空宏和未定义的宏都展开为空字符串，但定义为空字符串的宏被视为是在预处理表达式中定义的。

Both null and undefined macros expand to null strings, but a macro defined as a null string is considered defined in preprocessing expressions.

youdao

空宏和未定义的宏都展开为空字符串，但定义为空字符串的宏被视为是在预处理表达式中定义的。

Both null and undefined macros expand to null strings, but a macro defined as a null string is considered defined in preprocessing expressions.

youdao