STL源码剖析——基本算法copy

copy算法可将指定区间[first, last)内的元素复制到输出区间[result, result + (last - first))内。它时常被调用，所以一个需要有一个高效率的算法。
SGI STL copy算法用了各种办法，包括函数重载、型别特性（type traits）、偏特化（partial specilization）等编程技巧加强效率。

SGI STL的copy算法的完整流程

看不进去了，直接复制代码过来，头晕！！！

// 这是不支持随机访问的情况
template <class InputIterator, class OutputIterator>
inline OutputIterator __copy(InputIterator first, InputIterator last,
                             OutputIterator result, input_iterator_tag)
{
  // first != last导致要进行迭代器的比较, 效率低
  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*)
{
  // 不进行迭代器间的比较, 直接指定循环次数, 高效
  for (Distance n = last - first; n > 0; --n, ++result, ++first)
    *result = *first;
  return result;
}
 
// 这是支持随机访问的情况
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
{
  // 这里是一个仿函数. 再次派发
  OutputIterator operator()(InputIterator first, InputIterator last,
                            OutputIterator result) {
    return __copy(first, last, result, iterator_category(first));
  }
};
 
// 提供兼容
#ifdef __STL_CLASS_PARTIAL_SPECIALIZATION
 
// 可以直接移动, 不需要额外操作
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);
}
 
// 需要进行一些处理, 保证对象复制的正确性
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);
}
 
// 对指针提供特化
template <class T>
struct __copy_dispatch<T*, T*>
{
  T* operator()(T* first, T* last, T* result)
  {
    // 判断其内部是否具有trivial_assignment_operator, 以进行派发
    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) {
    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)拷贝到result处
template <class InputIterator, class OutputIterator>
inline OutputIterator copy(InputIterator first, InputIterator last,
                           OutputIterator result)
{
  // 此处进行函数派发操作
  return __copy_dispatch<InputIterator,OutputIterator>()(first, last, result);
}
 
// 针对char字符串的特化, 效率至上, C++的设计理念
inline char* copy(const char* first, const char* last, char* result)
{
  memmove(result, first, last - first);
  return result + (last - first);
}
 
// 针对wchar_t字符串的特化, 效率至上, C++的设计理念
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 */
 
// 将[first, last)的元素反向拷贝到(..., last)处, 其机制和copy非常接近, 不做说明
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)
{
  // 使用copy()以选择最高效的拷贝算法
  RandomAccessIterator last = first + count;
  return pair<RandomAccessIterator, OutputIterator>(last,
                                                    copy(first, last, result));
}
 
// 从first拷贝n个值到result处
template <class InputIterator, class Size, class OutputIterator>
inline pair<InputIterator, OutputIterator>
copy_n(InputIterator first, Size count,
       OutputIterator result)
{
  // 进行函数派发, 选咋高效版本
  return __copy_n(first, count, result, iterator_category(first));
}

参考：
STL源码分析之copy算法