[boost] Re: implicit_cast issues

From: Valentin Bonnard (Bonnard.V_at_[hidden])
Date: 1999-11-05 10:32:52

Beman Dawes wrote:

> But implicit_cast<> needs more than reactivating the warnings.  There
> are other issues.  (For reference, the current definition is given:)
> 
>         template< typename Target, typename Source >
>         inline Target implicit_cast( Source s ) { return s; }

> Issue 1:  Valentin Bonnard has pointed out the conversion should be
> done at the argument, in case it requires an access privilege which
> implicit_cast doesn't enjoy.  I guess this would be formulated as:
> 
>         template< typename T >
>         inline T implicit_cast( T s ) { return s; }
> 
> Issue 2:  Ivan Johnson has pointed out a implicit_cast slicing
> problem:
> 
>     class Base {};
>     class Derived : public Base {};
> 
>     int main()
>     {
>         Derived d;
>         Derived& rd = d;
>         Base& rb = implicit_cast<Base&>(rd);
> 
>         if ( &rd != &rb )
>             cout << "oops\n";
> 
>         return 0;
>     }

It's worse than a slicing problem: it's a reference 
binding in a return statement to an automatic variable, 
ie a dandling reference.

> Ivan suggests fixing this by changing the arg to pass-by-reference:
> 
>         template< typename Target, typename Source >
>         inline Target implicit_cast( Source& s ) { return s; }
> 
> Issue 3:  Issue 1 and 2 interact.  The proposed resolution for 2
> doesn't fix 1.  The proposed solution for 1 also fixes 2 for the
> example given, but silently slices if the template parameter is
> omitted:
> 
>         Base& rb = implicit_cast(rd);  // slices

I would say that it copies, not slices. This is ill-formed 
anyway (as it should be).

-- 
Valentin Bonnard