + My_Type (3) - ...
> 
> apparently written by people who thought that "3" has type Integer (or
> perhaps used a compiler with that "feature").

Well, there *are* languages where "3" is an int, and "3L" is
a long (or even "3l").  Yuck.

> 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.)

- Bob