Thanks, I didn't know (or didn't remember) about this, but it's not clear from the docs that it bounds checks []. I don't find it difficult to use .at(). I'll try the debug mode too, but when I bother writing something in C++ it's usually because I actually care about its speed, so I don't want the overhead to be too bad.

https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_macros...