Re: Ada Smileys in C++ lib Conversion

[Brian Drummond <brian_drummond@btconnect.com>]

On Mon, 16 Aug 2010 05:39:34 -0700 (PDT), Maciej Sobczak
<see.my.homepage@gmail.com> wrote:

>On 16 Sie, 14:30, Brian Drummond <brian_drumm...@btconnect.com> wrote:

>> So a "const" is not constant, says the compiler.
>
>The above is guaranteed to work by the C++ standard (nerds see
>5.19/1).

And it does work in C++. 
(It was clear I was talking about C, yes?).

>The problem will be when the initializer for "size" above will indeed
>be a function call - this would not be a constant expression, even
>though the object "size" is declared as const.
>The reason is that constant expression should always evaluate to the
>same value, whereas function calls are not required (and cannot be
>verified) to do so. 
... even though the function is only called once to initialise the const - and
it is a "pure function" in Ada or VHDL terms - and all its arguments are consts
(or even constants!). Does make me wonder what _can_ be verified in C++.

>So, there is a difference between a "constant expression" (roughly
>equivalent to Ada's "static expression") and a "const object".
>Array bounds require constant expression.

So we have - even in C++ - a const object that cannot be used 
where a constant expression is required.

Interesting, Thanks.

To get back almost on-topic, I am frequently amazed in Ada, after years of
bruising by various {}-languages, how often I can program from the BNF, (i.e.
"the BNF doesn't stop me doing this, but surely it can't be THAT simple..." 
and it just works. 

- Brian ... smiling