[boost] Re: safe_bool alternative?

From: Nicola Musatti (nmusatti_at_[hidden])
Date: 2003-11-27 02:59:22

Joel de Guzman wrote:
[...]
> Can you spot some problems with this too? The point is that it would be
> nice to have one *correct* and generic  implementation that everyone
> can use. It's easy to get it wrong (I did). There are some subtle issues
> that make it an ideal candidate for a small library component (e.g. 
> Borland does not like the pointer to member stuff, maybe there's a
> better formulation for Borland? e.g. While the code above is fine with
> g++3.2 and Comeau, VC7.1 does not like it in some cases, etc...).

How about something like:

template <typename T> class bool_convertible {
   public:
     typedef void (bool_convertible::* safe_bool )();
   protected:
     safe_bool make_safe_bool(bool cond) const {
       return cond ? &bool_convertible::dummy : 0;
     }
   private:
     void dummy() {}
};

To be used as:

class A : public boost::bool_convertible<A>
{
   public:
     A(int i) : a(i) {}
     operator safe_bool() const { return make_safe_bool(a != 0); }
   private:
     int a;
};

It has the drawback that class templates require a redefinition of the 
safe_bool typedef. I only made a very simple test, but it appears to be 
accepted by Borland 5.6.4 .

Cheers,
Nicola Musatti