stl_iterator_base.h

traits关键技术:内嵌类型，函数模板参数推断，偏特化获取原生指针的类型

/* NOTE: This is an internal header file, included by other STL headers.
 *   You should not attempt to use it directly.
 */

#ifndef __SGI_STL_INTERNAL_ITERATOR_BASE_H
#define __SGI_STL_INTERNAL_ITERATOR_BASE_H

// This file contains all of the general iterator-related utilities.
// The internal file stl_iterator.h contains predefined iterators, 
// such as front_insert_iterator and istream_iterator.

#include <concept_checks.h>

__STL_BEGIN_NAMESPACE
//用于重载函数的选择
// STL中有五种迭代器类型
// Input Iterator               read only  
// Output Iterator              write only  
// Forward Iterator             允许写入型"算法在其指向区间进行操作 
// Bidirectional Iterator       提供双向访问能力
// Random Access Iterator       支持原生指针具有的全部能力
struct input_iterator_tag {};
struct output_iterator_tag {};
struct forward_iterator_tag : public input_iterator_tag {};
struct bidirectional_iterator_tag : public forward_iterator_tag {};
struct random_access_iterator_tag : public bidirectional_iterator_tag {};

// The base classes input_iterator, output_iterator, forward_iterator,
// bidirectional_iterator, and random_access_iterator are not part of
// the C++ standard.  (They have been replaced by struct iterator.)
// They are included for backward compatibility with the HP STL.

template <class _Tp, class _Distance> struct input_iterator {
  typedef input_iterator_tag iterator_category;
  typedef _Tp                value_type;
  typedef _Distance          difference_type;
  typedef _Tp*               pointer;
  typedef _Tp&               reference;
};

struct output_iterator {
  typedef output_iterator_tag iterator_category;
  typedef void                value_type;
  typedef void                difference_type;
  typedef void                pointer;
  typedef void                reference;
};

template <class _Tp, class _Distance> struct forward_iterator {
  typedef forward_iterator_tag iterator_category;
  typedef _Tp                  value_type;
  typedef _Distance            difference_type;
  typedef _Tp*                 pointer;
  typedef _Tp&                 reference;
};


template <class _Tp, class _Distance> struct bidirectional_iterator {
  typedef bidirectional_iterator_tag iterator_category;
  typedef _Tp                        value_type;
  typedef _Distance                  difference_type;
  typedef _Tp*                       pointer;
  typedef _Tp&                       reference;
};

template <class _Tp, class _Distance> struct random_access_iterator {
  typedef random_access_iterator_tag iterator_category;
  typedef _Tp                        value_type;
  typedef _Distance                  difference_type;
  typedef _Tp*                       pointer;
  typedef _Tp&                       reference;
};
//为了符合规范任何迭代器都应该有个内嵌类型，否则和其他STL组件不兼容
//新设计的迭代器可简单继承自iterator
#ifdef __STL_USE_NAMESPACES
template <class _Category, class _Tp, class _Distance = ptrdiff_t,
          class _Pointer = _Tp*, class _Reference = _Tp&>
struct iterator {
  typedef _Category  iterator_category;
  typedef _Tp        value_type;
  typedef _Distance  difference_type;
  typedef _Pointer   pointer;
  typedef _Reference reference;
};
#endif /* __STL_USE_NAMESPACES */

#ifdef __STL_CLASS_PARTIAL_SPECIALIZATION
//为什么多一层iterator_traits来访问迭代器的内嵌类型
//因为可以偏特化，使int*等非class type也可以取到value type
template <class _Iterator>
struct iterator_traits {
  typedef typename _Iterator::iterator_category iterator_category;
  typedef typename _Iterator::value_type        value_type;
  typedef typename _Iterator::difference_type   difference_type;
  typedef typename _Iterator::pointer           pointer;
  typedef typename _Iterator::reference         reference;
};

template <class _Tp>
struct iterator_traits<_Tp*> {  //偏特化，_Tp为原生指针
  typedef random_access_iterator_tag iterator_category;  //原生指针为随机迭代器
  typedef _Tp                         value_type;
  typedef ptrdiff_t                   difference_type;
  typedef _Tp*                        pointer;
  typedef _Tp&                        reference;
};

template <class _Tp>
struct iterator_traits<const _Tp*> {
  typedef random_access_iterator_tag iterator_category;
  typedef _Tp                         value_type;
  typedef ptrdiff_t                   difference_type;
  typedef const _Tp*                  pointer;
  typedef const _Tp&                  reference;
};

// The overloaded functions iterator_category, distance_type, and
// value_type are not part of the C++ standard.  (They have been
// replaced by struct iterator_traits.)  They are included for
// backward compatibility with the HP STL.

