Re: [boost] [safebool] Can we generalize it and put it into utilities?

Subject: Re: [boost] [safebool] Can we generalize it and put it into utilities?
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2009-03-30 09:19:49

On Monday, March 30, 2009 7:34 AM
Vladimir Batov wrote:
> > From: "Steven Watanabe" <watanabesj_at_[hidden]>
> > So, here's another take on it:
> >
> > template<class Tag>
> > struct safe_bool {
> >    typedef void (safe_bool::*result_type)();
> >    result_type operator()(bool b) { return b?
> &safe_bool::true_ : 0 }
> > private:
> >    void true_() {}
> > };
>
> Yes, "templatazing" makes sense. The deployment will probably

It isn't immediately obvious why it is sensible, so that must be documented.

> be as follows:
>
> class Foo
> { ...
>     operator safe_bool<Foo>::result_type() const { return
> safe_bool<Foo>()(my_condition); }
> };

The extra parentheses to invoke the function call operator will trip up many would-be users, so a named function would be better.

A function template can make this even easier:

   template <class Tag>
   inline safe_bool<Tag>::result_type
   make_safe_bool(bool _value)
   {
      return safe_bool<Tag>::apply(_value);
   }

Usage is now:

   class Foo
   {
      ...
      operator safe_bool<Foo>::result_type() const
      {
         return make_safe_bool<Foo>(condition_);
      }
   };

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer, Core Software      using std::disclaimer;
Susquehanna International Group, LLP  http://www.sig.com

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