stl_algobase.h中的算法，比较值得学习的是copy()，它“无所不用其极”的改善效率。copy的目的是复制一段元素到指定区间，复制操作最容易想到赋值操作符=，但是有的赋值操作符=是trivial的，可以直接拷贝。关于赋值操作符=是不是trivial的，可以参考“Memberwise copy(深拷贝)与Bitwise copy(浅拷贝)的区别”。

下面是copy调用过程的讲解。

G++ 2.91.57，cygnus\cygwin-b20\include\g++\stl_algobase.h 完整列表
/*
 *
 * Copyright (c) 1994
 * Hewlett-Packard Company
 *
 * Permission to use, copy, modify, distribute and sell this software
 * and its documentation 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.  Hewlett-Packard Company makes no
 * representations about the suitability of this software for any
 * purpose.  It is provided "as is" without express or implied warranty.
 *
 *
 * Copyright (c) 1996
 * Silicon Graphics Computer Systems, Inc.
 *
 * Permission to use, copy, modify, distribute and sell this software
 * and its documentation 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.  Silicon Graphics makes no
 * representations about the suitability of this software for any
 * purpose.  It is provided "as is" without express or implied warranty.
 */

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


#ifndef __SGI_STL_INTERNAL_ALGOBASE_H
#define __SGI_STL_INTERNAL_ALGOBASE_H

#ifndef __STL_CONFIG_H
#include <stl_config.h>
#endif
#ifndef __SGI_STL_INTERNAL_RELOPS
#include <stl_relops.h>
#endif
#ifndef __SGI_STL_INTERNAL_PAIR_H
#include <stl_pair.h>
#endif
#ifndef __TYPE_TRAITS_H_
#include <type_traits.h>
#endif

#include <string.h>
#include <limits.h>
#include <stdlib.h>
#include <stddef.h>
#include <new.h>
#include <iostream.h>

#ifndef __SGI_STL_INTERNAL_ITERATOR_H
#include <stl_iterator.h>
#endif

__STL_BEGIN_NAMESPACE

template <class ForwardIterator1, class ForwardIterator2, class T>
inline void __iter_swap(ForwardIterator1 a, ForwardIterator2 b, T*) {//T是迭代器，T*就是
  T tmp = *a;														//迭代器指向的类型了
  *a = *b;
  *b = tmp;
}
//对调两个迭代器指向元素的值
template <class ForwardIterator1, class ForwardIterator2>
inline void iter_swap(ForwardIterator1 a, ForwardIterator2 b) {
  // iter_swap() 是「有必要运用迭代器之 value type」的一个好例子。
  // 是的，它必须知道迭代器的 value type，才能够据此宣告一个物件，用來
  // 暂时放置迭代器所指的物件。
  __iter_swap(a, b, value_type(a));	// 注意第三參數的型別!
  /*
  // 以下定义于 <stl_iterator.h>
  template <class Iterator>
  inline typename iterator_traits<Iterator>::value_type*
  value_type(const Iterator&) {
    return static_cast<typename iterator_traits<Iterator>::value_type*>(0);
  }
  */

  // 侯捷认为（并予实证），不需像上行那样调用，可改用以下写法：
  // typename iterator_traits<ForwardIterator1>::value_type tmp = *a;
  // *a = *b;
  // *b = tmp;
}

template <class T>
inline void swap(T& a, T& b) {
  T tmp = a;
  a = b;
  b = tmp;
}

#ifndef __BORLANDC__

#undef min
#undef max

template <class T>
inline const T& min(const T& a, const T& b) {
  return b < a ? b : a;
}

template <class T>
inline const T& max(const T& a, const T& b) {
  return  a < b ? b : a;
}

#endif /* __BORLANDC__ */

template <class T, class Compare>
inline const T& min(const T& a, const T& b, Compare comp) {
  return comp(b, a) ? b : a;	// 由 comp 決定「大小比较」标准
}

template <class T, class Compare>
inline const T& max(const T& a, const T& b, Compare comp) {
  return comp(a, b) ? b : a;	// 由 comp 決定「大小比较」标准
}

template <class InputIterator, class OutputIterator>
inline OutputIterator __copy(InputIterator first, InputIterator last,
                             OutputIterator result, input_iterator_tag)
{
  for ( ; first != last; ++result, ++first)
    *result = *first;
  return result;
}

