tuple_basic.hpp

C++的一个好库。。。现在很流行

//  tuple_basic.hpp -----------------------------------------------------

// Copyright (C) 1999, 2000 Jaakko J鋜vi ([e-mail])
//
// Distributed under the Boost Software License, Version 1.0. (See
// accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)

// For more information, see http://www.boost.org

// Outside help:
// This and that, Gary Powell.
// Fixed return types for get_head/get_tail
// ( and other bugs ) per suggestion of Jens Maurer
// simplified element type accessors + bug fix  (Jeremy Siek)
// Several changes/additions according to suggestions by Douglas Gregor,
// William Kempf, Vesa Karvonen, John Max Skaller, Ed Brey, Beman Dawes,
// David Abrahams.

// Revision history:
// 2002 05 01 Hugo Duncan: Fix for Borland after Jaakko's previous changes
// 2002 04 18 Jaakko: tuple element types can be void or plain function
//                    types, as long as no object is created.
//                    Tuple objects can no hold even noncopyable types
//                    such as arrays.
// 2001 10 22 John Maddock
//      Fixes for Borland C++
// 2001 08 30 David Abrahams
//      Added default constructor for cons<>.
// -----------------------------------------------------------------

#ifndef BOOST_TUPLE_BASIC_HPP
#define BOOST_TUPLE_BASIC_HPP


#include <utility> // needed for the assignment from pair to tuple

#include "boost/type_traits/cv_traits.hpp"
#include "boost/type_traits/function_traits.hpp"

#include "boost/detail/workaround.hpp" // needed for BOOST_WORKAROUND

namespace boost {
namespace tuples {

// -- null_type --------------------------------------------------------
struct null_type {};

// a helper function to provide a const null_type type temporary
namespace detail {
  inline const null_type cnull() { return null_type(); }


// -- if construct ------------------------------------------------
// Proposed by Krzysztof Czarnecki and Ulrich Eisenecker

template <bool If, class Then, class Else> struct IF { typedef Then RET; };

template <class Then, class Else> struct IF<false, Then, Else> {
  typedef Else RET;
};

} // end detail

// - cons forward declaration -----------------------------------------------
template <class HT, class TT> struct cons;


// - tuple forward declaration -----------------------------------------------
template <
  class T0 = null_type, class T1 = null_type, class T2 = null_type,
  class T3 = null_type, class T4 = null_type, class T5 = null_type,
  class T6 = null_type, class T7 = null_type, class T8 = null_type,
  class T9 = null_type>
class tuple;

// tuple_length forward declaration
template<class T> struct length;



namespace detail {

// -- generate error template, referencing to non-existing members of this
// template is used to produce compilation errors intentionally
template<class T>
class generate_error;

// - cons getters --------------------------------------------------------
// called: get_class<N>::get<RETURN_TYPE>(aTuple)

template< int N >
struct get_class {
  template<class RET, class HT, class TT >
  inline static RET get(const cons<HT, TT>& t)
  {
#if BOOST_WORKAROUND(__IBMCPP__,==600)
    // vacpp 6.0 is not very consistent regarding the member template keyword
    // Here it generates an error when the template keyword is used.
    return get_class<N-1>::get<RET>(t.tail);
#else
    return get_class<N-1>::BOOST_NESTED_TEMPLATE get<RET>(t.tail);
#endif
  }
  template<class RET, class HT, class TT >
  inline static RET get(cons<HT, TT>& t)
  {
#if BOOST_WORKAROUND(__IBMCPP__,==600)
    return get_class<N-1>::get<RET>(t.tail);
#else
    return get_class<N-1>::BOOST_NESTED_TEMPLATE get<RET>(t.tail);
#endif
  }
};

template<>
struct get_class<0> {
  template<class RET, class HT, class TT>
  inline static RET get(const cons<HT, TT>& t)
  {
    return t.head;
  }
  template<class RET, class HT, class TT>
  inline static RET get(cons<HT, TT>& t)
  {
    return t.head;
  }
};

} // end of namespace detail


// -cons type accessors ----------------------------------------
// typename tuples::element<N,T>::type gets the type of the
// Nth element ot T, first element is at index 0
// -------------------------------------------------------

