[Boost-users] Problems with boost::variant

From: Alex Drummond (a.drummond_at_[hidden])
Date: 2004-11-16 05:11:38

Hi,

I'm using boost 1.31.0 and I'm having some trouble using type sequences with 
boost::variant. There seems to be an arbitrary limit on their length. For 
example, the following code compiles:

[----------------------------------------
#include <boost/variant.hpp>
#include <boost/mpl/vector.hpp>

typedef boost::mpl::vector< boost::recursive_wrapper<struct A> > T0;
typedef boost::mpl::push_front<T0,  boost::recursive_wrapper<struct B> >::type 
T1;
typedef boost::mpl::push_front<T1,  boost::recursive_wrapper<struct C> >::type 
T2;
typedef boost::mpl::push_front<T2,  boost::recursive_wrapper<struct D> >::type 
T3;
typedef boost::mpl::push_front<T3,  boost::recursive_wrapper<struct E> >::type 
T4;
typedef boost::mpl::push_front<T4,  boost::recursive_wrapper<struct F> >::type 
T5;
typedef boost::mpl::push_front<T5,  boost::recursive_wrapper<struct G> >::type 
T6;
typedef boost::mpl::push_front<T6,  boost::recursive_wrapper<struct H> >::type 
T7;
typedef boost::mpl::push_front<T7,  boost::recursive_wrapper<struct I> >::type 
T8;

typedef boost::make_variant_over<T8>::type Var;

struct A { Var v; };
struct B { Var v; };
struct C { Var v; };
struct D { Var v; };
struct E { Var v; };
struct F { Var v; };
struct G { Var v; };
struct H { Var v; };
struct I { Var v; };

int main() { return 0; }
----------------------------------------]

But with the addition of one more type to the sequence:

[----------------------------------------
typedef boost::mpl::push_front<T8,  boost::recursive_wrapper<struct J> >::type 
T9;
......
struct J { Var v; }
----------------------------------------]

And an appropriate redefinition of Var:

typedef boost::make_variant_over<T9>::type Var;

gcc 3.3.3 gives various obscure template error messages:

/usr/include/boost/mpl/apply_if.hpp: In instantiation of 
`boost::mpl::push_front<boost::mpl::vector10<boost::recursive_wrapper<J>, 
boost::recursive_wrapper<I>, boost::recursive_wrapper<H>, 
boost::recursive_wrapper<G>, boost::recursive_wrapper<F>, 
boost::recursive_wrapper<E>, boost::recursive_wrapper<D>, 
boost::recursive_wrapper<C>, boost::recursive_wrapper<B>, 
boost::recursive_wrapper<A> >, boost::detail::variant::backup_holder<void*> 
>':
/usr/include/boost/mpl/apply_if.hpp:39:   instantiated from 
`boost::mpl::apply_if<boost::variant<boost::detail::variant::over_sequence<T9>, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_>::has_fallback_type_, boost::mpl::identity<T9>, 
boost::mpl::push_front<boost::mpl::vector10<boost::recursive_wrapper<J>, 
boost::recursive_wrapper<I>, boost::recursive_wrapper<H>, 
boost::recursive_wrapper<G>, boost::recursive_wrapper<F>, 
boost::recursive_wrapper<E>, boost::recursive_wrapper<D>, 
boost::recursive_wrapper<C>, boost::recursive_wrapper<B>, 
boost::recursive_wrapper<A> >, boost::detail::variant::backup_holder<void*> > 
>'
/usr/include/boost/variant/variant.hpp:226:   instantiated from 
`boost::detail::variant::make_storage<boost::mpl::vector10<boost::recursive_wrapper<J>, 
boost::recursive_wrapper<I>, boost::recursive_wrapper<H>, 
boost::recursive_wrapper<G>, boost::recursive_wrapper<F>, 
boost::recursive_wrapper<E>, boost::recursive_wrapper<D>, 
boost::recursive_wrapper<C>, boost::recursive_wrapper<B>, 
boost::recursive_wrapper<A> >, 
boost::variant<boost::detail::variant::over_sequence<T9>, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_>::has_fallback_type_>'
/usr/include/boost/variant/variant.hpp:1079:   instantiated from 
`boost::variant<boost::detail::variant::over_sequence<T9>, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_>'
test.cc:19:   instantiated from here
/usr/include/boost/mpl/apply_if.hpp:39: error: base class `
   boost::mpl::push_front_traits<boost::mpl::aux::vector_tag<10> 
   >::algorithm<boost::mpl::vector10<boost::recursive_wrapper<J>, 
   boost::recursive_wrapper<I>, boost::recursive_wrapper<H>, 
   boost::recursive_wrapper<G>, boost::recursive_wrapper<F>, 
   boost::recursive_wrapper<E>, boost::recursive_wrapper<D>, 
   boost::recursive_wrapper<C>, boost::recursive_wrapper<B>, 
   boost::recursive_wrapper<A> >, boost::detail::variant::backup_holder<void*> 
   >' has incomplete type
/usr/include/boost/mpl/apply_if.hpp: In instantiation of 
`boost::mpl::apply_if<boost::variant<boost::detail::variant::over_sequence<T9>, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_>::has_fallback_type_, boost::mpl::identity<T9>, 
boost::mpl::push_front<boost::mpl::vector10<boost::recursive_wrapper<J>, 
boost::recursive_wrapper<I>, boost::recursive_wrapper<H>, 
boost::recursive_wrapper<G>, boost::recursive_wrapper<F>, 
boost::recursive_wrapper<E>, boost::recursive_wrapper<D>, 
boost::recursive_wrapper<C>, boost::recursive_wrapper<B>, 
boost::recursive_wrapper<A> >, boost::detail::variant::backup_holder<void*> > 
>':
/usr/include/boost/variant/variant.hpp:226:   instantiated from 
`boost::detail::variant::make_storage<boost::mpl::vector10<boost::recursive_wrapper<J>, 
boost::recursive_wrapper<I>, boost::recursive_wrapper<H>, 
boost::recursive_wrapper<G>, boost::recursive_wrapper<F>, 
boost::recursive_wrapper<E>, boost::recursive_wrapper<D>, 
boost::recursive_wrapper<C>, boost::recursive_wrapper<B>, 
boost::recursive_wrapper<A> >, 
boost::variant<boost::detail::variant::over_sequence<T9>, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_>::has_fallback_type_>'
/usr/include/boost/variant/variant.hpp:1079:   instantiated from 
`boost::variant<boost::detail::variant::over_sequence<T9>, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_>'
test.cc:19:   instantiated from here
/usr/include/boost/mpl/apply_if.hpp:39: error: no type named `type' in `struct 
   boost::mpl::push_front<boost::mpl::vector10<boost::recursive_wrapper<J>, 
   boost::recursive_wrapper<I>, boost::recursive_wrapper<H>, 
   boost::recursive_wrapper<G>, boost::recursive_wrapper<F>, 
   boost::recursive_wrapper<E>, boost::recursive_wrapper<D>, 
   boost::recursive_wrapper<C>, boost::recursive_wrapper<B>, 
   boost::recursive_wrapper<A> >, boost::detail::variant::backup_holder<void*> 
   >'
/usr/include/boost/variant/variant.hpp: In instantiation of 
`boost::detail::variant::make_storage<boost::mpl::vector10<boost::recursive_wrapper<J>, 
boost::recursive_wrapper<I>, boost::recursive_wrapper<H>, 
boost::recursive_wrapper<G>, boost::recursive_wrapper<F>, 
boost::recursive_wrapper<E>, boost::recursive_wrapper<D>, 
boost::recursive_wrapper<C>, boost::recursive_wrapper<B>, 
boost::recursive_wrapper<A> >, 