template <class RandomAccessIterator, class OutputIterator, class Distance>
inline OutputIterator
__copy_d(RandomAccessIterator first, RandomAccessIterator last,
         OutputIterator result, Distance*)
{
  for (Distance n = last - first; n > 0; --n, ++result, ++first) 
    *result = *first;//一个一个赋值
  return result;
}

template <class RandomAccessIterator, class OutputIterator>
inline OutputIterator 
__copy(RandomAccessIterator first, RandomAccessIterator last,
       OutputIterator result, random_access_iterator_tag)
{
  return __copy_d(first, last, result, distance_type(first));
}

template <class InputIterator, class OutputIterator>
struct __copy_dispatch
{
  OutputIterator operator()(InputIterator first, InputIterator last,
                            OutputIterator result) {
    return __copy(first, last, result, iterator_category(first));
  }
};

#ifdef __STL_CLASS_PARTIAL_SPECIALIZATION 
//__true_type意味着has trivial operator=。可以直接拷贝
template <class T>
inline T* __copy_t(const T* first, const T* last, T* result, __true_type) {
  memmove(result, first, sizeof(T) * (last - first));
  return result + (last - first);
}

//__false_type意味着has non-rivial operator=
template <class T>
inline T* __copy_t(const T* first, const T* last, T* result, __false_type) {
  return __copy_d(first, last, result, (ptrdiff_t*) 0);
}

template <class T>
struct __copy_dispatch<T*, T*>
{
  T* operator()(T* first, T* last, T* result) {
    typedef typename __type_traits<T>::has_trivial_assignment_operator t; 
    return __copy_t(first, last, result, t());
  }
};

template <class T>
struct __copy_dispatch<const T*, T*>
{
  T* operator()(const T* first, const T* last, T* result) {
  //赋值操作符=是否是trivial的
    typedef typename __type_traits<T>::has_trivial_assignment_operator t; 
    return __copy_t(first, last, result, t());
  }
};

#endif /* __STL_CLASS_PARTIAL_SPECIALIZATION */
//将[first last)区间元素复制到[result result+(last-first))
// copy 函数运用了 function overloading, type traits, partial 
// specialization, 无所不用其极的改善效率。

//下面还有个copy_backward，它是从后向前复制。防止两个区间
//[first last)和[result result+(last-first))可能会有重叠。
//如果一一赋值，会有覆盖，但是用memmove则不会，因为它会先将
//值拷贝下来。
template <class InputIterator, class OutputIterator>
inline OutputIterator copy(InputIterator first, InputIterator last,
                           OutputIterator result)
{
  return __copy_dispatch<InputIterator,OutputIterator>()(first, last, result);
}

inline char* copy(const char* first, const char* last, char* result) {
  memmove(result, first, last - first);
  return result + (last - first);
}

inline wchar_t* copy(const wchar_t* first, const wchar_t* last,
                     wchar_t* result) {
  memmove(result, first, sizeof(wchar_t) * (last - first));
  return result + (last - first);
}

template <class BidirectionalIterator1, class BidirectionalIterator2>
inline BidirectionalIterator2 __copy_backward(BidirectionalIterator1 first, 
                                              BidirectionalIterator1 last, 
                                              BidirectionalIterator2 result) {
  while (first != last) *--result = *--last;
  return result;
}


template <class BidirectionalIterator1, class BidirectionalIterator2>
struct __copy_backward_dispatch
{
  BidirectionalIterator2 operator()(BidirectionalIterator1 first, 
                                    BidirectionalIterator1 last, 
                                    BidirectionalIterator2 result) {
    return __copy_backward(first, last, result);
  }
};

#ifdef __STL_CLASS_PARTIAL_SPECIALIZATION 

template <class T>
inline T* __copy_backward_t(const T* first, const T* last, T* result,
                            __true_type) {
  const ptrdiff_t N = last - first;
  memmove(result - N, first, sizeof(T) * N);
  return result - N;
}

template <class T>
inline T* __copy_backward_t(const T* first, const T* last, T* result,
                            __false_type) {
  return __copy_backward(first, last, result);
}

template <class T>
struct __copy_backward_dispatch<T*, T*>
{
  T* operator()(T* first, T* last, T* result) {
    typedef typename __type_traits<T>::has_trivial_assignment_operator t; 
    return __copy_backward_t(first, last, result, t());
  }
};

template <class T>
struct __copy_backward_dispatch<const T*, T*>
{
  T* operator()(const T* first, const T* last, T* result) {
    typedef typename __type_traits<T>::has_trivial_assignment_operator t; 
    return __copy_backward_t(first, last, result, t());
  }
};

