411 REFERENCE · Wikibooks
C++ Language/Preprocessing/ConditionalCompilation
Reference material by Wikibooks contributors. This retains its source identity and is not 411 news reporting.
The traditional way to conditionally-compile code was to use #ifdef or #if.
But the preprocessor cannot reason about types, so modern code sometimes uses if constexpr instead.
Additional information about conditional-compilation (includes interactive examples)