opaque-holder.hpp

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/*
  OPAQUE-HOLDER.hpp  -  buffer holding an object inline while hiding the concrete type

   Copyright (C)
     2009,            Hermann Vosseler <Ichthyostega@web.de>

  **Lumiera** 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. See the file COPYING for further details.

*/

#ifndef LIB_OPAQUE_HOLDER_H
#define LIB_OPAQUE_HOLDER_H


#include "lib/error.hpp"
#include "lib/nocopy.hpp"
#include "lib/access-casted.hpp"
#include "lib/meta/util.hpp"
#include "lib/util.hpp"

#include <boost/lexical_cast.hpp>

#include <type_traits>
#include <utility>


namespace lib {
  
  namespace error = lumiera::error;
  using LERR_(BOTTOM_VALUE);
  using LERR_(WRONG_TYPE);
  
  using util::isSameObject;
  using util::isSameAdr;
  using util::unConst;
  
  
  namespace { // implementation helpers...
    
    using lib::meta::enable_if;
    using lib::meta::disable_if;
    using std::is_constructible;
    
    template<typename X>
                       enable_if< is_constructible<bool,X>,
    bool              >
    validitySelfCheck (X const& boolConvertible)
      {
        return bool(boolConvertible);
      }
    
    template<typename X>
                       disable_if< is_constructible<bool,X>,
    bool              >
    validitySelfCheck (X const&)
      {
        return true; // just pass if this type doesn't provide a validity check...
      }
    
  }
  
  
  
  /* ==== Policy classes controlling re-Access ==== */
  
  template<class BA>
  struct InPlaceAnyHolder_useCommonBase
    {
      typedef BA Base;
      
      template<class SUB>
      static Base*
      convert2base (SUB& obj)
        {
          SUB* oPtr = &obj;
          BA* asBase = util::AccessCasted<BA*>::access (oPtr);
          if (asBase)
            return asBase;
          
          throw error::Logic ("Unable to convert concrete object to Base interface"
                             , LERR_(WRONG_TYPE)
                             );
        }
    };
  
  struct InPlaceAnyHolder_unrelatedTypes
    {
      typedef void Base;
      
      template<class SUB>
      static void*
      convert2base (SUB& obj)
        {
          return static_cast<void*> (&obj);
        }
    };
  
  
  
  
  
  
  
  template
    < size_t siz           
    , class AccessPolicy = InPlaceAnyHolder_unrelatedTypes
    >
  class InPlaceAnyHolder
    {
      typedef typename AccessPolicy::Base * BaseP;
      
      struct Buffer
        {
          alignas(size_t) std::byte content_[siz];
          
          void* ptr() { return &content_; }
          
          virtual ~Buffer() {}           
          virtual bool  isValid()  const =0;
          virtual bool  empty()    const =0;
          virtual BaseP getBase()  const =0;
          
          virtual void  clone (void* targetStorage)  const =0;
        };
      
      struct EmptyBuff : Buffer
        {
          virtual bool isValid()  const { return false; }
          virtual bool empty()    const { return true; }
          
          BaseP
          getBase()  const
            {
              throw error::Invalid("accessing empty holder"
                                  , LERR_(BOTTOM_VALUE));
            }
          
          virtual void
          clone (void* targetStorage)  const
            {
              new(targetStorage) EmptyBuff();
            }
        };
      
      
      template<typename SUB>
      struct Buff : Buffer
        {
          static_assert (siz >= sizeof(SUB), "InPlaceAnyHolder: insufficient Buffer size");
          
          SUB&
          get()  const  
            {
              return * std::launder (reinterpret_cast<SUB*> (unConst(this)->ptr()));
            }
          
          ~Buff()
            {
              get().~SUB();
            }
          
          explicit
          Buff (SUB const& obj)
            {
              new(Buffer::ptr()) SUB (obj);
            }
          
          Buff (Buff const& oBuff)
            {
              new(Buffer::ptr()) SUB (oBuff.get());
            }
          
          Buff&
          operator= (Buff const& ref) 
            {
              if (&ref != this)
                get() = ref.get();
              return *this;
            }
          
          /* == virtual access functions == */
          
          virtual void
          clone (void* targetStorage)  const
            {
              new(targetStorage) Buff(get());
            }
          
          virtual BaseP
          getBase()  const
            {
              return AccessPolicy::convert2base (get());
            }
          
          virtual bool
          empty()  const
            {
              return false;
            }
          
          virtual bool
          isValid()  const
            {
              return validitySelfCheck (this->get());
            }
        };
      
      
      
      enum{ BUFFSIZE = sizeof(Buffer) };
      
      char storage_[BUFFSIZE];
      
      
      
