Is it safe to serialize a raw boost::variant?

Try just including boost/serialization/variant.hpp; it does the work for you.

Regarding serialisation: It should work, yes. But why don't you use boost::variant's visitation mechanism to write out the actual type contained in the variant?

struct variant_serializer : boost::static_visitor<void> {
    template <typename T>
    typename boost::enable_if< boost::is_pod<T>, void>::type
    operator()( const T & t ) const {
        // ... serialize here, e.g.
        std::cout << t;
    }
};

int main() {

    const boost::variant<int,char,float,double> v( '1' );

    variant_serializer s;
    boost::apply_visitor( s, v );

    return 0;
}

Regarding shared memory: boost::variant does not perform heap allocations, so you can place it into shared memory just like an int, assuming proper synchronisation, of course.

Needless to say, as you said, the above is only valid if the variant can only contain POD types.