关于容器vector的insert实现问题，求教大神
我想问的是在备用空间足够的情况下，为什么要分两种情况啊
template <class _Tp, class _Alloc>
void
vector<_Tp, _Alloc>::insert (iterator __position,
const_iterator __first,
const_iterator __last)
{
if (__first != __last) {
size_type __n = 0;
distance(__first, __last, __n);
if (size_type(_M_end_of_storage - _M_finish) >= __n) {
const size_type __elems_after = _M_finish - __position;
iterator __old_finish = _M_finish;
if (__elems_after > __n) {//我想问的就是这里，为什么要分两种情况呢？
//为啥不直接copy_backward(__position, __old_finish, __old_finish+n-1);
//_M_finish=__old_finish+n;
uninitialized_copy(_M_finish - __n, _M_finish, _M_finish);
_M_finish += __n;
copy_backward(__position, __old_finish - __n, __old_finish);
copy(__first, __last, __position);
}
else {
uninitialized_copy(__first + __elems_after, __last, _M_finish);
_M_finish += __n - __elems_after;
uninitialized_copy(__position, __old_finish, _M_finish);
_M_finish += __elems_after;
copy(__first, __first + __elems_after, __position);
}
}
else {
const size_type __old_size = size();
const size_type __len = __old_size + max(__old_size, __n);
iterator __new_start = _M_allocate(__len);
iterator __new_finish = __new_start;
__STL_TRY {
__new_finish = uninitialized_copy(_M_start, __position, __new_start);
__new_finish = uninitialized_copy(__first, __last, __new_finish);
__new_finish
= uninitialized_copy(__position, _M_finish, __new_finish);
}
__STL_UNWIND((destroy(__new_start,__new_finish),
_M_deallocate(__new_start,__len)));
destroy(_M_start, _M_finish);
_M_deallocate(_M_start, _M_end_of_storage - _M_start);
_M_start = __new_start;
_M_finish = __new_finish;
_M_end_of_storage = __new_start + __len;
}
}
}
