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Subject: Re: [boost] [variant]compile-time-checked boost::get<T>?
From: Arno Schödl (aschoedl_at_[hidden])
Date: 2008-12-11 04:05:11
> > As a further extension, boost::get<S> where S is a base class of a T contained in the variant, should also work.
> > I believe this would also be implementable with enable_if/disable_if, but we have not done it, so no guarantee.
> Actually, it should work with all types that are convertible from any of
> the types of the variant.
Arguably, only conversion to an L-value are o.k., do you agree? But boost::is_convertible<T const&, S const&> compiles on MSVC9 even if the conversion is to an R-value:
class S {};
class T {
public:
operator S() const{
return S(); // returns a value!
}
};
// but cast to const reference works
T t;
static_cast<S const&>(t);
// and is_convertible says it does
BOOST_STATIC_ASSERT( boost::is_convertible<T const& BOOST_PP_COMMA() S const&>::value );
I think we must require boost::is_convertible<T &, S &> and also make sure static_cast<S &>(T) is actually used. The cast inside boost::get for a constant variant would then look like this:
static_cast<S const&>( static_cast<S &>( const_cast<T &>( t ) ) );
I am not sure whether this always does the intended thing. The problem to ensure that a cast is to L-value seems quite general, so maybe someone knows a standard solution?
P.S. Did I get the L-value vs. R-value terminology right :-) ?
-- Dr. Arno Schoedl · aschoedl_at_[hidden] Technical Director think-cell Software GmbH · Invalidenstr. 34 · 10115 Berlin, Germany http://www.think-cell.com · phone +49-30-666473-10 · toll-free (US) +1-800-891-8091 Directors: Dr. Markus Hannebauer, Dr. Arno Schoedl · Amtsgericht Charlottenburg, HRB 85229
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