Re: [Boost-users] Indirectly 'recursive' type erasure

AMDG On 03/06/2014 04:01 PM, Samuel Christie wrote:

I'm trying to create a type erasure concept for a type that has a member function which can return a custom collection of itself.

Having had little luck with the general formulation, I tried simplifying it to:

BOOST_TYPE_ERASURE_MEMBER((has_collection), collection, 0);

typedef any< mpl::vector< copy_constructible<>, typeid_<>, relaxed, has_collection()> >

...

test;

class Test { public: vector<test> collection(); };

Try this: struct my_requirements; typedef any test; struct my_requirements mpl::vector<..., has_collection > {}; The easiest way to handle a cyclic dependency is to forward declare something, in this case my_requirements, should do the trick.

However, I get conversion errors if the Test::collection method returns either vector<Test> or vector<test>; it only seems to match if I have it literally return vector<_self>, which implies to me that I'm not understanding what I'm doing, and that _self isn't getting substituted in this case.

That is the expected behavior. See http://www.boost.org/doc/libs/1_55_0/doc/html/boost_typeerasure/conceptdef.h... "The template parameters of the concept may involve placeholders. The following are considered. Each template argument may be a cv and/or reference qualified placeholder type. If a template argument is a function type, its arguments and return type may be cv/reference qualified placeholders. Any other placeholders are ignored."

What do I need to do in order to be able to return types that are dependent on the erasure type?

It's technically possible for me to make has_collection()> work, but it would require a rather messy customization layer. It can't be made to work automatically for any X<T> without help.

My actual code uses a more indirect relationship, with the collection method returning a custom erased collection type that has an erased iterator that can dereference to the equivalent of Test. However, since there is still a form of cyclic dependency, I'm not really sure what I can do about it. I've tried using a custom struct based concept that had direct template arguments in the place of 'boost type erasure member', but I think I understand that technique even less, and it didn't seem to help much.

It looked like there might be something useful under the Associated Types section of the type erasure documentation, but I couldn't quite figure it out.

It sounds like Associated Types may be what you want for your real use case. It would look something like this: // Calculates the return type of T::collection() template<class T> struct get_collection_type { typedef std::vector<T> type; }; typedef deduced > _collection; typedef mpl::vector< // requirements on test copy_constructible<>, typeid_<>, has_collection<_collection()>, // requirements on _collection copy_constructible<_collection>, // has_begin/has_end<_collection> // requirements on the iterator (using _self as the value_type)

requirements;

typedef any<requirements> test; typedef any erased_collection; typedef any erased_iterator; Note that since test, the erased collection, and the erased iterator have a cyclic dependency, the requirements for them all need to be defined at once, for the library to resolve it. In Christ, Steven Watanabe