Is there any way to fix the vector size So that it automatically deletes elements from front?

Question

I am trying to figure out the best c++ container before I decide to define my own data structure.

Here is What I am trying to do :

For example, In Vectors (C++) Is there any way to fix the size so that I can keep pushing and popping elements from one end and if I push more elements than the size, It should automatically remove the old elements from front.

If not vectors, any thing else ?

The Boost Library has [`boost::circular_buffer`](https://www.boost.org/doc/libs/1_66_0/doc/html/circular_buffer.html) — jogojapan, Mar 31 '18 at 08:48
@jogojapan Thanks! , I think that's exactly what I wanted. But Can I dynamically change the size if required ? . i.e, After using size 7 circular buffer for a while, If I wanted it to resize it to hold 9 elements and continue its behaviour , is it possible ? — Severus Tux, Mar 31 '18 at 08:56
Yes, definitely. The Boost version has a [`resize()`](https://www.boost.org/doc/libs/1_66_0/doc/html/boost/circular_buffer.html#idp39122944-bb) function. And if you implement the circular buffer yourself, you just have to make sure you use an underlying data structure that is flexible in size, e.g. a `std::vector` rather than a `std::array`. — jogojapan, Mar 31 '18 at 09:04
Possible duplicate of [limit size of Queue in C++](https://stackoverflow.com/questions/1273026/limit-size-of-queuet-in-c) — xskxzr, Mar 31 '18 at 09:12
@xskxzr: Beware, for the highest voted answer of that question is terrible (I just downvoted it). However, the accepted answer is correct (so I just upvoted it). — Christian Hackl, Mar 31 '18 at 09:56

score 4 · Answer 1 · answered Mar 31 '18 at 09:02

There is no such container in the C++ standard library, but you can easily create your own by wrapping an existing container. std::deque may be appropriate, because it's designed to allow fast insertion and deletion at both ends.

template <class T, int MaxSize>
class CircularContainer
{
static_assert(MaxSize >= 0);

public:
    void Add(T const& t)
    {
        if (MaxSize > 0)
        {
            if (size(data) == MaxSize)
            {
                data.pop_front();
            }
            data.push_back(t);
        }
    }

// ...

private:
    std::deque<T> data;
};

There are a lot of design decisions to make here, for example:

Should be the maximum size be completely static, set in the constructor or possibly be modifiable even after a container object has been created?
Which iterators and references to elements are allowed to become invalid by operations like Add?
Which parts of the wrapped container's interface does your container need to expose? Does it need its own iterator classes, does it have to work with range-based for loops, and so on.
Do you wish to allow a maximum size of zero, and do you need to optimise the corresponding checks with template specialisation or constexpr if?
Should the wrapped container class itself be a template parameter, just like std::stack does it?

Most of those design decisions will depend on whether the container will be used in library code or in application code.

Is there any way to fix the vector size So that it automatically deletes elements from front?

1 Answers1