Question

I’ve got a strange problem with templates and namespaces…

I have the following code which compiles fine..

using namespace boost::multi_index;

template < typename OT, typename KT, KT (OT::* KM)() const, typename KC, typename CMP >
class OrderBook
{
public:
    OrderBook() {}
    ~OrderBook() {}

    typedef multi_index_container<
        OT,
        indexed_by<
            ordered_unique<
                const_mem_fun< OT, KT, KM >,
                KC
            >,
            ordered_unique<
                identity< OT >,
                CMP
            >
        >
    > Container;

    typedef typename Container::template nth_index< 0 >::type index_0;
    typedef typename Container::template nth_index< 1 >::type index_1;

    typedef typename index_0::const_iterator const_iterator_0;
    typedef typename index_1::const_iterator const_iterator_1;

    const_iterator_0 begin0() const { return _container.get<0>().begin(); }
    const_iterator_0 end0() const { return _container.get<0>().end(); }


public:
    Container _container;
};

However, due to a namespace collision when I insert this code into another project I have to have… (Notice how I’ve had to remove the using namespace boost::multi_index and manually specify it where needed

template < typename OT, typename KT, KT (OT::* KM)() const, typename KC, typename CMP >
class OrderBook
{
public:
    OrderBook() {}
    ~OrderBook() {}

    typedef boost::multi_index::multi_index_container<
        OT,
        boost::multi_index::indexed_by<
            boost::multi_index::ordered_unique<
                boost::multi_index::const_mem_fun< OT, KT, KM >,
                KC
            >,
            boost::multi_index::ordered_unique<
                boost::multi_index::identity< OT >,
                CMP
            >
        >
    > Container;

    typedef typename Container::template nth_index< 0 >::type index_0;
    typedef typename Container::template nth_index< 1 >::type index_1;

    typedef typename index_0::const_iterator const_iterator_0;
    typedef typename index_1::const_iterator const_iterator_1;

    const_iterator_0 begin0() const { return _container.get<0>().begin(); }
    const_iterator_0 end0() const { return _container.get<0>().end(); }


public:
    Container _container;
};

Which gives me the following error from g++.

In member function 'typename boost::multi_index::multi_index_container<OT, boost::multi_index::indexed_by<boost::multi_index::ordered_unique<boost::multi_index::const_mem_fun<OT, KT, KM>, KC, mpl_::na>, boost::multi_index::ordered_unique<boost::multi_index::identity<Value>, CMP, mpl_::na>, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, std::allocator<_CharT> >::nth_index<0>::type::const_iterator OrderBook<OT, KT, KM, KC, CMP>::begin0() const':

error: expected primary-expression before ')' token


In member function 'typename boost::multi_index::multi_index_container<OT, boost::multi_index::indexed_by<boost::multi_index::ordered_unique<boost::multi_index::const_mem_fun<OT, KT, KM>, KC, mpl_::na>, boost::multi_index::ordered_unique<boost::multi_index::identity<Value>, CMP, mpl_::na>, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, std::allocator<_CharT> >::nth_index<0>::type::const_iterator OrderBook<OT, KT, KM, KC, CMP>::end0() const':

error: expected primary-expression before ')' token

Sorry for the long error messages, I did consider cleaning them up but I thought I’d better leave them intact in case I removed something crucial.

I tried this…

typedef typename Container::template boost::multi_index::nth_index< 0 >::type index_0;
typedef typename Container::template boost::multi_index::nth_index< 1 >::type index_1;

and it just made g++ even madder 🙁

Any ideas?

Answer 1

Prefix get<0>() with template:

const_iterator_0 begin0() const { return _container.template get<0>().begin(); }
const_iterator_0 end0  () const { return _container.template get<0>().end();   }

Similar to typename for dependent types, dependent templates have to be prefixed by template:

struct X {
    template<class T> void f();
};

template<class T>
void test() {
    T::f<int>(); // ill-formed
    T::template f<int>(); // ok
}

// ...
test<X>();

And for the curious, that is §14.2/4:

When the name of a member template
specialization appears after . or ->
in a postfix-expression, or after
nested-name-specifier in a
qualified-id, and the
postfix-expression or qualified-id
explicitly depends on a
template-parameter (14.6.2), the
member template name must be prefixed
by the keyword template. Otherwise the
name is assumed to name a
non-template.