如何通过at（）函数访问2D向量的索引？

I am going to initialize a 2D vector in a member function, where the input arguments are planned to be fed into determined indexes within the function argument. Furthermore, I'm not gonna use [] operator to member access due to safety stuffs. How can I use at() function in order to access to the index of a 2D vector, as below?I am going to initialize a 2D vector in a membe