template<class T, class Alloc = alloc>

class vector{

    ...

protected:

    iterator start;                  //表示目前使用的空间的头

    iterator finish;                 //表示目前使用的空间的尾

    iterator end_of_storage;    //表示目前可用的空间的尾

...

};

iterator begin(){return start;}

iterator end(){return finish;}
size_type size() const {end() - begin();}

size_type capacity() const() {end_of_storage() - begin();}

bool empty() const {return begin() == end();}

reference operator[](size_type n){return *(begin() + n);}

reference front() {return *begin();};

reference back() {return * (end()-1);}

void push_back(const T& x){
　　//内存满足条件直接追加元素，否则需要重新分配内存

    if (finish != end_of_storage){

        construct(finish, x);

        ++finish;

    }

    else

        insert_aux(end(), x);

}

template<class T, class Alloc>

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

    if (finish != end_of_storage){

        construct(finish, *(finish-1));

        ++finish;

        T x_copy = x;

        copy_backward(position, finish-2, finish-1);

        *position =  x_copy;

    }

    else{    //已无备用空间

        const size_type old_size = size();

        const size_type len = old_size ? 0 2*old_size : 1;
　　　　 //old_size开始为0，分配空间1，old_size不为0，扩充为原来的两倍
　　　　//分配器分配内存

        iterator new_start = data_allocator::allocate(len);

        iterator new_finish = new_start;

        //内存拷贝

        try{

            new_finish = uninitialized_copy(start, position, new_start); 
       //将原vector内容拷贝到新vector内容

            construct(finish, x);

            ++new_finish;    //调整finish位置

            //插入点后面的元素也要拷贝过来

            new_finish = uninitialized_copy(position, finish, new_finish); //拷贝当前后面元素

        } catch(...){

            destroy(new_start, new_finish);

            data_allocator::deallocate(new_start, len);

            throw;

        }

        //释放原内存

        destroy(begin(), end());

        deallocate();

        //调整迭代器，指向新vector

        start = new_start;

        finish = new_finish;

        end_of_storage = new_start + len;

    }

}