// We introduce internal names for these functions.

template <class _Iter>
inline typename iterator_traits<_Iter>::iterator_category
__iterator_category(const _Iter&)
{
  typedef typename iterator_traits<_Iter>::iterator_category _Category;
  return _Category();
}

template <class _Iter>
inline typename iterator_traits<_Iter>::difference_type*
__distance_type(const _Iter&)
{
  return static_cast<typename iterator_traits<_Iter>::difference_type*>(0);
}

template <class _Iter>
inline typename iterator_traits<_Iter>::value_type*
__value_type(const _Iter&)
{
  return static_cast<typename iterator_traits<_Iter>::value_type*>(0);
}
 //以下利用函数模板的参数推断功能，萃取迭代器类型，difference_type，value_type
template <class _Iter>
inline typename iterator_traits<_Iter>::iterator_category
iterator_category(const _Iter& __i) { return __iterator_category(__i); } 


template <class _Iter>
inline typename iterator_traits<_Iter>::difference_type*
distance_type(const _Iter& __i) { return __distance_type(__i); }

template <class _Iter>
inline typename iterator_traits<_Iter>::value_type*
value_type(const _Iter& __i) { return __value_type(__i); }

#define __ITERATOR_CATEGORY(__i) __iterator_category(__i)
#define __DISTANCE_TYPE(__i)     __distance_type(__i)
#define __VALUE_TYPE(__i)        __value_type(__i)

#else /* __STL_CLASS_PARTIAL_SPECIALIZATION */
//不支持模板偏特化的情况，不能traits
template <class _Tp, class _Distance> 
inline input_iterator_tag 
iterator_category(const input_iterator<_Tp, _Distance>&)
  { return input_iterator_tag(); }

inline output_iterator_tag iterator_category(const output_iterator&)
  { return output_iterator_tag(); }

template <class _Tp, class _Distance> 
inline forward_iterator_tag
iterator_category(const forward_iterator<_Tp, _Distance>&)
  { return forward_iterator_tag(); }

template <class _Tp, class _Distance> 
inline bidirectional_iterator_tag
iterator_category(const bidirectional_iterator<_Tp, _Distance>&)
  { return bidirectional_iterator_tag(); }

template <class _Tp, class _Distance> 
inline random_access_iterator_tag
iterator_category(const random_access_iterator<_Tp, _Distance>&)
  { return random_access_iterator_tag(); }

template <class _Tp>
inline random_access_iterator_tag iterator_category(const _Tp*)
  { return random_access_iterator_tag(); }

template <class _Tp, class _Distance> 
inline _Tp* value_type(const input_iterator<_Tp, _Distance>&)
  { return (_Tp*)(0); }

template <class _Tp, class _Distance> 
inline _Tp* value_type(const forward_iterator<_Tp, _Distance>&)
  { return (_Tp*)(0); }

template <class _Tp, class _Distance> 
inline _Tp* value_type(const bidirectional_iterator<_Tp, _Distance>&)
  { return (_Tp*)(0); }

template <class _Tp, class _Distance> 
inline _Tp* value_type(const random_access_iterator<_Tp, _Distance>&)
  { return (_Tp*)(0); }

template <class _Tp>
inline _Tp* value_type(const _Tp*) { return (_Tp*)(0); }

template <class _Tp, class _Distance> 
inline _Distance* distance_type(const input_iterator<_Tp, _Distance>&)
{
  return (_Distance*)(0);
}

template <class _Tp, class _Distance> 
inline _Distance* distance_type(const forward_iterator<_Tp, _Distance>&)
{
  return (_Distance*)(0);
}

template <class _Tp, class _Distance> 
inline _Distance* 
distance_type(const bidirectional_iterator<_Tp, _Distance>&)
{
  return (_Distance*)(0);
}

template <class _Tp, class _Distance> 
inline _Distance* 
distance_type(const random_access_iterator<_Tp, _Distance>&)
{
  return (_Distance*)(0);
}

template <class _Tp>
inline ptrdiff_t* distance_type(const _Tp*) { return (ptrdiff_t*)(0); }

// Without partial specialization we can't use iterator_traits, so
// we must keep the old iterator query functions around.  

