Hah, you learn something new everyday.

In c++, variables can be changed by passing them to functions, thanks to the reference notation.

I checked the validation.cpp file and could only see one place where there is a change to the variable.

It is also changed using this code.


ReadLastBlockFile takes int& nFile as its input.

I thought references like that were inherently constants.

I think this effect is due to the changes to blockstorage.cpp (including some refactoring away from validation.cpp).  The call to ReadLastBlockFile doesn't change the nLastBlockFile global variable.  It just targets a local variable in the function.

This means the global variable stays at its default of zero.

This is what allows the node to write to previously completed blk*.dat files rather than only looking at new blk files.

In the Bitcoin Core client, the call to ReadLastBlockFile has a side effect that initializes the nLastBlockFile variable.

This is a Bitcoin Unlimited only behavior, I think, since Bitcoin Core and Bitcoin ABC haven't made the changes.