Ichthyostega
710ae8fa0f
Using a policy based switch in case the client interface collaborates and provides copy operations; in this case, a direct static downcast instead of the expensive indirect (dynamic) cast to the management interface can be employed.
390 lines
14 KiB
C++
390 lines
14 KiB
C++
/*
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POLYMORPHIC-VALUE.hpp - building opaque polymorphic value objects
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Copyright (C) Lumiera.org
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2011, Hermann Vosseler <Ichthyostega@web.de>
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This program is free software; you can redistribute it and/or
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modify it under the terms of the GNU General Public License as
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published by the Free Software Foundation; either version 2 of
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the License, or (at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, write to the Free Software
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Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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*/
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/** @file polymorphic-value.hpp
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** A mechanism to allow for opaque polymorphic value objects.
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** This template helps to overcome a problem frequently encountered in
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** C++ programming, based on the fundamental design of C++, which favours
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** explicit low-level control, copying of values and strict ctor-dtor pairs.
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** Many object oriented design patterns build on polymorphism, where the
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** actual type of an object isn't disclosed and collaborations rely on
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** common interfaces. This doesn't mix well with the emphasis the C/C++
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** language puts on efficient handling of small data elements as values
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** and explicit control of the used storage; indeed several of the modern
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** object oriented and functional programming techniques more or less
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** assume the presence of a garbage collector or similar mechanism,
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** so 'objects' need just to be mentioned by reference.
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**
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** In C++ to employ many of the well known techniques, you're more or less
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** bound to explicitly put the objects somewhere in heap allocated memory
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** and then pass an interface pointer or reference into the actual algorithm.
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** Often, this hinders a design based on constant values and small descriptor
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** objects used inline, thus forcing into unnecessarily complex and heavyweight
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** alternatives. While it's certainly pointless to fight the fundamental nature
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** of the programming language, we may try to pull some (template based) trickery
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** to make polymorphic objects fit better with the handling of small copyable
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** value objects. Especially, C++ gives a special meaning to passing parameters
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** as \c const& -- typically constructing an anonymous temporary object conveniently
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** just for passing an abstraction barrier (while the optimiser can be expected to
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** remove this barrier and the accompanying nominal copy operations altogether in
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** the generated code). Consequently the ability to return a polymorphic object
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** from a factory or configuration function <i>by value</i> would open a lot of
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** straight forward design possibilities and concise formulations.
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**
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** \par how to build a copyable value without knowing it's layout in detail
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**
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** So the goal is to build a copyable and assignable type with value semantics,
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** without disclosing the actual implementation and object layout at the usage site.
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** This seemingly contradictory goal can be achieved, provided that
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** - the space occupied by the actual implementation object is limited,
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** so it can be placed as binary data into an otherwise opaque holder buffer
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** - and the actual implementation object assists with copying and cloning
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** itself, observing the actual implementation data layout
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**
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** The PolymorphicValue template implements this idea, by exposing an copyable
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** container with value semantics to the client code. On instantiation, a common
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** base interface for the actual value objects needs to be provided; the resulting
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** instance will be automatically convertible to this interface. Obviously this
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** common interface must be an ABC or at least contain some virtual functions.
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** Moreover, the PolymorphicValue container provides static builder functions,
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** allowing to place a concrete instance of a subclass into the content buffer.
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** After construction, the actual type of this instance will be forgotten
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** (``type erasure''), but because the embedded vtable, on access the
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** proper implementation functions will be invoked.
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**
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** Expanding on that pattern, the copying and cloning operations of the whole
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** container can be implemented by forwarding to appropriate virtual functions
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** on the embedded payload (implementation) object -- the concrete implementation
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** of these virtual functions can be assumed to know the real type and thus be
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** able to invoke the correct copy ctor or assignment operator. For this to
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** work, the interface needs to expose those copy and clone operations somehow
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** as virtual functions. There are two alternatives to get at this point:
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** - in the general case, the common base interface doesn't expose such operations.
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** Thus we need to <i>mix in</i> an additional \em management interface; this
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** can be done by \em subclassing the desired implementation type, because
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** this concrete type is irrelevant after finishing the placement constructor.