#endif /* __STL_CLASS_PARTIAL_SPECIALIZATION */

template <class BidirectionalIterator1, class BidirectionalIterator2>
inline BidirectionalIterator2 copy_backward(BidirectionalIterator1 first, 
                                            BidirectionalIterator1 last, 
                                            BidirectionalIterator2 result) {
  return __copy_backward_dispatch<BidirectionalIterator1, 
                                  BidirectionalIterator2>()(first, last, 
                                                            result);
}

template <class InputIterator, class Size, class OutputIterator>
pair<InputIterator, OutputIterator> __copy_n(InputIterator first, Size count,
                                             OutputIterator result,
                                             input_iterator_tag) {
  for ( ; count > 0; --count, ++first, ++result)
    *result = *first;
  return pair<InputIterator, OutputIterator>(first, result);
}

template <class RandomAccessIterator, class Size, class OutputIterator>
inline pair<RandomAccessIterator, OutputIterator>
__copy_n(RandomAccessIterator first, Size count,
         OutputIterator result,
         random_access_iterator_tag) {
  RandomAccessIterator last = first + count;
  return pair<RandomAccessIterator, OutputIterator>(last,
                                                    copy(first, last, result));
}

// 以下为 SGI STL 专属，从 first 开始复制 count 个元素到 result 以后的空间。
template <class InputIterator, class Size, class OutputIterator>
inline pair<InputIterator, OutputIterator>
copy_n(InputIterator first, Size count,
       OutputIterator result) {
  return __copy_n(first, count, result, iterator_category(first));
}

//将[first last)内所有元素改填新值value
template <class ForwardIterator, class T>
void fill(ForwardIterator first, ForwardIterator last, const T& value) {
  for ( ; first != last; ++first)	// 迭代走过整个范围
    *first = value;
}
//注意:n不能超过区间长度
template <class OutputIterator, class Size, class T>
OutputIterator fill_n(OutputIterator first, Size n, const T& value) {
  for ( ; n > 0; --n, ++first)		// 经过n个元素
    *first = value;	// 注意，assignment 是覆盖（overwrite）而不是插入（insert）
  return first;
}

//用于比较两个区间，指出两者之间的第一个不匹配点。返回一对迭代器，分别
//指出两个区间的不匹配点。如果都匹配，返回的是指向两个区间的last迭代器。
template <class InputIterator1, class InputIterator2>
pair<InputIterator1, InputIterator2> mismatch(InputIterator1 first1,
					      InputIterator1 last1,
					      InputIterator2 first2) {
  // 以下，如果序列一走完，就结束。
  // 以下，如果序列一和序列二的对应元素相等，就结束。
  // 显然，区间一的元素个数必须多过序列二的元素个数，否则结果不可预期。
  while (first1 != last1 && *first1 == *first2) {
    ++first1;
    ++first2;
  }
  return pair<InputIterator1, InputIterator2>(first1, first2);
}

template <class InputIterator1, class InputIterator2, class BinaryPredicate>
pair<InputIterator1, InputIterator2> mismatch(InputIterator1 first1,
					      InputIterator1 last1,
					      InputIterator2 first2,
					      BinaryPredicate binary_pred) {
  while (first1 != last1 && binary_pred(*first1, *first2)) {
    ++first1;
    ++first2;
  }
  return pair<InputIterator1, InputIterator2>(first1, first2);
}
//如果两个区间在[first last)区间相等，返回true。如果第二个区间元素比较多，多出来的
//不予考虑。如果第二个区间元素少，会有不可预测的结果。因此在使用前最好判断区间大小。
template <class InputIterator1, class InputIterator2>
inline bool equal(InputIterator1 first1, InputIterator1 last1,
		  InputIterator2 first2) {
  // 遍历一遍区间的元素
  // 如果区间一的元素个数多过区间二的元素个数，就糟糕了。
  for ( ; first1 != last1; ++first1, ++first2)
    if (*first1 != *first2)		// 只要对应元素不相等，
      return false;			// 就结束并返回 false。
  return true;				// 至此，全部相等，返回true。
}

