@AndrewMeadows - let me explain this a little more clearly.

I want to suppress any echoes that are within _combFilterWindow milliseconds from any other already known echoes. I don't care which one I keep, so first one in wins, the others that are too close get suppressed.

So I needed a data-structure that I could store a list of values in, and easily search to see if any values between newValue - _combFilterWindow  to newValue + _combFilterWindow already exist and NOT insert the newValue if any other value already exists in that range.

I could have build my own structure that kept sorted values, and then did a range search on that. But it turns out that QMap already maintains its elements in a sorted manner. And it also has this "find closest match" feature... which is what I needed. So that's why I used QMap<key, key>...