#ifndef BOOST_NO_CV_SPECIALIZATIONS

template<int N, class T>
struct element
{
private:
  typedef typename T::tail_type Next;
public:
  typedef typename element<N-1, Next>::type type;
};
template<class T>
struct element<0,T>
{
  typedef typename T::head_type type;
};

template<int N, class T>
struct element<N, const T>
{
private:
  typedef typename T::tail_type Next;
  typedef typename element<N-1, Next>::type unqualified_type;
public:
#if BOOST_WORKAROUND(__BORLANDC__,<0x600)
  typedef const unqualified_type type;
#else
  typedef typename boost::add_const<unqualified_type>::type type;
#endif

};
template<class T>
struct element<0,const T>
{
#if BOOST_WORKAROUND(__BORLANDC__,<0x600)
  typedef const typename T::head_type type;
#else
  typedef typename boost::add_const<typename T::head_type>::type type;
#endif
};

#else // def BOOST_NO_CV_SPECIALIZATIONS

namespace detail {

template<int N, class T, bool IsConst>
struct element_impl
{
private:
  typedef typename T::tail_type Next;
public:
  typedef typename element_impl<N-1, Next, IsConst>::type type;
};

template<int N, class T>
struct element_impl<N, T, true /* IsConst */>
{
private:
  typedef typename T::tail_type Next;
public:
  typedef const typename element_impl<N-1, Next, true>::type type;
};

template<class T>
struct element_impl<0, T, false /* IsConst */>
{
  typedef typename T::head_type type;
};

template<class T>
struct element_impl<0, T, true /* IsConst */>
{
  typedef const typename T::head_type type;
};

} // end of namespace detail


template<int N, class T>
struct element: 
  public detail::element_impl<N, T, ::boost::is_const<T>::value>
{
};

#endif


// -get function templates -----------------------------------------------
// Usage: get<N>(aTuple)

// -- some traits classes for get functions

// access traits lifted from detail namespace to be part of the interface,
// (Joel de Guzman's suggestion). Rationale: get functions are part of the
// interface, so should the way to express their return types be.

template <class T> struct access_traits {
  typedef const T& const_type;
  typedef T& non_const_type;

  typedef const typename boost::remove_cv<T>::type& parameter_type;

// used as the tuple constructors parameter types
// Rationale: non-reference tuple element types can be cv-qualified.
// It should be possible to initialize such types with temporaries,
// and when binding temporaries to references, the reference must
// be non-volatile and const. 8.5.3. (5)
};

template <class T> struct access_traits<T&> {

  typedef T& const_type;
  typedef T& non_const_type;

  typedef T& parameter_type;
};

// get function for non-const cons-lists, returns a reference to the element

template<int N, class HT, class TT>
inline typename access_traits<
                  typename element<N, cons<HT, TT> >::type
                >::non_const_type
get(cons<HT, TT>& c BOOST_APPEND_EXPLICIT_TEMPLATE_NON_TYPE(int, N)) {
#if BOOST_WORKAROUND(__IBMCPP__,==600 )
  return detail::get_class<N>::
#else
  return detail::get_class<N>::BOOST_NESTED_TEMPLATE
#endif
         get<
           typename access_traits<
             typename element<N, cons<HT, TT> >::type
           >::non_const_type,
           HT,TT
         >(c);
}

// get function for const cons-lists, returns a const reference to
// the element. If the element is a reference, returns the reference
// as such (that is, can return a non-const reference)
template<int N, class HT, class TT>
inline typename access_traits<
                  typename element<N, cons<HT, TT> >::type
                >::const_type
get(const cons<HT, TT>& c BOOST_APPEND_EXPLICIT_TEMPLATE_NON_TYPE(int, N)) {
#if BOOST_WORKAROUND(__IBMCPP__,==600)
  return detail::get_class<N>::
#else
  return detail::get_class<N>::BOOST_NESTED_TEMPLATE
#endif
         get<
           typename access_traits<
             typename element<N, cons<HT, TT> >::type
           >::const_type,
           HT,TT
         >(c);
}

// -- the cons template  --------------------------------------------------
namespace detail {

//  These helper templates wrap void types and plain function types.
//  The reationale is to allow one to write tuple types with those types
//  as elements, even though it is not possible to instantiate such object.
//  E.g: typedef tuple<void> some_type; // ok
//  but: some_type x; // fails

template <class T> class non_storeable_type {
  non_storeable_type();
};

template <class T> struct wrap_non_storeable_type {
  typedef typename IF<
    ::boost::is_function<T>::value, non_storeable_type<T>, T
  >::RET type;
};
template <> struct wrap_non_storeable_type<void> {
  typedef non_storeable_type<void> type;
};