#define __ITERATOR_CATEGORY(__i) iterator_category(__i)
#define __DISTANCE_TYPE(__i)     distance_type(__i)
#define __VALUE_TYPE(__i)        value_type(__i)

#endif /* __STL_CLASS_PARTIAL_SPECIALIZATION */
//继承关系传递调用,Forward Iterator,Bidirectional Iterator都会使用此Input Iterator版本
template <class _InputIterator, class _Distance>
inline void __distance(_InputIterator __first, _InputIterator __last,
                       _Distance& __n, input_iterator_tag)
{
  while (__first != __last) { ++__first; ++__n; }
}

template <class _RandomAccessIterator, class _Distance>
inline void __distance(_RandomAccessIterator __first, 
                       _RandomAccessIterator __last, 
                       _Distance& __n, random_access_iterator_tag)
{
  __STL_REQUIRES(_RandomAccessIterator, _RandomAccessIterator);   //concept check
  __n += __last - __first;
}

template <class _InputIterator, class _Distance>
inline void distance(_InputIterator __first, 
                     _InputIterator __last, _Distance& __n)
{
  __STL_REQUIRES(_InputIterator, _InputIterator);
  __distance(__first, __last, __n, iterator_category(__first));
}

#ifdef __STL_CLASS_PARTIAL_SPECIALIZATION

template <class _InputIterator>
inline typename iterator_traits<_InputIterator>::difference_type
__distance(_InputIterator __first, _InputIterator __last, input_iterator_tag)
{
  typename iterator_traits<_InputIterator>::difference_type __n = 0;
  while (__first != __last) {
    ++__first; ++__n;
  }
  return __n;
}

template <class _RandomAccessIterator>
inline typename iterator_traits<_RandomAccessIterator>::difference_type
__distance(_RandomAccessIterator __first, _RandomAccessIterator __last,
           random_access_iterator_tag) {
  __STL_REQUIRES(_RandomAccessIterator, _RandomAccessIterator);
  return __last - __first;
}

template <class _InputIterator>
inline typename iterator_traits<_InputIterator>::difference_type
distance(_InputIterator __first, _InputIterator __last) {
  typedef typename iterator_traits<_InputIterator>::iterator_category 
    _Category;
  __STL_REQUIRES(_InputIterator, _InputIterator);
  return __distance(__first, __last, _Category());
}

#endif /* __STL_CLASS_PARTIAL_SPECIALIZATION */

template <class _InputIter, class _Distance>
inline void __advance(_InputIter& __i, _Distance __n, input_iterator_tag) {
  while (__n--) ++__i;
}

#if defined(__sgi) && !defined(__GNUC__) && (_MIPS_SIM != _MIPS_SIM_ABI32)
#pragma set woff 1183
#endif

template <class _BidirectionalIterator, class _Distance>
inline void __advance(_BidirectionalIterator& __i, _Distance __n, 
                      bidirectional_iterator_tag) {
  __STL_REQUIRES(_BidirectionalIterator, _BidirectionalIterator);
  if (__n >= 0)
    while (__n--) ++__i;
  else
    while (__n++) --__i;
}

#if defined(__sgi) && !defined(__GNUC__) && (_MIPS_SIM != _MIPS_SIM_ABI32)
#pragma reset woff 1183
#endif

template <class _RandomAccessIterator, class _Distance>
inline void __advance(_RandomAccessIterator& __i, _Distance __n, 
                      random_access_iterator_tag) {
  __STL_REQUIRES(_RandomAccessIterator, _RandomAccessIterator);
  __i += __n;
}

template <class _InputIterator, class _Distance>
inline void advance(_InputIterator& __i, _Distance __n) {
  __STL_REQUIRES(_InputIterator, _InputIterator);
  __advance(__i, __n, iterator_category(__i));
}

__STL_END_NAMESPACE

#endif /* __SGI_STL_INTERNAL_ITERATOR_BASE_H */