[virgilp]

TBH, that depends HIGHLY on the context. If I'm not familiar with Lua, do you not hire me? I'd answer "flex and bison" because they're (for me) extremely easy to work with, rest on solid theoretical foundations, have great tools for checking & understanding you grammars, and produce blazingly fast parsers - probably orders of magnitude faster than whatever you do in Lua.

Oh, and the parser code is C, so it integrates perfectly.

[spc476]

I would accept that (maybe with a follow up question if the code generated by flex and bison is re-entrant; I know that the older lex and yacc aren't). I also would accept "I would first look for an existing library that can parse SIP messages in C or C++." I might seriously question you if you answered "I would write code to parse SIP messages in C or C++".

And for the record, flex and bison might be faster than Lua and LPeg, but so far, it hasn't been an issue.

[virgilp]

> if the code generated by flex and bison is re-entrant

This has been fixed long ago, you can definitely get reentrant parsers if you need to. Also, I don't know the SIP grammar, but fwiw, I'm able to write an extremely clean recursive descent parser in C++ (1); for simple grammars, it's often an overkill to go the flex/bison route. Also by "writing code to parse messages in C++" someone familiar with boost might be thinking about using boost::spirit. That's not necessarily a bad choice.

(1) Just looked - it's in fact too complex for that; but the point is, the right tools are dependent on the skillset of the person who answers, which may be different from your own/ your team's. Just because you find writing parser by hand daunting, doesn't mean that I do (I taught compiler design at the university, worked on a commercial C++ compiler, wrote many parsers).

Your interview question is good - but the key to it is the "why" part, not the answer of "how would you do it". And it's simply a different sort of question - your question tests the mindset + domain knowledge; The "reverse a linked list" question aims at filtering-out the developers that are not worth loosing time with. A developer that can't reverse a linked list is completely useless in a C++ project (in fact, his contribution is likely to be negative - doing more damage than adding value).