仅当作为头名称的一部分，或在字符文字量或字符串文字量的引号字符之间时，空白才可以出现在预处理标记内部。
White space can appear within a preprocessing token only as a part of a header name or between the quotation characters in a character literal or string literal.
文字量包括字符串，字符和数字文字量。
Literals include strings and character and numeric literals.
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