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////////////////////////////////////////////////////////////////////////////////
// The Loki Library
// Copyright (c) 2001 by Andrei Alexandrescu
// This code accompanies the book:
// Alexandrescu, Andrei. "Modern C++ Design: Generic Programming and Design
// Patterns Applied". Copyright (c) 2001. Addison-Wesley.
// Permission to use, copy, modify, distribute and sell this software for any
// purpose is hereby granted without fee, provided that the above copyright
// notice appear in all copies and that both that copyright notice and this
// permission notice appear in supporting documentation.
// The author or Addison-Wesley Longman make no representations about the
// suitability of this software for any purpose. It is provided "as is"
// without express or implied warranty.
////////////////////////////////////////////////////////////////////////////////
// Last update: June 20, 2001
#ifndef VISITOR_INC_
#define VISITOR_INC_
#include "Typelist.h"
#include "HierarchyGenerators.h"
namespace Loki
{
////////////////////////////////////////////////////////////////////////////////
// class template BaseVisitor
// The base class of any Acyclic Visitor
////////////////////////////////////////////////////////////////////////////////
class BaseVisitor
{
public:
virtual ~BaseVisitor() {}
};
////////////////////////////////////////////////////////////////////////////////
// class template Visitor
// The building block of Acyclic Visitor
////////////////////////////////////////////////////////////////////////////////
template <class T, typename R = void>
class Visitor
{
public:
typedef R ReturnType;
virtual ReturnType Visit(T&) = 0;
};
////////////////////////////////////////////////////////////////////////////////
// class template Visitor (specialization)
// This specialization is not present in the book. It makes it easier to define
// Visitors for multiple types in a shot by using a typelist. Example:
//
// class SomeVisitor :
// public BaseVisitor // required
// public Visitor<TYPELIST_2(RasterBitmap, Paragraph)>,
// public Visitor<Paragraph>
// {
// public:
// void Visit(RasterBitmap&); // visit a RasterBitmap
// void Visit(Paragraph &); // visit a Paragraph
// };
////////////////////////////////////////////////////////////////////////////////
template <class Head, class Tail, typename R>
class Visitor<Typelist<Head, Tail>, R>
: public Visitor<Head, R>, public Visitor<Tail, R>
{
public:
typedef R ReturnType;
// using Visitor<Head, R>::Visit;
// using Visitor<Tail, R>::Visit;
};
template <class Head, typename R>
class Visitor<Typelist<Head, NullType>, R> : public Visitor<Head, R>
{
public:
typedef R ReturnType;
using Visitor<Head, R>::Visit;
};
////////////////////////////////////////////////////////////////////////////////
// class template BaseVisitorImpl
// Implements non-strict visitation (you can implement only part of the Visit
// functions)
////////////////////////////////////////////////////////////////////////////////
template <class TList, typename R = void> class BaseVisitorImpl;
template <class Head, class Tail, typename R>
class BaseVisitorImpl<Typelist<Head, Tail>, R>
: public Visitor<Head, R>
, public BaseVisitorImpl<Tail, R>
{
public:
// using BaseVisitorImpl<Tail, R>::Visit;
virtual R Visit(Head&)
{ return R(); }
};
template <class Head, typename R>
class BaseVisitorImpl<Typelist<Head, NullType>, R>
: public Visitor<Head, R>
{
public:
virtual R Visit(Head&)
{ return R(); }
};
////////////////////////////////////////////////////////////////////////////////
// class template BaseVisitable
////////////////////////////////////////////////////////////////////////////////
template <typename R, typename Visited>
struct DefaultCatchAll
{
static R OnUnknownVisitor(Visited&, BaseVisitor&)
{ return R(); }
};
////////////////////////////////////////////////////////////////////////////////
// class template BaseVisitable
////////////////////////////////////////////////////////////////////////////////
template
<
typename R = void,
template <typename, class> class CatchAll = DefaultCatchAll
>
class BaseVisitable
{
public:
typedef R ReturnType;
virtual ~BaseVisitable() {}
virtual ReturnType Accept(BaseVisitor&) = 0;
protected: // give access only to the hierarchy
template <class T>
static ReturnType AcceptImpl(T& visited, BaseVisitor& guest)
{
// Apply the Acyclic Visitor
if (Visitor<T,R>* p = dynamic_cast<Visitor<T,R>*>(&guest))
{
return p->Visit(visited);
}
return CatchAll<R, T>::OnUnknownVisitor(visited, guest);
}
};
////////////////////////////////////////////////////////////////////////////////
// macro DEFINE_VISITABLE
// Put it in every class that you want to make visitable (in addition to
// deriving it from BaseVisitable<R>
////////////////////////////////////////////////////////////////////////////////
#define DEFINE_VISITABLE() \
virtual ReturnType Accept(Loki::BaseVisitor& guest) \
{ return AcceptImpl(*this, guest); }
////////////////////////////////////////////////////////////////////////////////
// class template CyclicVisitor
// Put it in every class that you want to make visitable (in addition to
// deriving it from BaseVisitable<R>
////////////////////////////////////////////////////////////////////////////////
template <typename R, class TList>
class CyclicVisitor : public Visitor<TList, R>
{
public:
typedef R ReturnType;
// using Visitor<TList, R>::Visit;
template <class Visited>
ReturnType GenericVisit(Visited& host)
{
Visitor<Visited, ReturnType>& subObj = *this;
return subObj.Visit(host);
}
};
////////////////////////////////////////////////////////////////////////////////
// macro DEFINE_CYCLIC_VISITABLE
// Put it in every class that you want to make visitable by a cyclic visitor
////////////////////////////////////////////////////////////////////////////////
#define DEFINE_CYCLIC_VISITABLE(SomeVisitor) \
virtual SomeVisitor::ReturnType Accept(SomeVisitor& guest) \
{ return guest.GenericVisit(*this); }
} // namespace Loki
////////////////////////////////////////////////////////////////////////////////
// Change log:
// March 20: add default argument DefaultCatchAll to BaseVisitable
// June 20, 2001: ported by Nick Thurn to gcc 2.95.3. Kudos, Nick!!!
////////////////////////////////////////////////////////////////////////////////
#endif // VISITOR_INC_
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