Re: [boost] yomm2 - open methods

Subject: Re: [boost] yomm2 - open methods
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2018-01-19 21:25:36

AMDG

On 01/19/2018 01:50 PM, Jean-Louis Leroy via Boost wrote:
> I have zero experience using TypeErasure so what I am going to say
> here may miss the point entirely.
> 
> A priori it's two different worlds because open methods suppose that
> there is an is-a relationship between the virtual parameters in the
> method declaration and the corresponding arguments in the method
> definitions (aka overrides or specializations). It also supposes that
> virtual parameters are polymorphic types.
> 

But why exactly do you require it?
a) You can do a (static|dynamic)_cast from the base to the
   derived type.  Replace this with any_cast<Derived*>(&a) or
   static_cast<Derived*>(any_cast<void*>(&a)) for any.
b) You can determine the dynamic type of the object with typeid.
   Replace this with typeid_of(a) for any.

Plain boost::any has similar features, although I don't think
it supports any_cast<void*>.

> But wait! yomm2 supports virtual_<std::shared_ptr<Animal>> parameters
> in addition to virtual_<Animal&> and virtual_<Animal*> - and
> shared_ptrs are not polymorphic types (neither is Animal* in fact) so
> there is some flexibility here.
> 
> Do you mean something like (inspired from my synopsis):
> 

  Yes, that's exactly what I mean.  (Note that
if you were to use virtual_<any<typeid_<> >, _self&>>,
it would essentially be able to accept anything.)

> using AnyAnimal = any< mpl::vector< ... > >;
> 
> declare_method(string, kick, (virtual_<AnyAnimal>));
> 
> define_method(string, kick, (Dog* dog)) {
>     return "bark";
> }
> 
> define_method(string, kick, (Bulldog* dog)) {
>     return "bark and bite";
> }
> 
> ...or even (following Steven's Basic Usage example):
> 
> using AnyCounter = any<
>     mpl::vector<
>         copy_constructible<>,
>         typeid_<>,
>         incrementable<>,
>         ostreamable<>
>     >
>> ;
> 
> declare_method(string, describe, (virtual_<AnyCounter>));
> 
> define_method(string, describe, (int* num)) {
>     return "it's an integer";
> }
> 
> define_method(string, describe, (double* dog)) {
>     return "it's a double";
> }
> 
> If I'm off the mark maybe you can provide an example to illustrate
> your question?
> 

In Christ,
Steven Watanabe