      
      
      
    protected: /* === internal interface for managing the storage === */
      
      Buffer&
      buff()
        {
          return * std::launder (reinterpret_cast<Buffer*> (&storage_));
        }
      const Buffer&
      buff()  const
        {
          return * std::launder (reinterpret_cast<const Buffer *> (&storage_));
        }
      
      
      void
      killBuffer()
        {
          buff().~Buffer();
        }
      
      void
      make_emptyBuff()
        {
          new(&storage_) EmptyBuff();
        }
      
      template<class SUB>
      void
      place_inBuff (SUB const& obj)
        {
          new(&storage_) Buff<SUB> (obj);
        }
      
      void
      clone_inBuff (InPlaceAnyHolder const& ref)
        {
          ref.buff().clone (storage_);
        }
      
      BaseP
      asBase ()  const   
        {
          BaseP asBase = buff().getBase();
          ASSERT (asBase);
          return asBase;
        }
      
      
      
      
    public:
      
      ~InPlaceAnyHolder()
        {
          killBuffer();
        }
      
      void
      clear ()
        {
          killBuffer();
          make_emptyBuff();
        }
      
      
      InPlaceAnyHolder()
        {
          make_emptyBuff();
        }
      
      template<class SUB>
      InPlaceAnyHolder(SUB const& obj)
        {
          place_inBuff (obj);
        }
      
      InPlaceAnyHolder (InPlaceAnyHolder const& ref)
        {
          clone_inBuff (ref);
        }
      
      InPlaceAnyHolder&
      operator= (InPlaceAnyHolder const& ref)
        {
          if (not isSameObject (*this, ref))
            {
              killBuffer();
              try
                {
                  clone_inBuff (ref);
                }
              catch (...)
                {
                  make_emptyBuff();
                  throw;
                }
            }
          return *this;
        }
      
      template<class SUB>
      InPlaceAnyHolder&
      operator= (SUB const& newContent)
        {
          if (empty()
             or not isSameAdr (buff().getBase(), &newContent)    // caution: BaseP may be void* and SUB might be a pointer
             )
            {
              killBuffer();
              try
                {
                  place_inBuff (newContent);
                }
              catch (...)
                {
                  make_emptyBuff();
                  throw;
                }
            }
          return *this;
        }
      
      
      
      template<class SUB>
      SUB& get()  const
        {
          typedef const Buffer* Iface;
          typedef const Buff<SUB> * Actual;
          Iface interface = &buff();
          Actual actual = dynamic_cast<Actual> (interface);
          if (actual)
            return actual->get();
          
          if (this->empty())
            throw error::Invalid("accessing empty holder"
                                ,LERR_(BOTTOM_VALUE));
          else
            throw error::Logic ("Attempt to access OpaqueHolder's contents "
                                "specifying incompatible target type"
                               , LERR_(WRONG_TYPE)
                               );
        }
      
      
      
      bool
      empty() const
        {
          return buff().empty();
        }
      
      
      bool
      isValid() const
        {
          return buff().isValid();
        }
      
      explicit
      operator bool() const
        {
          return isValid();
        }
    };
  
  
  
  
  
  
  template
    < class BA
    , size_t siz = sizeof(BA)
    >
  class OpaqueHolder
    : public InPlaceAnyHolder<siz, InPlaceAnyHolder_useCommonBase<BA>>
    {
      typedef InPlaceAnyHolder<siz, InPlaceAnyHolder_useCommonBase<BA>> InPlaceHolder;
      
    public:
      OpaqueHolder() : InPlaceHolder() {}
      
      template<class SUB>
      OpaqueHolder(SUB const& obj) : InPlaceHolder(obj) {}
      
      template<class SUB>
      OpaqueHolder&
      operator= (SUB const& newContent)
        {
          static_cast<InPlaceHolder&>(*this) = newContent;
          return *this;
        }
      
      // note: using standard copy operations
      
      
      
      /* === smart-ptr style access === */
      
      BA&
      operator* ()  const
        {
          ASSERT (!InPlaceHolder::empty());
          return *InPlaceHolder::buff().getBase();
        }
      
      BA*
      operator-> ()  const
        {
          ASSERT (!InPlaceHolder::empty());
          return InPlaceHolder::buff().getBase();
        }
      
    };
  
  
  
  
  
  
  template<class BA, class DEFAULT>
  class PlantingHandle;
  
  template
    < class BA
    , size_t siz = sizeof(BA)
    , class DEFAULT = BA
    >
  class InPlaceBuffer
    : util::NonCopyable
    {
      
      alignas(BA) mutable
        std::byte buf_[siz];
      
