[boost] Re: implicit_cast issues

From: Valentin Bonnard (Bonnard.V_at_[hidden])
Date: 1999-11-05 11:36:01

C. Green wrote:
> 
> At 10:15 AM -0500 1999.11.05, Beman Dawes wrote:
> >Darin Adler wrote:
> >
> [Darin's comments on implicit conversions in implicit_cast]
> 
> >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:
> >
> [snip]
> >        Derived d;
> >        Derived& rd = d;
> >        Base& rb = implicit_cast<Base&>(rd);
> 
> I assume this must be for the first (two-typename template func), because
> the second one (single typename) will work fine here too.
> 
> >        if ( &rd != &rb )
> >            cout << "oops\n";
> [snip]
> >Issue 3:  Issue 1 and 2 interact.  The proposed resolution for 2
> >[take Source& instead of plain Source] 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
> 
> First, let's assume you show conforming code that compiles:
> 
>          Base const& rb = implicit_cast(rd);
> 
> In other words, this bug is not caught. implicit_cast without a
> specification of the destination type is meaningless.
> 
> >Anyone have a resolution that solves all three issues?
> 
>     template<typename T>
>     struct mirror_type {
>         typedef T type;
>     };
> 
>     template<typename T>
>     inline T implicit_cast( typename mirror_type<T>::type t ) {
>         return t;
>     }
> 
> Will this work?

Oups ! yes !

> Side comment, implicit_cast seems to break code that casts to a const ref type:
> 
>     int const& ir = 1.0; // OK - temp int lives as long as ir lives
>     int j = ir; // OK
> 
>     int const& ir2 = implicit_cast<int const&> (1.0); // temp destroyed at
> EOS, right?

Yes

>     int j2 = ir2; // undefined

Yes

> This can be safe if no temp is generated, but how do we know that one is
> not being generated? (user-defined conversions, built-in conversion like
> above)

If implicit_cast cannot be used on references types, it just 
isn't a general purpose implicit cast anymore and shouldn't 
be called implicit_cast.

-- 
Valentin Bonnard