LUMIERA.clone/src/lib/variant.hpp

/*
  VARIANT.hpp  -  lightweight typesafe union record

  Copyright (C)         Lumiera.org
    2015,               Hermann Vosseler <Ichthyostega@web.de>

  This program is free software; you can redistribute it and/or
  modify it under the terms of the GNU General Public License as
  published by the Free Software Foundation; either version 2 of
  the License, or (at your option) any later version.

  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with this program; if not, write to the Free Software
  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

*/


/** @file variant.hpp
 ** A typesafe union record to carry embedded values of unrelated type.
 ** This file defines a simple alternative to boost::variant. It pulls in
 ** fewer headers, has a shorter code path and is hopefully more readable,
 ** but also doesn't deal with alignment issues and is <b>not threadsafe</b>.
 **  
 ** Deliberately, the design rules out re-binding of the contained type. Thus,
 ** once created, a variant \em must hold a valid element and always an element
 ** of the same type. Beyond that, variant elements are copyable and mutable.
 ** Direct access requires knowledge of the embedded type (no switch-on type).
 ** Type mismatch is checked at runtime. As a fallback, we provide a visitor
 ** scheme for generic access.
 ** 
 ** \par implementation notes
 ** We use a similar "double capsule" implementation technique as for lib::OpaqueHolder.
 ** In fact, Variant is almost identical to the latter, just omitting unnecessary flexibility.
 ** The outer capsule exposes the public handling interface, while the inner, private capsule
 ** is a polymorphic value holder. Since C++ as such does not support polymorphic values,
 ** the inner capsule is placed "piggyback" into a char buffer. The actual value is carried
 ** within yet another, nested char buffer. Thus, effectively the first "slot" of the storage
 ** will hold the VTable pointer, thereby encoding the actual type information -- leading to
 ** a storage requirement of MAX<TYPES...> plus one "slot" for the VTable. (with "slot" we
 ** denote the smallest disposable storage size for the given platform after alignment,
 ** typically the size of a size_t).
 ** 
 ** @see Veriant_test
 ** @see lib::diff::GenNode
 ** 
 */


#ifndef LIB_VARIANT_H
#define LIB_VARIANT_H


#include "lib/meta/typelist.hpp"
#include "lib/meta/typelist-util.hpp"
//#include "lib/util.hpp"

#include <type_traits>
#include <typeindex>
//#include <utility>
//#include <string>
//#include <array>


namespace lib {
  
  using std::string;
  
  using std::move;
  using std::forward;
  using util::unConst;
  
  namespace error = lumiera::error;
  
  template<typename TYPES>
  class Variant;
  
  namespace { // implementation helpers
    
    using std::is_constructible;
    using std::remove_reference;
    using std::remove_const;
    using std::is_same;
    using std::enable_if;
    using meta::NullType;
    using meta::Node;
    
    
    template<typename X, typename TYPES>
    struct CanBuildFrom
      : CanBuildFrom<typename remove_reference<X>::type
                    ,typename TYPES::List
                    >
      { };
    
    template<typename X, typename TYPES>
    struct CanBuildFrom<X, Node<X, TYPES>>
      {
        using Type = X;
      };
    
    template<typename X, typename T,typename TYPES>
    struct CanBuildFrom<X, Node<T, TYPES>>
      {
        using Type = typename CanBuildFrom<X,TYPES>::Type;
      };
    
    template<typename X>
    struct CanBuildFrom<X, NullType>
      {
        static_assert (0 > sizeof(X), "No type in Typelist can be built from the given argument");
      };
    
    
    template<typename T>
    struct Bare
      {
        using Type = typename remove_const<
                     typename remove_reference<T>::type>::type;
      };
    
    template<class T>
    struct is_Variant
      {
        static constexpr bool value = false;
      };
    
    template<class TYPES>
    struct is_Variant<Variant<TYPES>>
      {
        static constexpr bool value = true;
      };
    
    template<class V>
    struct use_if_is_Variant
      : enable_if<is_Variant<typename Bare<V>::Type>::value, V>
      { };
    
    
    template<class B, class D>
    class FullCopySupport
      : public B
      {
        virtual void
        copyInto (void* targetStorage)  const override
          {
            D const& src = static_cast<D const&> (*this);
            new(targetStorage) D(src);
          }
        
        virtual void
        moveInto (void* targetStorage)  override
          {
            D& src = static_cast<D&> (*this);
            new(targetStorage) D(move(src));
          }
        
        virtual void
        copyInto (B& target)  const override
          {
            D&       t = D::downcast(target);
            D const& s = static_cast<D const&> (*this);
            t = s;
          }
        