      
      BA&
      getObj()  const
        {
          return * std::launder (reinterpret_cast<BA*> (&buf_));
        }
      
      void
      placeDefault()
        {
          static_assert (siz >= sizeof(DEFAULT), "InPlaceBuffer too small");
          
          new(&buf_) DEFAULT();
        }
      
      void
      destroy()
        {
          getObj().~BA();
        }
      
      
    public:
     ~InPlaceBuffer ()
        {
          destroy();
        }
      
      InPlaceBuffer ()
        {
          placeDefault();
        }
      
      template<class SUB>
      InPlaceBuffer (SUB&& instance)
        {
          static_assert (siz >= sizeof(SUB), "InPlaceBuffer too small");
          
          new(&buf_) SUB (std::forward<SUB> (instance));
        }
      
      template<typename TY>
      struct TypeTag{ };
      template<typename SUB>
      static auto embedType() { return TypeTag<SUB>{}; }
      
      template<class TY, typename...ARGS>
      InPlaceBuffer (TypeTag<TY>, ARGS&& ...args)
        {
          static_assert (siz >= sizeof(TY), "InPlaceBuffer too small");
          
          new(&buf_) TY (std::forward<ARGS> (args)...);
        }
      
      
      
      using Handle = PlantingHandle<BA, DEFAULT>;
      
      
      template<class TY, typename...ARGS>
      TY&
      create (ARGS&& ...args)
        {
          static_assert (siz >= sizeof(TY), "InPlaceBuffer too small");
          
          destroy();
          try {
              return *new(&buf_) TY {std::forward<ARGS> (args)...};
            }
          catch (...)
            {
              placeDefault();
              throw;
            }
        }
      
      template<class SUB>
      SUB&
      emplace (SUB&& implementation)
        {
          static_assert (siz >= sizeof(SUB), "InPlaceBuffer too small");
          
          destroy();
          try {
              return *new(&buf_) SUB {std::forward<SUB> (implementation)};
            }
          catch (...)
            {
              placeDefault();
              throw;
            }
        }
      
      DEFAULT&
      reset()
        {
          destroy();
          placeDefault();
          return static_cast<DEFAULT&> (getObj());
        }
      
      
      
      
      /* === smart-ptr style access === */
      
      BA&
      operator* ()  const
        {
          return getObj();
        }
      
      BA*
      operator-> ()  const
        {
          return &getObj();
        }
      
      
      template<class SUB>
      SUB*
      access ()
        {
          BA * asBase = &getObj();
          SUB* content = util::AccessCasted<SUB*>::access (asBase);
          return content;
        }     // NOTE: might be null.
    };
  
  
  
  template<class BA, class DEFAULT = BA>
  class PlantingHandle
    {
      void* buffer_;
      size_t maxSiz_;
      
      static_assert (std::has_virtual_destructor<BA>(),
                     "target interface BA must provide virtual dtor, "
                     "since InPlaceBuffer needs to take ownership.");
      
      template<class SUB>
      void __ensure_can_create();
      
      
    public:
      template<size_t maxSiz>
      PlantingHandle (InPlaceBuffer<BA, maxSiz, DEFAULT>& targetBuffer)
        : buffer_(&targetBuffer)
        , maxSiz_(maxSiz)
        { }
      
      // default copy acceptable...
      
      
      template<class SUB>
      bool
      canCreate()  const
        {
          static_assert(std::is_base_of<BA,SUB>(), "concrete object implanted into the opaque "
                                                   "buffer must implement the defined interface");
          return sizeof(SUB) <= maxSiz_;
        }
      
      template<class SUB>
      SUB&
      emplace (SUB&& implementation)
        {
          __ensure_can_create<SUB>();
          
          using Holder = InPlaceBuffer<BA, sizeof(SUB), DEFAULT>;
          Holder& holder = *static_cast<Holder*> (buffer_);
          
          return holder.template emplace (std::forward<SUB> (implementation));
        }
      
      template<class SUB, typename...ARGS>
      SUB&
      create (ARGS&& ...args)
        {
          __ensure_can_create<SUB>();
          
          using Holder = InPlaceBuffer<BA, sizeof(SUB), DEFAULT>;
          Holder& holder = *static_cast<Holder*> (buffer_);
          
          return holder.template create<SUB> (std::forward<ARGS> (args)...);
        }
      
      
      BA*
      get()  const
        {
          ENSURE (buffer_);
          BA& bufferContent = **static_cast<InPlaceBuffer<BA>*> (buffer_);
          return &bufferContent;
        }
    };
  
  
  
  template<class BA, class B0>
  template<class SUB>
  inline void
  PlantingHandle<BA,B0>::__ensure_can_create()
  {
    if (not this->canCreate<SUB>())
      throw error::Fatal("Unable to implant implementation object of size "
                         "exceeding the pre-established storage buffer capacity. "
                         +boost::lexical_cast<std::string>(sizeof(SUB)) + " > "
                         +boost::lexical_cast<std::string>(maxSiz_)
                        ,error::LUMIERA_ERROR_CAPACITY);
  }
  
  
  
} // namespace lib
#endif