Is there a bug in circular_q::size()?

[SaltedReed]

Now circular_q::size() is implemented as:

    // Return number of elements actually stored
    size_t size() const
    {
        if (tail_ >= head_)
        {
            return tail_ - head_;
        }
        else
        {
            return max_items_ - (head_ - tail_); // is it wrong?
        }
    }

At where I commented, is it supposed to be "return max_items_ - 1 - (head_ - tail_);"?

An example is, when users call the following constructor to create a circular queue with max_items=3:

explicit circular_q(size_t max_items)
        : max_items_(max_items + 1) // one item is reserved as marker for full q
        , v_(max_items_)
    {}

max_items_=4. So If the circular queue looks like this:
index:             0   1   2
has element: no no yes
                    tail_     head_
then size() is supposed to return 1. But with current implementation, the return value is 4-(2-0)=2. I think the marker for full q should be minused here, which isn't yet.

[tt4g]

It has been changed in this commit (bad7284), but maybe missed the max_items_ = max_items + 1 in the constructor.
There is no test, so I think it is most likely a bug.

[gabime]

Good catch @SaltedReed . PR to fix is welcome.
@tt4g You are right, we need to add some tests for it.