        virtual void
        moveInto (B& target)  override
          {
            D& t = D::downcast(target);
            D& s = static_cast<D&> (*this);
            t = move(s);
          }
      };
    
    
    template<class B, class D>
    class CloneSupport
      : public B
      {
        virtual void
        copyInto (void* targetStorage)  const override
          {
            D const& src = static_cast<D const&> (*this);
            new(targetStorage) D(src);
          }
        
        virtual void
        moveInto (void* targetStorage)  override
          {
            D& src = static_cast<D&> (*this);
            new(targetStorage) D(move(src));
          }
        
        virtual void
        copyInto (B& target)  const override
          {
            throw error::Logic("Assignment invoked but target is not assignable");
          }
        
        virtual void
        moveInto (B& target)  override
          {
            throw error::Logic("Assignment invoked but target is not assignable");
          }
      };
    
    
    template<class B, class D>
    class MoveSupport
      : public B
      {
        virtual void
        copyInto (void* targetStorage)  const override
          {
            throw error::Logic("Copy construction invoked but target allows only move construction");
          }
        
        virtual void
        moveInto (void* targetStorage)  override
          {
            D& src = static_cast<D&> (*this);
            new(targetStorage) D(move(src));
          }
        
        virtual void
        copyInto (B& target)  const override
          {
            throw error::Logic("Assignment invoked but target is not assignable");
          }
        
        virtual void
        moveInto (B& target)  override
          {
            throw error::Logic("Assignment invoked but target is not assignable");
          }
      };
    
    
    template<class B, class D>
    class NoCopyMoveSupport
      : public B
      {
        virtual void
        copyInto (void* targetStorage)  const override
          {
            throw error::Logic("Copy construction invoked but target is noncopyable");
          }
        
        virtual void
        moveInto (void* targetStorage)  override
          {
            throw error::Logic("Move construction invoked but target is noncopyable");
          }
        
        virtual void
        copyInto (B& target)  const override
          {
            throw error::Logic("Assignment invoked but target is not assignable");
          }
        
        virtual void
        moveInto (B& target)  override
          {
            throw error::Logic("Assignment invoked but target is not assignable");
          }
      };
    
    
    template<class X, class SEL=void>
    struct CopySupport
      {
        template<class B, class D>
        using Policy = NoCopyMoveSupport<B,D>;
      };
    
    
  }//(End) implementation helpers
  
  
  /**
   * Typesafe union record.
   * A Variant element may carry an embedded value of any of the predefined types.
   * The type may not be rebound: It must be created holding some value and each
   * instance is fixed to the specific type used at construction time.
   * Yet within the same type, variant elements are copyable and assignable.
   * The embedded type is erased on the signature, but knowledge about the
   * actual type is retained, encoded into the embedded VTable. Thus,
   * any access to the variant's value requires knowledge of the type
   * in question, but type mismatch will provoke an exception at runtime.
   * Generic access is possible using a visitor.
   */
  template<typename TYPES>
  class Variant
    {
      enum { SIZ = meta::maxSize<typename TYPES::List>::value };
      
      /** Inner capsule managing the contained object (interface) */
      struct Buffer
        {
          char content_[SIZ];
          
          void* ptr() { return &content_; }
          
          
          virtual ~Buffer() {}           ///< this is an ABC with VTable
          
          virtual void  copyInto (void* targetStorage)  const =0;
          virtual void  moveInto (void* targetStorage)        =0;
          virtual void  copyInto (Buffer&      target)  const =0;
          virtual void  moveInto (Buffer&      target)        =0;
//        virtual void  operator= (Buffer const&)             =0;
//        virtual void  operator= (Buffer&&)                  =0;
          
        };
      
      
      /** concrete inner capsule specialised for a given type */
      template<typename TY>
      struct Buff
        : CopySupport<TY>::template Policy<Buffer,Buff<TY>>
        {
          static_assert (SIZ >= sizeof(TY), "Variant record: insufficient embedded Buffer size");
          
          TY&
          access()  const  ///< core operation: target is contained within the inline buffer
            {
              return *reinterpret_cast<TY*> (unConst(this)->ptr());
            }
          
         ~Buff()
            {
              access().~TY();
            }
          
          Buff (TY const& obj)
            {
              new(Buffer::ptr()) TY(obj);
            }
          
          Buff (TY && robj)
            {
              new(Buffer::ptr()) TY(move(robj));
            }
          
          Buff (Buff const& oBuff)
            {
              new(Buffer::ptr()) TY(oBuff.access());
            }
          
          Buff (Buff && rBuff)
            {
              new(Buffer::ptr()) TY(move (rBuff.access()));
            }
          
          void
          assignValue (TY const& ob)
            {
              if (&ob != Buffer::ptr())
                this->access() = ob;
            }
          
          void
          assignValue (TY && rob)
            {
              this->access() = move(rob);
            }
          
