Re: [boost] safe bool not safe enough ?

Subject: Re: [boost] safe bool not safe enough ?
From: Krzysztof Czainski (1czajnik_at_[hidden])
Date: 2013-03-14 13:13:40

2013/3/14 John Maddock <boost.regex_at_[hidden]>

>  class Testable
>>> {
>>>   bool ok_;
>>>   typedef void (Testable::*bool_type)() const;
>>>   void this_type_does_not_support_****comparisons() const {}
>>>
>>> public:
>>>   explicit Testable(bool b=true):ok_(b) {}
>>>
>>>   operator bool_type() const
>>>   {
>>>      return ok_==true ?
>>>         &Testable::this_type_does_not_****support_comparisons : 0;
>>>
>>>   }
>>> };
>>>
>>> Testable t(true);
>>>
>>> bool b = t; // Compiles OK - Ooops!
>>>
>>>
>> I'm curious, why is this bad? I personally hate the fact, that the above
>> doesn't compile for explicit operator bool(), which leads to idioms like
>> !!t.
>>
>
> It's bad because the type isn't logically convertible to bool, BTW the !!t
> idiom isn't required in C++11 mode, the following works just fine:
>
> #include <iostream>
>
>
> class Testable
> {
>   bool ok_;
>   typedef void (Testable::*bool_type)() const;
>   void this_type_does_not_support_**comparisons() const {}
> public:
>   explicit Testable(bool b=true):ok_(b) {}
>
>   explicit operator bool() const
>   {
>      return ok_;
>   }
> };
>
> int main()
> {
>   Testable t(true);
>
>   if(t) std::cout << "Yeh\n";
>
>   if(!t) std::cout << "Oh\n";
>
>   bool b = t ? true : false;
>
>   return 0;
>
> }
>

bool do_something_with_a_Testable()
{
  Testable t;
  [...]
  // return t; doesn't work
  return !!t;
}

So
bool b = t; // I wish it worked with Testable::explicit operator bool, but
doesn't.

And the explicitness of operator bool should block situations, where bool
is treated as an arithmetic type, like:
t + 1
which compiles if Testable provides an implicit conversion to bool.

John Maddock wrote:
>
>> It's not directly connected - except that what should have failed to
>> compile - a function that looks like:
>>
>> bool functor(number const& a, number const& b)
>> {
>>   return a - b;
>> }
>>
>
However, I now see why allowing
bool b = t
can do harm, as in your example above.

2013/3/14 Peter Dimov <lists_at_[hidden]>

> I see; yes, this is something that works with "safe bool" and has been
> explicitly disallowed for the new explicit operator bool.

Too bad, IMHO.

Regards,
Kris