//删除尾部元素，调整大小

void pop_back(){

    --finish; //尾端标记往前一格，表示放弃尾部元素

    destroy(finish);

}

//清除(first,last)中的所有元素

iterator erase(iterator first,iterator last){

    iterator i=copy(last,finish,first);//将last到finish的元素往前复制，从first位置开始

    destory(i,finish);

    finish=finish-(last-first);

    return first;

}

//清除某个位置上的元素

iterator erase(iterator position){

    if(position+!=end()){

        copy(position+,finish,position);

    }

    --finish;

    destory(finish);

    return position;

}

void clear(){

    erase(begin(),end());

}

template <class T,class Alloc>

void vector<T,Alloc>::insert(iterator position,size_type n,const T& x){

    if(n!=){

        if(size_type(end_of_storage-finish)>=n){

            //备用空间大于等于"新增元素"

            T x_copy=x;

            //以下计算插入点之后的现有元素个数

            const size_type elems_after=finish-position;

            iterator old_finish=finish;

            if(elems_after>n){

                //(1)、"插入点之后的现有元素个数"大于"新增元素个数"(见图1)
//①从finish处开始复制范围(finish-n,finish)的元素

                uninitialized_copy(finish-n,finish,finish);

                finish+=n; //②vector尾部后移

                //③将范围(position,old_finish-n)的元素移到(,old_finish)处

                copy_backward(position,old_finish-n,old_finish);
//④从插入点开始填入元素

                fill(position,position+n,x_copy);

            }

            else{

                //(2)、"插入点之后的现有元素个数"小于"新增元素个数"（见图2）
//①从finish处复制n-elems_after个元素x_copy

                uninitialized_fill_n(finish,n-elems_after,x_copy);

                finish+=n-elems_after; //②vector尾部后移

                //③从finish处开始复制范围(position,old_finish)的元素

                uninitialized_copy(position,old_finish,finish);

                finish+=elems_after; //④vector尾部后移

                //⑤从插入点开始填入元素

                fill(position,old_finish,x_copy);

            }

        }

        else{

            //备用空间大于等于"新增元素",即必须配置额外空间

            //首先决定新长度，旧长度的两倍，或者旧长度+新长度（见图3）

            const size_type old_size=size();

            const size_type len=old_size+max(old_size,n);

            //以下配置新的vector空间

            iterator new_start=data_allocator::allocate(len);

            iterator new_finish=new_start;

            __STL_TRY{
//①先将旧的vector的插入点之前的元素复制到新的空间

                new_finish=uninitialized_copy(start,position,new_start);
//②再将新增的元素填入新空间

                new_finish=uninitialized_fill_n(new_finish,n,x);
//③再将旧vector的插入点之后的元素复制到新空间

                new_finish=uninitialized_copy(position,finish,new_finish);

            }

#ifdef __STL_USE_EXCEPTIONS

            catch(...){

                //如果发现异常，实现rollback

                destory(new_start,new_finish);

                data_allocator::deallocate(new_satrt,len);

                throw;

            }

#endif /*__STL_USE_EXCEPTIONS*/

            //以下清除并释放旧的vector

            destory(start,finish);

            deallocate();

            //调整水位标记

            start=new_start;

            finish=new_finish;

            end_of_storage=new_start+len;

            }

        }

    }

}