lumiera_/src/lib/replaceable-item.hpp

/*
  REPLACEABLE-ITEM.hpp  -  adapter to take snapshot from non-assignable values

  Copyright (C)         Lumiera.org
    2017,               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 replaceable-item.hpp
 ** Adapter wrapper to treat non-assignable values as if they were assignable.
 ** Prerequisite is for the type to be copy constructible (maybe even just move constructible).
 ** The typical usage is when we want to take a _snapshot_ from some value, but do not actually
 ** need the ability to assign something to a specific object instance. In such a situation,
 ** we may just destroy a previous snapshot and then place a new copy initialised value into
 ** the same storage space. For all really assignable types we fall back to a trivial
 ** implementation.
 ** 
 ** @see proc::control::MementoTie
 ** 
 */


#ifndef LIB_REPLACABLE_ITEM_H
#define LIB_REPLACABLE_ITEM_H

#include "lib/error.hpp"
#include "lib/null-value.hpp"
#include "lib/meta/util.hpp"
#include "lib/util.hpp"

#include <utility>
#include <type_traits>



namespace lib {
namespace wrapper {
  
//  using util::unConst;
  using util::isSameObject;
//  using lumiera::error::LUMIERA_ERROR_BOTTOM_VALUE;
  
  
  
  
  /**
   * 
   */
  template<typename X, typename COND =void>
  class ReplaceableItem
    {
      char content_[sizeof(X)];
      
    public:
      ReplaceableItem()
        {
          emplace (NullValue<X>::get());
        }
     ~ReplaceableItem()
        {
          destroy();
        }
      
      template<typename Z>
      ReplaceableItem (Z&& otherValue)
        {
          emplace (forward<Z> (otherValue));
        }
      
      template<typename Z>
      ReplaceableItem&
      operator= (Z&& otherValue)
        {
          if (not isSameObject(*this, otherValue))
            reAssign (forward<Z> (otherValue));
          return *this;
        }
      
      operator X&()
        {
          return access();
        }
      operator X const&()  const
        {
          return access();
        }
      
    private:
      X&
      access()  const
        {
          return reinterpret_cast<X&>(content_);
        }
      
      template<typename Z>
      void
      emplace (Z&& otherValue)
        try {
          new(&content_) X{forward<Z> (otherValue)};
        } 
        catch(...) {
          NullValue<X>::build (&content_);
        }
      
      void
      destroy()
        {
          access().~X();
        }
      
      template<typename Z>
      void
      reAssign (Z&& otherValue)
        {
          destroy();
          emplace (forward<Z> (otherValue));
        }
    };
  
  template<typename X>
  class ReplaceableItem<X,   meta::enable_if<std::is_copy_assignable<X>>>
    : public X
    { 
      using X::X;
    };
  
  
  
  
  
}} // namespace lib::wrap
#endif /*LIB_REPLACABLE_ITEM_H*/