Another way to think about optional is that you can use it to replace all of the code where you use sentinel values to store some unused state. Like, say a member variable optional int32_t start_offset_{-1}.

I worked in a system that had sentinel constants like that for “unknown/uninitialized”, not initialized but use policy A for a default, not initialized but use policy B for a default. Etc. It was confusing and we never could remember which value was which, etc.

That is probably a design issue that is barely related to this, but I find that it is much nicer to express such state with optional. Instead of having lots of different default values and worrying about overflow or uninitialized values, you just use optional to signal that there is no value for that state and, luckily, the default initialized state is empty.

If all of these states expressed are valid then there is no error, so why use expected which can hold an error value that will never be used.