Re: [boost] [signals2] Test failure in C++11 (trivial fix for incorrect usage of boost::optional)

Subject: Re: [boost] [signals2] Test failure in C++11 (trivial fix for incorrect usage of boost::optional)
From: Frank Mori Hess (fmh6jj_at_[hidden])
Date: 2014-02-28 10:38:57

On Thu, Feb 27, 2014 at 8:45 PM, Gavin Lambert <gavinl_at_[hidden]> wrote:
> On 28/02/2014 12:33, Quoth Frank Mori Hess:
>>
>> Ah, I wasn't aware optional already had overloaded the comparison
>> operators with comparisons of optional<T> vs T.  A little unfortunate
>> IMO, it makes optional<bool> a bit of a disaster when any sort of
>> conversion to bool at all is supported.
>
>
> Not really; tristate bool logic with optional<bool> is pretty
> straightforward.  It usually falls into the pattern:
>
>   if (value == true) {
>     /* do something when true */
>   } else if (value == false) {
>     /* do something when false */
>   } else {
>     /* do something when null/none/unknown/unspecified */
>   }
>
> There is a little trap for the unwary where:
>
>   if (value) {
>     /* either true or false */
>   } else {
>     /* only null, not false */
>   }
>
> But once you get used to it, I'd hardly call it a "disaster".

Nothing is a problem once you get used to it.  I apologize for using a
bit of hyperbole, let me try again.  I consider the following obvious:
given two expressions like "!x" and "x==false" we have:

desirable behavior: they both compile and mean the same thing.
neutral behavior: only one compiles.
undesirable behavior: they both compile and mean completely different things.

-- 
Frank