[Qt-interest] circular buffer container

[Andre Somers]

On 23-4-2010 11:38, Sean Harmer wrote:
> Hi All,
>
> Just in case anybody is doing data acquisition/logging type work, I have
> just submitted a QCircularBuffer class as a merge request to gitorious.
> The merge request is here:
>
> http://qt.gitorious.org/qt/qt/merge_requests/2364
>
> This is my first merge request of any size so I would really appreciate
> any feedback that you can give on API, implementation and documentation
> omissions etc.
>
> Many thanks in advance,
>
>    

First of all, I like the idea of adding a circular buffer to Qt. Very nice.
I did wonder when I glanced over your implementation: would it not have 
been much easier to implement this based on one of the exising container 
classes instead? That would result, I think, in an easier to maintain 
solution, as you have to do the heavy lifting of memory operations only 
once. This is how QQueue and QStack are implemented: a bit of sugar on 
top of QList and QVector respectively. To me, it seems like a circular 
buffer is not much more than a QQueue with a limited number of items in 
it, right? Is there a reason you chose a from-scratch aproach?

I have created similar datastructures myself (based on the existing Qt 
datastructures) in the past. I found that often there are other criteria 
that are relevant than just what the oldest bit of data is. Wouldn't it 
make your data structure more versatile if you would provide an optional 
way to determine which item to drop first?

Let me give you an example: A long time ago, I implemented a system to 
store measurements of the windvector for a flight computer for glider 
pilots. These measurements had different quality, and we had an 
indication for that quality. Because of memory constraints, we only kept 
a limited number of such measurements in memory to determine the current 
wind. The selection of what item to drop, was a function of that quality 
*and* the age of the measurement. I can image similar constraints in 
other applications. You talk about data logging or visualisation 
applications in your merge request. Would also there not only the age of 
a piece of data, but also it's importance play a role? For instance, say 
you want to keep a log of events on a server. These come in the form of 
errors, severe warnings, warnings and information messages. Personally, 
I would build the application in a way that it would keep te errors and 
the severe warnings (perhaps detections of attemted server breakins?) 
longer in the displayed log than the informational and low-importantance 
warnings.
However, such a feature may be out of scope :-)

André

- just another Qt user