Although KJ style prefers exceptions, the library is carefully written (and tested) to work with -fno-exceptions. KJ_ASSERT/KJ_REQUIRE invocations (which normally throw exceptions) can define a fallback path to use in the case that exceptions are disabled, usually involving returning some sort of dummy value. You can also register a per-thread exception callback which will be called on all exceptions before the fallback starts (or before aborting, if there is no fallback).