[boost] implicit_cast issues

From: Beman Dawes (beman_at_[hidden])
Date: 1999-11-05 10:15:20

Darin Adler wrote:

>We should change the Boost implementation of implicit_cast to a
simple
>implementation that does not attempt to disable any warnings.
>
> ... [snip rationale]
>
>A cast that can do only implicit conversions, but disables implicit
numeric
>conversion warnings, is not as useful as a cast that can do only
implicit
>conversions with all the same warnings that you get in any other
context
>where an implicit conversion is done.

I agree with the above, particularly now that we have a strong
numeric_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:

    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;
    }

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

Anyone have a resolution that solves all three issues?

--Beman