Usually, if I have a member constant I want to have configurable in my constructor, I have it by the same name in the constructor parameters. In this case, ModePattern* backingMode was passed in, and then saved for later use in the constructor list via backingMode(backingMode). This pattern has worked fine for about two years () at this point.

Today, it stopped working because I had a lambda function which captured by reference, [&](){...}-like. I had copied out the pattern sampling function, backingMode->sample(samplePoint, Layer::Primary) from another function in my class. Copy-paste-done, works perfectly, moving on. Except.

I'd copied it into my constructor. The way precedence works, backingMode is looked up first in function args, then in class members. So when I moved the line of code up to the constructor, inside the lambda function, it neatly captured a reference to the constructor parameter instead of the class member function which was being used previously. And this worked fine, until I actually went and looked up a member variable in the lambda which was run after the function returned. A few seconds later, the memory was overwritten with some value that pointed into the zero-page, and I get the error.

It could have been hours. We got lucky this time. 

Anyway, I just learned how addr2line works in Linux, so that's useful going forward. I'm... new to this yet. Another entry in the #ravelights development ongoing saga