[Boost-bugs] [Boost C++ Libraries] #11276: Ability to hold already type erased object into a type erased reference.

Subject: [Boost-bugs] [Boost C++ Libraries] #11276: Ability to hold already type erased object into a type erased reference.
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2015-05-08 11:34:55

#11276: Ability to hold already type erased object into a type erased reference.
------------------------------+-----------------------------
 Reporter:  germandiago       |      Owner:  steven_watanabe
     Type:  Bugs              |     Status:  new
Milestone:  To Be Determined  |  Component:  type_erasure
  Version:  Boost 1.55.0      |   Severity:  Problem
 Keywords:                    |
------------------------------+-----------------------------
 I would like to ask for an enhancement to this fantastic library, that is
 making me so happy in the absence of non-library solutions in c++ :)

 This problem came up in my code:

 {{{

 class my_concept :  public any<requirements> {
    using base = any<requirements>;
 public:
    using base::base;
 };

 class my_concept_ref : public any<requirements, _self &> {
    using base = any<requirements, _self&>;
 public:
    template <class... Args>
    my_concept_ref(Args &&... args) : base(std::forward<Args>(args)...) {}
 };

 //client code
 vector<my_concept> vec;
 //
 vec.push_back(my_concrete_model{});

 my_concept_ref get_a_type_erased_type_in_a_ref(vector<my_concept> & vec) {
     return vec[0];
 }

 //Throws bad_cast:
 auto & want_my_model =
 any_cast<my_concrete_model&>(get_a_type_erased_type_in_a_ref());

 //I must do this:
 auto & will_give_me_my_model =
 any_cast<my_concrete_model&>(any_cast<my_concept &>(
   get_a_type_erased_type_in_a_ref());

 }}}


 The problem here is that when the constructor for my_concept_ref takes an
 already type erased
 type from the vector (my_concept), it wraps that concrete type, which is a
 model of my_concept, but it was already type_erased, and not the internal
 concrete model (my_concrete_model).

 What could be implemented is automatic unwrapping of a type erased type so
 that I can
 do this:

 {{{

 //Throws bad_cast:
 auto & want_my_model =
 any_cast<my_concrete_model&>(get_a_type_erased_type_in_a_ref());

 }}}

 I think this should work. There is no easy workaround but to cast twice. I
 think
 the runtime concept constructor should check wether the class being taking
 in the
 constructor is actually already a type erased one based on detecting
 any<requirements>.
 I do not want to use in my code both
 my_concept & and my_concept_ref depending on wether I wrapped my concrete
 class or not
 in a type erased wrapper. This would give the library more uniformity and
 makes
 sense conceptually, because my_concept & can only be used in case I
 wrapped my
 type in a my_concept &, which is not always the case.

 Great work, by the way! I am trying to make heavy use of the library and
 so far it is serving me very well.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/11276>
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