std::vector<int> a {1,2,3,4,5,6}, b {9,10,11};
 
 size_t a_len = a.size(), b_len = b.size();
 a.resize(a_len+b_len);
 memcpy(&(a[a_len]), &(b[0]), b_len*sizeof(b[0]));
 b.clear();

  reference
      operator[](size_type __n) _GLIBCXX_NOEXCEPT
      {
	__glibcxx_requires_subscript(__n);
	return *(this->_M_impl._M_start + __n);
      }

     /**
       *   Returns a pointer such that [data(), data() + size()) is a valid
       *   range.  For a non-empty %vector, data() == &front().
       */
      _Tp*
      data() _GLIBCXX_NOEXCEPT
      { return _M_data_ptr(this->_M_impl._M_start); }

      void
      _M_create_storage(size_t __n)
      {
	this->_M_impl._M_start = this->_M_allocate(__n);
	this->_M_impl._M_finish = this->_M_impl._M_start;
	this->_M_impl._M_end_of_storage = this->_M_impl._M_start + __n;
      }
    };