Set of Warnings as Error g++

If you just give -Werror all warnings become errors. Aside from listing the ones you (don't) want to make into errors as -W(no-)error=xxx, I don't believe there is a way to "make some warnings into errors".

Of course, one solution might be to compile the code twice - once with -Wall and -Wextra, and once with -Werror, but not -Wall and -Wextra.

In the long term, I'm sure it will be worth the extra effort of marking which errors you (don't) want -Werror to see as errors [although I'd say the better solution is probably to use -Wno-xxx, to disable any warnings that you deem acceptable, as opposed to "warn but don't make it an error" - after all, the purpose of -Werror in my view is to prevent code from being submitted to a project with warnings in it - and that should mean one of two things: the warning is fixed, or the warning is disabled. Whichever makes sense for that project].

Rather than using -Werror=... with a long list of warnings you can use -Werror -Wno-error=... with a much shorter list of warnings (only the ones enabled by -Wall -Wextra). The manual lists which warnings are enabled by -Wall -Wextra so it's a no-brainer.