[boost] [poly_collection] Custom RTTI with PolyCollection (was: Small review)

Subject: [boost] [poly_collection] Custom RTTI with PolyCollection (was: Small review)
From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2017-05-11 09:02:54

On 11/05/2017 8:26, Joaquin M López Muñoz via Boost wrote:
> El 09/05/2017 a las 15:46, Ion Gaztañaga via Boost escribió:
>> On 09/05/2017 13:39, Joaquin M López Muñoz via Boost wrote:
>>> typeid is used extensively throughout the library, both the static
>>> (typeid(T)) and the dynamic
>>> version (typeid(x)) --the latter only in base_collection and in one
>>> spot in
>>> boost/poly_collection/detail/segment.hpp. std::type_index is part of the
>>> public interface.
>>> Boost.TypeIndex could be in principle optionally used for RTTI-less
>>> scenarios, but I don't really
>>> know whether this is a common enough / requested scenario.
>>
>> [...]
>>
>> Basic examples in the documentation are based on games concepts
>> (sprite, warrior, goblin, ...) and usually in game development builds
>> RTTI is disabled although polymorphism is widely used. My comment is
>> about making the runtime-identification configurable by the user
>>
>> I can imagine an application where all classes are known and a custom
>> typeid can be generated and stored in the abstract base class of every
>> type, so that it can be retrieved via an inline function, without
>> noticeable run-time overhead. It could just cache a std::type_info
>> pointer.
>>
>> I think a lot of game engines use their own RTTI framework that could
>> be easily plugged into PolyCollection (maybe as an additional
>> TypeIdTraits template parameter). Something like:
>>
>> class typeid_traits
>> {
>>    typedef /**/ type_info_t;
>>    typedef /**/ type_index_t;
>>
>>    template<class T>
>>    static type_info_t typeid_of(T *p);
>>
>>    //Usage: const type_info_t &ti = typeid_of<MyClass>();
>>    template<class T>
>>    static type_info_t typeid_of();
>> };
>>
>> type_index_t should be constructible from type_info, hashable and
>> comparable.
>>
>> This type of abstraction could support Boost.TypeInfo and many other
>> approaches. Anyone sees this customization useful?
>
> I'm no expert in game development so I can't really assess how common
> this custom
> typeid thing is. In any case, if the community deems this worth
> including I can
> definitely work on it; one thing I don't get about your proposed design
> is the
> insistence on the type_info_t/type_index_t distinction: to the best of
> my knowledge
> std::type_index should have been what typeid() returned and was added as
> a patch
> over less useful typeinfo so as not to break backwards compatibility, so
> here we can just
> as well learn from the past and simply use type_index_t (with typeid_of
> directly
> returning type_index_'s rather than  type_info_t's), right?

Yes, it was just a quick sketch, a single opaque type would work better. 
The idea is to synthesize the minimum abstraction that PolyCollection needs.

Two further
> doubts:
>
> * How does this combine with usage of Boost.TypeIndex? I mean, can usage of
> Boost.TypeIndex be shoehorned into this typeid_traits framework as an
> alternative

I have no previous experience but Boost.TypeIndex claims to be a drop-in 
replacement for std::type_index:

typeid(T) -> boost::typeindex::type_id<T>()
typeid(variable) -> boost::typeindex::type_id_runtime(variable)

so I expect that Boost.TypeIndex integration will be fine. In general, a 
very simple scheme of:

- Obtaining the typeid from a type
- Obtaining the typeid from a variable
- Be able to compare and hash the typeid.

could work with most alternatives. The scheme where the abstract base 
class stores the id (say, an integer or a pointer) and offers it using 
an inline function is certainly easily implementable with this 
straightforward concept.

> * function_collection and any_collection share 99% of the code with
> base_collection
> (the three containers are basically the same with different so-called
> "model"
> policies), so this traits thing could extend almost automatically to
> them. But I fail
> to see what the use of it would be. Except when the selected traits are
> those of
> Boost.TypeIndex... well, possibilities abound here.

In general, if RTTI is disabled and any heterogeneous callback container 
is needed (which is also typical in some environments) an alternative 
must be used to distinguish between callable types at runtime. I have 
never used a custom RTTI with callable types but I can imagine an scheme 
where different function objects are required to have some kind of 
protocol (say, base class) that returns the typeid. I don't know if raw 
function pointers or "member to function types + this" can be easily 
detected and an implicit typeid can be used. But I haven't thought much 
about it, the main use case, I think, is related to inheritance 
polymorphism.

The "model" policy is a very interesting design that is (until now) 
hidden in the implementation. It seems extremely powerful. I will study 
it a bit further because it might provide a radical generalization 
opportunity for user-defined polymorphism models beyond the three 
provided by default by PolyCollection.

Ion