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** In order to re-access this management interface, so to be able to invoke
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** the copy or clone operations, we need to do an elaborate re-cast operation,
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** first going down to the leaf type and then back up into the mixed in
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** management interface. Basically this operation is performed by using
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** an \c dynamic_cast<CopyAPI&>(bufferContents)
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** - but when the used client types provide some collaboration and implement
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** this management interface either directly on the API or as an immediate
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** sub-interface, then this copy/management interface is located within the
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** direct inheritance chain and can be reached by a simple \c static_cast.
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** Indeed, as we're just using a different meaning of the VTable, only a
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** single indirection (virtual function call) is required at runtime in
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** this case to invoke the copy ctor or assignment operator.
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**
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** @see polymorphic-value-test.cpp
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** @see opaque-holder.hpp other similar opaque inline buffer templates
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** @see lib::time::Mutation usage example
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*/
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#ifndef LIB_POLYMORPHIC_VALUE_H
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#define LIB_POLYMORPHIC_VALUE_H
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#include "lib/error.hpp"
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#include "lib/meta/duck-detector.hpp"
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//#include "lib/bool-checkable.hpp"
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//#include "lib/access-casted.hpp"
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//#include "lib/util.hpp"
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//#include <boost/noncopyable.hpp>
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#include <boost/utility/enable_if.hpp>
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namespace lib {
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// using lumiera::error::LUMIERA_ERROR_WRONG_TYPE;
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// using util::isSameObject;
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// using util::unConst;
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namespace polyvalue { // implementation helpers...
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using boost::enable_if;
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using lumiera::Yes_t;
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using lumiera::No_t;
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struct EmptyBase{ };
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template<class IFA
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,class BA = EmptyBase
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>
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class CopySupport
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: public BA
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{
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public:
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virtual ~CopySupport() { };
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virtual void cloneInto (void* targetBuffer) const =0;
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virtual void copyInto (IFA& targetBase) const =0;
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};
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template<typename T>
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class exposes_copySupportFunctions
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{
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META_DETECT_FUNCTION(void, cloneInto, (void*) const);
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public:
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enum{ value = HasFunSig_cloneInto<T>::value
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};
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};
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template <class TY, class YES = void>
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struct Trait
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{
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typedef CopySupport<TY> CopyAPI;
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enum{ ADMIN_OVERHEAD = 2 * sizeof(void*) };
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static CopyAPI&
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accessCopyHandlingInterface (TY& bufferContents)
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{
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REQUIRE (INSTANCEOF (CopyAPI, &bufferContents));
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return dynamic_cast<CopyAPI&> (bufferContents);
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}
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typedef CopyAPI AdapterAttachment;
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};
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template <class TY>
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struct Trait<TY, typename enable_if< exposes_copySupportFunctions<TY> >::type>
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{
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typedef TY CopyAPI;
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enum{ ADMIN_OVERHEAD = 1 * sizeof(void*) };
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template<class IFA>
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static CopyAPI&
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accessCopyHandlingInterface (IFA& bufferContents)
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{
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REQUIRE (INSTANCEOF (CopyAPI, &bufferContents));
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return static_cast<CopyAPI&> (bufferContents);
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}
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typedef EmptyBase AdapterAttachment;
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};
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}
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/**
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* Template to build polymorphic value objects.
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* Inline buffer with value semantics, yet holding and owning an object
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* while concealing the concrete type, exposing only the public interface.
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* Access to the contained object is by implicit conversion to this public
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* interface. The actual implementation object might be placed into the
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* buffer through a builder function; later, this buffer may be copied
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* and passed on without knowing the actual contained type.
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*
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* For using PolymorphicValue, several \b assumptions need to be fulfilled
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* - any instance placed into OpaqueHolder is below the specified maximum size
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* - the caller cares for thread safety. No concurrent get calls while in mutation!
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*
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* @warning when a create or copy-into operation fails with exception, the whole
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* PolymorphicValue object is in undefined state and must not be used further.
