A null string contains zero characters or only Spaces or tabs.
空字符串包含零个字符或只有空格或制表符。
To let users select the file to import, specify a null string for filename.
为了让用户选择要导入的文件,文件名指定一个空字符串。
Both null and undefined macros expand to null strings, but a macro defined as a null string is considered defined in preprocessing expressions.
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。
I already used this technique above in the split call, which matched the null string (it matches everywhere, so all the characters in the matched string are returned individually).