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: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2014-02-28 11:38:49

On Fri, Feb 28, 2014 at 5:45 AM, 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 */
>   }

I prefer to check for the value presence first:

  if (!value) {
    // value is absent
  } else if (value.get()) {
    // value is true
  } else {
    // value is false
  }

This way the semantics is pretty obvious.