for (int i = r; i < _size; i++) _elem[i + 1] = _elem[i];
ANo problem, just change it from front to back 没有问题,只是改为从前向后移动罢了
BOverwrite all elements in the original vector with rank greater than r 会覆盖原向量中秩大于r的所有元素
COverwrite all elements in the original vector with rank less than r 会覆盖原向量中秩小于r的所有元素
DWill cause vector expansion failure 会引起向量扩容失败