STL的sort算法,为什么VC的实现选择了用3中不同的实现,而GCC只用了插入排序?
STL的sort算法，实际调用的算法不管是哪一种，都应该让算法的复杂度保持最低，对吧。
VC的sort源代码实现是，当排序的个数小于8个的时间，直接插入排序，否则用快速排序。快速排序pivot如果递归层数太深，则余下的工作采用堆排序。这样就能达到算法的最优化结果，我贴了代码:
template<class _RanIt,
class _Diff,
class _Pr> inline
void _Sort(_RanIt _First, _RanIt _Last, _Diff _Ideal, _Pr _Pred)
{// order [_First, _Last), using _Pred
_Diff _Count;
for (; _ISORT_MAX < (_Count = _Last - _First) && 0 < _Ideal; )
{// divide and conquer by quicksort
_STD pair<_RanIt, _RanIt> _Mid =
_Unguarded_partition(_First, _Last, _Pred);
_Ideal /= 2, _Ideal += _Ideal / 2;// allow 1.5 log2(N) divisions
if (_Mid.first - _First < _Last - _Mid.second)
{// loop on second half
_Sort(_First, _Mid.first, _Ideal, _Pred);
_First = _Mid.second;
}
else
{// loop on first half
_Sort(_Mid.second, _Last, _Ideal, _Pred);
_Last = _Mid.first;
}
}
if (_ISORT_MAX < _Count)
{// heap sort if too many divisions
_STD make_heap(_First, _Last, _Pred);
_STD sort_heap(_First, _Last, _Pred);
}
else if (1 < _Count)
_Insertion_sort(_First, _Last, _Pred);// small
}
template<typename _RandomAccessIterator>
inline void
sort(_RandomAccessIterator __first, _RandomAccessIterator __last)
{
typedef typename iterator_traits<_RandomAccessIterator>::value_type
_ValueType;
// concept requirements
__glibcxx_function_requires(_Mutable_RandomAccessIteratorConcept<
_RandomAccessIterator>)
__glibcxx_function_requires(_LessThanComparableConcept<_ValueType>)
__glibcxx_requires_valid_range(__first, __last);
if (__first != __last)
{
std::__introsort_loop(__first, __last,
std::__lg(__last - __first) * 2);
std::__final_insertion_sort(__first, __last);
}
}
/// This is a helper function for the sort routine.
template<typename _RandomAccessIterator>
void
__final_insertion_sort(_RandomAccessIterator __first,
_RandomAccessIterator __last)
{
if (__last - __first > int(_S_threshold))
{
std::__insertion_sort(__first, __first + int(_S_threshold));
std::__unguarded_insertion_sort(__first + int(_S_threshold), __last);
}
else
std::__insertion_sort(__first, __last);
}