template <class InputIterator1, class InputIterator2, class BinaryPredicate>
inline bool equal(InputIterator1 first1, InputIterator1 last1,
		  InputIterator2 first2, BinaryPredicate binary_pred) {
  for ( ; first1 != last1; ++first1, ++first2)
    if (!binary_pred(*first1, *first2))
      return false;
  return true;
}
//以“字典排序方式”对两个区间[first1 last1)和[first2 last2)进行比较
template <class InputIterator1, class InputIterator2>
bool lexicographical_compare(InputIterator1 first1, InputIterator1 last1,
			     InputIterator2 first2, InputIterator2 last2) {
  // 以下，任何一个区间到达尾端，就结束。否则两个区间就相应元素一一进行比对。
  for ( ; first1 != last1 && first2 != last2; ++first1, ++first2) {
    if (*first1 < *first2)	// 第一序列元素值小于第二序列的相应元素值
      return true;
    if (*first2 < *first1) // 第二序列元素值小于第一序列的相应元素值
      return false;
    // 如果不符合以上两条件，表示两值相等，那就进行下一组相应元素值的比对。
  }
  // 运行到这里，如果第一区间到达尾端而第二区间尚有余额，那么第一区间小于第二区间。
  return first1 == last1 && first2 != last2;
}

template <class InputIterator1, class InputIterator2, class Compare>
bool lexicographical_compare(InputIterator1 first1, InputIterator1 last1,
			     InputIterator2 first2, InputIterator2 last2,
			     Compare comp) {
  for ( ; first1 != last1 && first2 != last2; ++first1, ++first2) {
    if (comp(*first1, *first2))
      return true;
    if (comp(*first2, *first1))
      return false;
  }
  return first1 == last1 && first2 != last2;
}

inline bool 
lexicographical_compare(const unsigned char* first1,
                        const unsigned char* last1,
                        const unsigned char* first2,
                        const unsigned char* last2)
{
  const size_t len1 = last1 - first1; 	// 第一区间長度
  const size_t len2 = last2 - first2; 	// 第二区间長度
  // 先比较相同长度的一截。memcmp() 速度极快。
  const int result = memcmp(first1, first2, min(len1, len2));
  // 如果不相上下，则长度较长者被视为比较大。
  return result != 0 ? result < 0 : len1 < len2;
}

inline bool lexicographical_compare(const char* first1, const char* last1,
                                    const char* first2, const char* last2)
{
#if CHAR_MAX == SCHAR_MAX
  // 转换为 const signed char*
  return lexicographical_compare((const signed char*) first1,
                                 (const signed char*) last1,
                                 (const signed char*) first2,
                                 (const signed char*) last2);
#else
  // 转换为 const unsigned char*
  return lexicographical_compare((const unsigned char*) first1,
                                 (const unsigned char*) last1,
                                 (const unsigned char*) first2,
                                 (const unsigned char*) last2);
#endif
}

template <class InputIterator1, class InputIterator2>
int lexicographical_compare_3way(InputIterator1 first1, InputIterator1 last1,
                                 InputIterator2 first2, InputIterator2 last2)
{
  while (first1 != last1 && first2 != last2) {
    if (*first1 < *first2) return -1;
    if (*first2 < *first1) return 1;
    ++first1; ++first2;
  }
  if (first2 == last2) {
    return !(first1 == last1);
  } else {
    return -1;
  }
}

inline int
lexicographical_compare_3way(const unsigned char* first1,
                             const unsigned char* last1,
                             const unsigned char* first2,
                             const unsigned char* last2)
{
  const ptrdiff_t len1 = last1 - first1;
  const ptrdiff_t len2 = last2 - first2;
  const int result = memcmp(first1, first2, min(len1, len2));
  return result != 0 ? result : (len1 == len2 ? 0 : (len1 < len2 ? -1 : 1));
}

inline int lexicographical_compare_3way(const char* first1, const char* last1,
                                        const char* first2, const char* last2)
{
#if CHAR_MAX == SCHAR_MAX
  return lexicographical_compare_3way(
				(const signed char*) first1,
                                (const signed char*) last1,
                                (const signed char*) first2,
                                (const signed char*) last2);
#else
  return lexicographical_compare_3way((const unsigned char*) first1,
                                      (const unsigned char*) last1,
                                      (const unsigned char*) first2,
                                      (const unsigned char*) last2);
#endif
}

__STL_END_NAMESPACE

#endif /* __SGI_STL_INTERNAL_ALGOBASE_H */

// Local Variables:
// mode:C++
// End: