[boost] Re: safe_bool alternative?

From: Daniel Frey (daniel.frey_at_[hidden])
Date: 2003-11-28 05:39:28

David Abrahams 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; }

Now add

       operator int() const { return 0; }

and see what happens. It compiles, but is definitely not what the user 
intended. "if(x)" calls "operator int()" now! The current bool_testable 
doesn't have this problem. Also, the current implementation of 
bool_testable expectes the user to provide operator bool(), it adds 
safety (and operator!()), which is IMHO the most obvious, preferable 
interface and matches the naming conventions of the operators library.

Regards, Daniel

-- 
Daniel Frey
aixigo AG - financial solutions & technology
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