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@think-cell.com 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