Better example: #define FLAG_63 (1ULL << 63) long long flags = FLAG_63; In this case, if (flags & FLAG_63) pass(); will pass, but typedef int BOOL;

Better example: #define FLAG_63 (1ULL << 63) long long flags = FLAG_63; In this case, if (flags & FLAG_63) pass(); will pass, but typedef int BOOL; BOOL set = flags & FLAG_63; if (set) pass(); won't pass, due to truncation. Question: Would you argue that a datatype that holds the smallest (1-bit) datum should be as wide as the largest integer type just to handle such cases? If so, that would be highly inefficient for storage purposes. Note that Win32 has 32-bit BOOL type, but internally NT uses 8-bit BOOLEAN type to store bools in structures.