          
          
          
          /* == virtual access functions == */
          
          virtual void
          copyInto (void* targetStorage)  const override
            {
              new(targetStorage) Buff(this->access());
            }
          
//        virtual void
//        operator= (Buffer const& ob)  override
//          {
//            assignValue (downcast(unConst(ob)).access());
//          }
//        
//        virtual void
//        operator= (Buffer&& rref)  override
//          {
//            assignValue (move (downcast(rref).access()));
//          }
          
          static Buff&
          downcast (Buffer& b)
            {
              Buff* buff = dynamic_cast<Buff*> (&b);
              
              if (!buff)
                throw error::Logic("Variant type mismatch: "
                                   "the given variant record does not hold "
                                   "a value of the type requested here"
                                  ,error::LUMIERA_ERROR_WRONG_TYPE);
              else
               return *buff;
            }
        };
      
      enum{ BUFFSIZE = sizeof(Buffer) };
      
      /** embedded buffer actually holding the concrete Buff object,
       *  which in turn holds and manages the target object.
       *  @note Invariant: always contains a valid Buffer subclass */
      char storage_[BUFFSIZE];
      
      
      
      template <class X, class SEL=void>
      struct Assign
        { 
          using RawType = typename remove_reference<X>::type;
          static_assert (meta::isInList<RawType, typename TYPES::List>(),
                         "Type error: the given variant could never hold the required type");
          
          
          static void
          assign (Buffer& buffer, X&& x)
            {
              buff<RawType>().assignValue (forward<X>(x));
            }
          
          
        };
      
      template<class V>
      struct Assign<V, typename use_if_is_Variant<V>::type>
        {
          
          static void
          assign (Buffer& buffer, V&& oVar)
            {
//              oVar.buffer()
//              buff<RawType>().assignValue (forward<X>(oVar));
            }
        };
      
      
    protected: /* === internal interface for managing the storage === */
      
      Buffer&
      buffer()
        {
          return *reinterpret_cast<Buffer*> (&storage_);
        }
      Buffer const&
      buffer()  const
        {
          return *reinterpret_cast<const Buffer*> (&storage_);
        }
      
      template<typename X>
      Buff<X>&
      buff()
        {
          return Buff<X>::downcast(this->buffer());
        }
      
      
    public:
     ~Variant()
        {
          buffer().~Buffer();
        }
      
      Variant()
        {
          using DefaultType = typename TYPES::List::Head;
          
          new(storage_) Buff<DefaultType> (DefaultType());
        }
      
      Variant (Variant& ref)
        {
          ref.buffer().copyInto (&storage_);
        }
      
      Variant (Variant const& ref)
        {
          ref.buffer().copyInto (&storage_);
        }
      
      Variant (Variant&& rref)
        {
          rref.buffer().copyInto (&storage_);
        }
      
      template<typename X>
      Variant(X&& x)
        {
          using StorageType = typename CanBuildFrom<X, TYPES>::Type;
          
          new(storage_) Buff<StorageType> (forward<X>(x));
        }
      
      template<typename X>
      Variant&
      operator= (X x)
        {
//        using RawType = typename remove_reference<X>::type;
//        static_assert (meta::isInList<RawType, typename TYPES::List>(),
//                       "Type error: the given variant could never hold the required type");
//        
//        buff<RawType>().assignValue (forward<X>(x));
          
          Assign<X>::assign (this->buffer(), forward<X> (x));
          return *this;
        }
/*      
      Variant&
      operator= (Variant& ovar)
        {
          buffer() = ovar.buffer();
          return *this;
        }
      
      Variant&
      operator= (Variant const& ovar)
        {
          buffer() = ovar.buffer();
          return *this;
        }
      
      Variant&
      operator= (Variant&& rvar)
        {
          buffer().operator= (move(rvar.buffer()));
          return *this;
        }
      */
      
#ifdef LIB_TEST_TEST_HELPER_H
      /* == diagnostic helper == */
      
      operator string()  const
        {
          UNIMPLEMENTED("diagnostic string conversion");
        }
#endif      
      
      /* === Access === */
      
      template<typename X>
      X&
      get()
        {
          static_assert (meta::isInList<X, typename TYPES::List>(),
                         "Type error: the given variant could never hold the required type");
          
          return buff<X>().access();
        }
      
      
      class Visitor
        {
        public:
          virtual ~Visitor() { } ///< this is an interface
        };
      
      void
      accept (Visitor& visitor)
        {
          UNIMPLEMENTED("visitor style value access");
        }
    };
  
  
  
} // namespace lib  
#endif /*LIB_VARIANT_H*/