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: Steven Watanabe (watanabesj_at_[hidden])
Date: 2009-03-29 21:53:54

AMDG

Vladimir.Batov_at_[hidden] wrote:
> Repeating safe-bool scaffolding over and over again is a real nuisance. It 
> is worse for the end users as not exactly everyone is intimately familiar 
> with the issue and the safe-bool workaround. Can we generalize it and put 
> something like the following into utilities?
>
> struct safebool
> {
>     typedef void (safebool::*unspecified_bool_type)() const;
>     typedef unspecified_bool_type result;
>
>     explicit safebool(bool v) : value_(v) {}
>
>     operator result() const { return value_ ? &safebool::internal_bool : 
> 0; }
>
>     private:
>
>     void internal_bool() const {};
>
>     bool value_;
> };
>
> That way all we (and the users) will have to do to deploy the technique 
> will be 
>
> class Foo
> { ...
>     operator safebool::result() const { return safebool(my_condition); }
> };
>
> This seems simple and hassle-free.
>   

I just noticed this in boost/detail/identifier.hpp

      typedef void (*unspecified_bool_type)(D); // without the D, 
unspecified_bool_type
      static void unspecified_bool_true(D){}    // conversion allows 
relational operators
                                                // between different 
identifier types

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_() {}
};

In Christ,
Steven Watanabe