Re: [boost] Re: safe_bool alternative?

From: Joel de Guzman (joel_at_[hidden])
Date: 2003-11-27 18:09:03

David Abrahams <dave_at_[hidden]> wrote:
> Daryle Walker <darylew_at_[hidden]> writes:
> 
>> All of the operator helpers need the final type to provide a core routine.
>> AFAIK, Peter's idiom is to use a pointer-to-member(-function) as the core
>> Boolean routine; it can't be provided by a library.
> 
> What am I missing?
> 
> template <class T>
> struct bool_testable
> {
>  private:
>     struct impl { int x; };
>     typedef int impl::*safe_bool;
>  public:
>     operator safe_bool() const
>     {
>         return !*static_cast<T const*>(this) ? 0 : &impl::x;
>     }
> };
> 
> struct X : bool_testable<X>
> {
>     bool operator!() const { return true; }
> };
> 
> 
> struct Y : bool_testable<Y>
> {
>     bool operator!() const { return false; }
> };
> 
> 
> int main()
> {
>     X x;
>     Y y;
>     if (x) { return 1; }
> 
>     if (y)
>         return 0;
> 
> //    if (x == true) return 1; // fails to compile
> //    if (x == y) return 1;    // fails to compile
> 
>     return 0;
> }

Indeed. A library solution is possible and better. The only thing
I do not like with the solution above is the inheritance, which implies
multi-inheritance when X or Y are already subclasses. Well, you can 
chain the inheritance, but... The solution that I posted a while ago
doesn't require any inheritance at all. 

-- 
Joel de Guzman
http://www.boost-consulting.com
http://spirit.sf.net