圆形数组的矢量实现

Question

I am confused. I was reading about the implementation of vectors using arrays.While we were on the simple dynamic array implementation everything was okay.

It was mentioned that you can implement the vector by using an array in a circular fashion and that that will make the addition and removal of elements (for first and last) run in constant time. Isn't that what a linked list is supposed to do?

I want to know how it works, but I couldn't really find an implementation or a proper explanation. Any information on what is the general idea and how to implement it is welcomed.

Edit: My guess is that new data should be written on the 'oldest' data and that the array has fixed size and that you must have a variable that stores the last used position.

Answer 1

You are talking about circular buffer

Unfortunately you cannot implement vector using it as you will not be able to insert in the middle or delete from the middle etc.

Answer 2

Since C++03 vector guarantees contiguous storage, that is to say for each 0 <= n < vec.size() :

&vec[n] == &vec[0] + n

This means that you cannot use a circular buffer to implement vector . At the point you "wrap around" from the end of the buffer to the start, you violate this restriction.

In case you think that operator[] could return some proxy with an overloaded operator& that satisfies this requirement -- no, it can't, it's required to return T& .

Aside from this requirement I don't think there's anything in the standard to prevent implementing vector using a circular buffer. In the case where the buffer is full you'd do the same thing vector actually does -- reallocate to a larger buffer. The only difference in behaviour would be what happens when you erase or insert at the start. With a circular buffer all the other elements would stay still assuming sufficient space, whereas normally they all move. But the standard doesn't explicitly require them to move, and since iterators and references to all elements are invalidated when you insert or erase at the start, a program cannot legitimately depend on them moving in any particular way.

It's true that std::list can insert and remove at both the start and the end in constant time. So can std::deque .