/* OPAQUE-HOLDER.hpp - buffer holding an object inline while hiding the concrete type Copyright (C) Lumiera.org 2009, Hermann Vosseler 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 opaque-holder.hpp ** Helper allowing type erasure while holding the actual object inline. ** Controlling the actual storage of objects usually binds us to commit ** to a specific type, thus ruling out polymorphism. But sometimes, when ** we are able to control the maximum storage for a family of classes, we ** can escape this dilemma by using the type erasure pattern combined with ** an inline buffer holding an object of the concrete subclass. Typically, ** this situation arises when dealing with functor objects. ** ** This template helps building custom objects and wrappers based on this ** pattern: it provides an buffer for the target objects and controls access ** through a two-layer capsule; while the outer container exposes a neutral ** interface, the inner container keeps track of the actual type by means ** of a vtable. OpaqueHolder can be empty; but re-accessing the concrete ** object requires knowledge of the actual type, similar to boost::any ** (but contrary to OpaqueHolder the latter uses heap storage). ** ** Using this approach is bound to specific stipulations regarding the ** properties of the contained object and the kind of access needed. ** When, to the contrary, the contained types are \em not related ** and you need to re-discover their concrete type, then maybe ** a visitor or variant record might be a better solution. ** ** @see opaque-holder-test.cpp ** @see function-erasure.hpp usage example ** @see variant.hpp */ #ifndef LIB_OPAQUE_HOLDER_H #define LIB_OPAQUE_HOLDER_H #include "lib/error.hpp" #include "lib/bool-checkable.hpp" #include "lib/access-casted.hpp" #include "lib/util.hpp" namespace lib { using lumiera::error::LUMIERA_ERROR_WRONG_TYPE; using util::isSameObject; using util::unConst; namespace { // implementation helpers... using boost::disable_if; using boost::is_convertible; bool validitySelfCheck (bool boolConvertible) { return boolConvertible; } template typename disable_if< is_convertible, bool >::type validitySelfCheck (X const&) { return true; // just pass if this type doesn't provide a validity check... } } /** * Inline buffer holding and owning an object while concealing the * concrete type. Access to the contained object is similar to a * smart-pointer, but the object isn't heap allocated. OpaqueHolder * may be created empty, which can be checked by a bool test. * The whole compound is copyable if and only if the contained object * is copyable. * * For using OpaqueHolder, several \b assumptions need to be fulfilled * - any instance placed into OpaqueHolder is below the specified maximum size * - the caller cares for thread safety. No concurrent get calls while in mutation! */ template < class BA ///< the nominal Base/Interface class for a family of types , size_t siz = sizeof(BA) ///< maximum storage required for the targets to be held inline > class OpaqueHolder : public BoolCheckable > { /** Inner capsule managing the contained object (interface) */ struct Buffer { char content_[siz]; void* ptr() { return &content_; } virtual ~Buffer() {} virtual bool isValid() const { return false; } virtual bool empty() const { return true; } virtual void clone (void* targetStorage) const { new(targetStorage) Buffer(); } virtual BA& get() const { throw lumiera::error::Logic ("get() called on empty Buffer"); } }; /** concrete subclass managing a specific kind of contained object. * @note invariant: content_ always contains a valid SUB object */ template struct Buff : Buffer { SUB& get() const ///< core operation: target is contained within the inline buffer { return *reinterpret_cast (unConst(this)->ptr()); } ~Buff() { get().~SUB(); } explicit Buff (SUB const& obj) { REQUIRE (siz >= sizeof(SUB)); new(Buffer::ptr()) SUB (obj); } Buff (Buff const& oBuff) { new(Buffer::ptr()) SUB (oBuff.get()); } Buff& operator= (Buff const& ref) ///< not used currently { if (&ref != this) get() = ref.get(); return *this; } void clone (void* targetStorage) const { new(targetStorage) Buff(this->get()); } bool empty() const { return false; } bool isValid() const { return validitySelfCheck (get()); } }; 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]; /* === internal interface for managing the storage === */ Buffer& buff() { return *reinterpret_cast (&storage_); } const Buffer& buff() const { return *reinterpret_cast (&storage_); } void killBuffer() { buff().~Buffer(); } void make_emptyBuff() { new(&storage_) Buffer(); } template void place_inBuff (SUB const& obj) { new(&storage_) Buff (obj); } void clone_inBuff (OpaqueHolder const& ref) { ref.buff().clone (storage_); } public: ~OpaqueHolder() { killBuffer(); } void clear () { killBuffer(); make_emptyBuff(); } OpaqueHolder() { make_emptyBuff(); } template OpaqueHolder(SUB const& obj) { place_inBuff (obj); } OpaqueHolder (OpaqueHolder const& ref) { clone_inBuff (ref); } OpaqueHolder& operator= (OpaqueHolder const& ref) { if (!isSameObject (*this, ref)) { killBuffer(); try { clone_inBuff (ref); } catch (...) { make_emptyBuff(); throw; } } return *this; } template OpaqueHolder& operator= (SUB const& newContent) { if ( !empty() && !isSameObject (buff().get(), newContent) ) { killBuffer(); try { place_inBuff (newContent); } catch (...) { make_emptyBuff(); throw; } } return *this; } /* === smart-ptr style access === */ BA& operator* () const { ASSERT (!empty()); return buff().get(); } BA* operator-> () const { ASSERT (!empty()); return &(buff().get()); } template SUB& get() const { typedef const Buffer* Iface; typedef const Buff * Actual; Iface interface = &buff(); Actual actual = dynamic_cast (interface); if (actual) return actual->get(); // second try: maybe we can perform a // dynamic downcast or direct conversion to the // actual target type. But we need to exclude a // brute force static cast (which might slice or reinterpret) if (!util::use_static_downcast::value) { BA* asBase = &(buff().get()); SUB* content = util::AccessCasted::access (asBase); if (content) return *content; } throw lumiera::error::Logic ("Attempt to access OpaqueHolder's contents " "specifying incompatible target type" , LUMIERA_ERROR_WRONG_TYPE ); } bool empty() const { return buff().empty(); } bool isValid() const { return buff().isValid(); } }; } // namespace lib #endif