Re: [boost] Re: More on the optional class.

From: Douglas Gregor (gregod_at_[hidden])
Date: 2001-09-17 14:57:37

On Monday 17 September 2001 11:35, you wrote:
[snip]
> With respect to the 'operator T*()', I haven't adopted it becuase it would
> allow the following:
>
>   optional<int> opt1 = 1234 ;
>   optional<int> opt2 = 1234 ;
>
>   if ( opt1 == opt2) // this will compile OK if there is an 'operator
> T*()', but won't evaluate as expected.
>
> the above boolean expression is ill-formed w.r.t the optional
> specification, but there is no compile-time nor runtime indication of that.

You can "poison" comparisons between optional<T>'s very easily, however, like 
this:

template<typename T1, typename T2>
void operator==(const optional<T1>&, const optional<T>&);

Since the operator == returns void, any attempt to use "opt1 == opt2" will 
generate a compile-time error. In the pointless case that the user writes the 
(harmless) statement expression "opt1==opt2;", the failure will occur at link 
time.

>  I decided to support only those idioms that would prevent the mistakes as
> above. AFAICT, these are (1) operator !(), (2) a friend 'T* get(opt)'
> function (thanks to Peter Dimov!).

Many of these concerns (how to use an object in a boolean context) were 
discussed prior to the review of Function. I settled on the use of Peter 
Dimov's pointer-to-member-of-useless-class technique for the boolean 
conversion, and on poisoning operator== and operator!= for Function class 
templates. The source file boost/function/function_base.hpp contains the code 
for both.

The fact that this discussion has come up again hints that these solutions 
should perhaps be documented somewhere. I would suggest the addition of a 
"safebool" type to utility.hpp that assists in the object-to-bool conversion. 
Similarly, operator poisoning would belong in operators.hpp: for instance, a 
type "not_equality_comparable" could be derived from to poison operators == 
and != for the given class.

If there is interest I can submit these for review.

        Doug