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*/
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template
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< class IFA ///< the nominal Base/Interface class for a family of types
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, size_t storage ///< maximum storage required for the target data to be held inline
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, class CPY = IFA ///< special sub-interface to support copy operations
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>
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class PolymorphicValue
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{
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typedef polyvalue::Trait<CPY> _Traits;
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typedef typename _Traits::CopyAPI _CopyHandlingAdapter;
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enum{
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siz = storage + _Traits::ADMIN_OVERHEAD
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};
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mutable char buf_[siz];
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template<class IMP>
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IMP&
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access() const
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{
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return reinterpret_cast<IMP&> (buf_);
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}
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void
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destroy()
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{
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access<IFA>().~IFA();
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}
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// REQUIRE (siz >= sizeof(IMP));
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template<class IMP>
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PolymorphicValue (IMP*)
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{
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new(&buf_) IMP();
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}
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template<class IMP, typename A1>
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PolymorphicValue (IMP*, A1& a1)
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{
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new(&buf_) IMP (a1);
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}
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template<class IMP, typename A1, typename A2>
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PolymorphicValue (IMP*, A1& a1, A2& a2)
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{
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new(&buf_) IMP (a1,a2);
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}
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template<class IMP, typename A1, typename A2, typename A3>
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PolymorphicValue (IMP*, A1& a1, A2& a2, A3& a3)
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{
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new(&buf_) IMP (a1,a2,a3);
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}
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template<class IMP>
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class Adapter
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: public IMP
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, public _Traits::AdapterAttachment
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{
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virtual void
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cloneInto (void* targetBuffer) const
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{
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new(targetBuffer) Adapter(*this);
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}
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virtual void
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copyInto (IFA& targetBase) const
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{
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REQUIRE (INSTANCEOF (Adapter, &targetBase));
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Adapter& target = static_cast<Adapter&> (targetBase);
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target = (*this);
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}
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public:
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/* using default copy and assignment */
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Adapter() : IMP() { }
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template<typename A1>
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Adapter (A1& a1) : IMP(a1) { }
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template<typename A1, typename A2>
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Adapter (A1& a1, A2& a2) : IMP(a1,a2) { }
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template<typename A1, typename A2, typename A3>
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Adapter (A1& a1, A2& a2, A3& a3) : IMP(a1,a2,a3) { }
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};
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_CopyHandlingAdapter&
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accessHandlingInterface () const
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{
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IFA& bufferContents = access<IFA>();
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_CopyHandlingAdapter& hap = _Traits::accessCopyHandlingInterface (bufferContents);
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return hap; ////TODO cleanup unnecessary temporary
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}
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public:
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operator IFA& ()
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{
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return access<IFA>();
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}
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operator IFA const& () const
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{
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return access<IFA>();
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}
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~PolymorphicValue()
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{
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destroy();
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}
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PolymorphicValue (PolymorphicValue const& o)
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{
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o.accessHandlingInterface().cloneInto (&buf_);
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}
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PolymorphicValue&
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operator= (PolymorphicValue const& o)
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{
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o.accessHandlingInterface().copyInto (this->access<IFA>());
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return *this;
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}
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template<class IMP>
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static PolymorphicValue
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build ()
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{
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Adapter<IMP>* type_to_build_in_buffer;
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return PolymorphicValue (type_to_build_in_buffer);
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}
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template<class IMP, typename A1>
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static PolymorphicValue
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build (A1& a1)
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{
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Adapter<IMP>* type_to_build_in_buffer;
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return PolymorphicValue (type_to_build_in_buffer, a1);
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}
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template<class IMP, typename A1, typename A2>
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static PolymorphicValue
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build (A1& a1, A2& a2)
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{
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Adapter<IMP>* type_to_build_in_buffer;
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return PolymorphicValue (type_to_build_in_buffer, a1,a2);
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}
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template<class IMP, typename A1, typename A2, typename A3>
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static PolymorphicValue
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build (A1& a1, A2& a2, A3& a3)
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{
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Adapter<IMP>* type_to_build_in_buffer;
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return PolymorphicValue (type_to_build_in_buffer, a1,a2,a3);
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}
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friend bool
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operator== (PolymorphicValue const& v1, PolymorphicValue const& v2)
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{
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return v1.access<IFA>() == v2.access<IFA>();
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}
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friend bool
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operator!= (PolymorphicValue const& v1, PolymorphicValue const& v2)
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{
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return ! (v1 == v2);
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}
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};
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} // namespace lib
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#endif
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