Compile error when using template aliases, inheriting template and using a "template parent's" types

Question

I have a data container which has following requirements:

Be fast: Hence templates and not normal inheritance
Use different implementations
Be able to extend those implementations with more methods
Data is specified via template argument and needs to be able to save pointers to data container items

The solution I have come up with is as follows:

    template<typename T, template<typename> class container_t>
class data_c
{
public:
    typedef data_c<T, container_t> value_type;
    typedef typename container_t<value_type>::ref container_ref;

    container_ref link;
    T c;
};

template<typename T>
class storage_container_impl
{
public:
    typedef T value_type;
    typedef storage_container_impl<T>* ref;
    T data;
};

template<typename _impl>
class storage_container : public _impl
{
public:
    typedef typename _impl::value_type value_type;
    typedef typename _impl::ref ref;
};

template<typename T>
using impl_storage_container = storage_container<storage_container_impl<T> >;

typedef impl_storage_container<data_c<int, impl_storage_container> > c_storage_container;

int main()
{
    c_storage_container tmp;
    tmp.data.c=5;
    tmp.data.link=&tmp;
    return tmp.data.c;
}

Which results in following error (gcc 4.7):

test1.cpp:6:48: error: no type named 'ref' in 'impl_storage_container<data_c<int, impl_storage_container> > {aka class storage_container<storage_container_impl<data_c<int, impl_storage_container> > >}'

It works if I use T *data instead of T data (But I do not want that indirection), or if I do not use storage_container_impl and have T data directly in storage_container. Using storage_container_impl mixin-style does not solve the problem as well. As container_ref is a pointer, there also should be no reason why it does not work, e.g. because there is a loop in the template declaration. This is a minimized version of the problem. Any help would be appreciated!

I'd guess that at the point where `data` is declared `data_c` is instantiated, which in turn refers to `storage_container`, which is not complete at that point yet. Clang doesn't accept it as well, which I take as indication that it's in fact illegal. — Philipp, Jul 18 '12 at 20:56
So ... your container should contain a single instance of a wrapper type, containing a single instance of the value type and a back pointer to the container. Is that really right? — Useless, Jul 18 '12 at 20:56
Obvious question: have you measured that virtual polymorphism would indeed be too slow in your case? — Philipp, Jul 18 '12 at 20:57
@Philipp No I didn't, but I'm working with someone else's code and he chose the template route. If this is illegal, I'll just save `link` as `void*` and cast, I think. — UrOni, Jul 18 '12 at 21:09
I can't quote the standard, but if Clang and GCC agree there's a change they're right :) When I tried to formulate `data_c` using the "normal" CRTP (without a template template parameter), I think I encountered an endless type recursion. That might indicate that C++ actually doesn't allow this structure (although it's possible that it allows more type recursion with template template parameters, I know too little about them to say for sure). — Philipp, Jul 18 '12 at 21:15
"*I'll just save `link` as `void*` and cast*" No, please don't -- this completely defeats the purpose of using templates for type safety. — ildjarn, Jul 18 '12 at 21:16

score 1 · Accepted Answer · answered Jul 19 '12 at 10:42

I know that it's not a perfect solution, but you can try to replace

typedef impl_storage_container<data_c<int, impl_storage_container> > c_storage_container;

with

typedef impl_storage_container<data_c<int, storage_container_impl> > c_storage_container;

It compiles and work. Otherwise you get endless type recursion.

Compile error when using template aliases, inheriting template and using a "template parent's" types

1 Answers1