[boost] Re: Formal Review Request: class optional<>

From: Dirk Gerrits (dirk_at_[hidden])
Date: 2002-11-22 10:36:34

Fernando Cacciola wrote:

> "Dirk Gerrits"  wrote in message
> news:arjgo5$o25$1_at_main.gmane.org...
>
> >Fernando Cacciola wrote:
> >
> >[snip]
> >
> >
> >>void recieve_async_message()
> >>{
> >>  optional rcv ;
> >>  while ( !!(rcv = get_async_input()) && !timeout() )
> >>    output(*rcv);
> >>}
> >
> >[snip]
> >
> >Maybe it's a minor point, but I think the !! is really ugly. Have you
> >considered the safe_bool idiom that's used in some other Boost
> >libraries? (smart_ptr to name one.)
> >
>
> Yes, I did.
> safe_bool (among other alternatives) was rejected because of the following
> (this specific point is explained in detail on the documentation)

Sorry, I only read your post, not the documentation.

> void foo()
> {
>   optional opt = get_some();
>   bool is_it = opt;
> }
>
> What is the intention of 'is_it'?
> Does it refer to the 'value' of 'opt', or to its initialized state?

Makes sense. Thanks for the explanation.

> The problem is that safe_bool allows for a conversion to bool in 
> certain contexts, but this violates the design premise that there is 
> no implicit conversion from  optional to T for the case when T=bool.
>
> With the current design, the above will fail to compile, requiring the 
> user to disambiguate the meaning by choosing either:
>
>   bool is_it_initialized = !!opt;
>   bool is_it_true         = *opt ;
>
> For similar reasons, the following is not supported either:
>
> if ( opt == 0 ) or if ( opt != 0 )
>
> But if you really dislike the syntax, there is an alternative spelling:
>
>   if ( peek(opt) ) ....
>
> peek() returns a pointer to the optional value if it is initialized, 
> or NULL if it isn't. Such a pointer can be directly used in a boolean 
> context with the expected meaning.

I guess I'd use   if (peek(opt) != 0)   or something. It's not that !! 
is so ugly, but it's not very clear IMHO.

Dirk Gerrits