"I work for a software development house that creates business software, maintaining legacy MFC applications," Graf writes. "We recently received an issue where a filter-toggle wouldn't switch back and forth, never changing from its default value. It's was a small utility function, rarely used, so we were a bit surprised to see it come up. Taking a quick glance at the code revealed the following:

#define ENABLE_FILTER void CReport::ToggleFilter() { #if defined(ENABLE_FILTER) #undef ENABLE_FILTER #else #define ENABLE_FILTER #endif } bool CReport::IsFilterEnabled() { #if defined(ENABLE_FILTER) return true; #else return false; #endif }