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Subject: Re: [boost] safe-bool CRTP class
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-05-27 15:55:13

Krzysztof Czainski wrote:
> 2011/5/27 Stewart, Robert <Robert.Stewart_at_[hidden]>
> > Krzysztof Czainski wrote:
> >
> > class boolable : public Base
> >
> > Why not s/boolable/safe_bool/?
> >
> I was trying to find a name than would fit nice into the
> operators library, together with andable for example. Under
> the name boolable I understand it may be tested for
> true/false like
> if ( x ) ...
> if ( !x ) ...
> But I am not against changing the name.

I see. Unfortunately, "boolable" just sounds horrible to me.

> > > operator unspecified_bool_t() const
> > > {
> > > return ! static_cast<Derived const&>( *this ) ? 0 :
> > > &boolable::unspecified_bool_t_f;
> >
> > This doesn't support types that have no other reason to
> > define operator !() or that have a ready-made is_valid()
> > sort of function. IOW, the "valid" or "invalid" function
> > on which to rely should be configurable.

Notice that I'm calling for the function to be configurable.

> (It should be possible to provide either one.)

Here I suggested that it should be possible to specify a function that indicates the object is valid *or* one that indicates the object is invalid.

> > I understand that you're assuming that if one can write if
> > (x), one might assume that if (!x) should also work, but
> > that should work anyway with the safe-bool operator, right?
> Right. So the question is what function should the user
> provide? I proposed operator!. Other options would be
> is_valid/is_invalid, to_bool . Or should this function's name
> be customizable?

The latter.

A policy class would permit installing code to call various derived class functions and to negate the result, if necessary. By default, you can specify a policy class that expects operator !().

That approach would give the default you're currently advocating and give users flexibility to use an existing function rather than create operator !(). That only makes sense if the library user has many classes with some particular function they can use in lieu of operator !() because otherwise, specializing the policy wouldn't be worth the trouble.

We have legacy code, for example, that specifies a virtual isValid() member function. A policy class that returns the result of calling derivate::isValid() would be just the ticket for that hierarchy.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer using std::disclaimer;
Dev Tools & Components
Susquehanna International Group, LLP

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