relational::operator bool()

[Richard B. Kreckel]

On Wed, 14 Nov 2001, Douglas Gregor wrote:
> Hello,
>   This is just a C++ nitpick, but operator bool() is often considered harmful 
> because of the plethora of implicit conversions (bool->int being the worst of 
> these implicit conversions, IMHO). I'm not suggesting that operator bool() be 
> removed from relational, but instead replace it with a safer construct.
>   Within the Boost C++ libraries (http://www.boost.org), we've adopted a 
> "safe_bool" conversion using of a pointer-to-member function. The trick can 
> be illustrated concisely:
> 
> // Add to class relational
>   private:
>     struct dummy {
>       void nonnull() {};
>     };
> 
>     typedef void (dummy::*safe_bool)();
> 
>    safe_bool make_safe_bool(bool cond) const
>       { return cond?  &dummy::nonnull : 0; }
> 
> Then in relational::operator safe_bool() const,  whereever there is a 
> "return <bool-expression>", it should be replaced with 
> "return make_safe_bool(<bool-expression>)".
> 
> The use of the pointer-to-member function eliminates implicit conversions, 
> but the relational class can still be used in a boolean context (since it is 
> evaluated as "is the pointer-to-member function null?"). All meaningless 
> operations that are allowed by a "bool" conversion except for == and != are 
> eleminated by this "safe_bool".
> 
> I can submit a patch against CVS if needed.

Can you please email that patch?  (I'm still trying to grok whether this
breaks compatibilities.  Binary yes, I guess.)

Regards
    -richy.
-- 
Richard Kreckel
<Richard.Kreckel at Uni-Mainz.DE>
<http://wwwthep.physik.uni-mainz.de/~kreckel/>