boost::variant<boost::detail::variant::over_sequence<T9>, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_>::has_fallback_type_>':
/usr/include/boost/variant/variant.hpp:1079:   instantiated from 
`boost::variant<boost::detail::variant::over_sequence<T9>, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_>'
test.cc:19:   instantiated from here
/usr/include/boost/variant/variant.hpp:226: error: no type named `type' in `
   struct 
   boost::mpl::apply_if<boost::variant<boost::detail::variant::over_sequence<T9>,
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_>::has_fallback_type_, 
   boost::mpl::identity<T9>, 
   boost::mpl::push_front<boost::mpl::vector10<boost::recursive_wrapper<J>, 
   boost::recursive_wrapper<I>, boost::recursive_wrapper<H>, 
   boost::recursive_wrapper<G>, boost::recursive_wrapper<F>, 
   boost::recursive_wrapper<E>, boost::recursive_wrapper<D>, 
   boost::recursive_wrapper<C>, boost::recursive_wrapper<B>, 
   boost::recursive_wrapper<A> >, boost::detail::variant::backup_holder<void*> 
   > >'
/usr/include/boost/variant/variant.hpp:230: error: no type named `type' in `
   struct 
   boost::mpl::apply_if<boost::variant<boost::detail::variant::over_sequence<T9>,
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_>::has_fallback_type_, 
   boost::mpl::identity<T9>, 
   boost::mpl::push_front<boost::mpl::vector10<boost::recursive_wrapper<J>, 
   boost::recursive_wrapper<I>, boost::recursive_wrapper<H>, 
   boost::recursive_wrapper<G>, boost::recursive_wrapper<F>, 
   boost::recursive_wrapper<E>, boost::recursive_wrapper<D>, 
   boost::recursive_wrapper<C>, boost::recursive_wrapper<B>, 
   boost::recursive_wrapper<A> >, boost::detail::variant::backup_holder<void*> 
   > >'
/usr/include/boost/variant/variant.hpp:236: error: no type named `type' in `
   struct 
   boost::mpl::apply_if<boost::variant<boost::detail::variant::over_sequence<T9>,
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_>::has_fallback_type_, 
   boost::mpl::identity<T9>, 
   boost::mpl::push_front<boost::mpl::vector10<boost::recursive_wrapper<J>, 
   boost::recursive_wrapper<I>, boost::recursive_wrapper<H>, 
   boost::recursive_wrapper<G>, boost::recursive_wrapper<F>, 
   boost::recursive_wrapper<E>, boost::recursive_wrapper<D>, 
   boost::recursive_wrapper<C>, boost::recursive_wrapper<B>, 
   boost::recursive_wrapper<A> >, boost::detail::variant::backup_holder<void*> 
   > >'
/usr/include/boost/variant/variant.hpp:252: error: no type named `type' in `
   struct 
   boost::mpl::apply_if<boost::variant<boost::detail::variant::over_sequence<T9>,
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_>::has_fallback_type_, 
   boost::mpl::identity<T9>, 
   boost::mpl::push_front<boost::mpl::vector10<boost::recursive_wrapper<J>, 
   boost::recursive_wrapper<I>, boost::recursive_wrapper<H>, 
   boost::recursive_wrapper<G>, boost::recursive_wrapper<F>, 
   boost::recursive_wrapper<E>, boost::recursive_wrapper<D>, 
   boost::recursive_wrapper<C>, boost::recursive_wrapper<B>, 
   boost::recursive_wrapper<A> >, boost::detail::variant::backup_holder<void*> 
   > >'
/usr/include/boost/variant/variant.hpp: In instantiation of 
`boost::variant<boost::detail::variant::over_sequence<T9>, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_, boost::detail::variant::void_, 
boost::detail::variant::void_>':
test.cc:19:   instantiated from here
/usr/include/boost/variant/variant.hpp:1079: error: no type named `type' in `
   struct 
   boost::detail::variant::make_storage<boost::mpl::vector10<boost::recursive_wrapper<J>,
   boost::recursive_wrapper<I>, boost::recursive_wrapper<H>, 
   boost::recursive_wrapper<G>, boost::recursive_wrapper<F>, 
   boost::recursive_wrapper<E>, boost::recursive_wrapper<D>, 
   boost::recursive_wrapper<C>, boost::recursive_wrapper<B>, 
   boost::recursive_wrapper<A> >, 
   boost::variant<boost::detail::variant::over_sequence<T9>, 
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_>::has_fallback_type_>'
/usr/include/boost/variant/variant.hpp:1118: error: no type named `type' in `
   struct 
   boost::detail::variant::make_storage<boost::mpl::vector10<boost::recursive_wrapper<J>,
   boost::recursive_wrapper<I>, boost::recursive_wrapper<H>, 
   boost::recursive_wrapper<G>, boost::recursive_wrapper<F>, 
   boost::recursive_wrapper<E>, boost::recursive_wrapper<D>, 
   boost::recursive_wrapper<C>, boost::recursive_wrapper<B>, 
   boost::recursive_wrapper<A> >, 
   boost::variant<boost::detail::variant::over_sequence<T9>, 
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_, boost::detail::variant::void_, 
   boost::detail::variant::void_>::has_fallback_type_>'

I'd be grateful for any help...
Thanks
Alex

-- 
"What possible harm could one insane mutant tentacle do?"