5 Lexical conventions [lex]

5.4 Preprocessing tokens [lex.pptoken]

1
#
Each preprocessing token that is converted to a token ([lex.token]) shall have the lexical form of a keyword, an identifier, a literal, or an operator or punctuator.
2
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A preprocessing token is the minimal lexical element of the language in translation phases 3 through 6.
In this document, glyphs are used to identify elements of the basic character set ([lex.charset]).
The categories of preprocessing token are: header names, placeholder tokens produced by preprocessing import and module directives (import-keyword, module-keyword, and export-keyword), identifiers, preprocessing numbers, character literals (including user-defined character literals), string literals (including user-defined string literals), preprocessing operators and punctuators, and single non-whitespace characters that do not lexically match the other preprocessing token categories.
If a U+0027 apostrophe or a U+0022 quotation mark character matches the last category, the program is ill-formed.
If any character not in the basic character set matches the last category, the program is ill-formed.
Preprocessing tokens can be separated by whitespace; this consists of comments ([lex.comment]), or whitespace characters (U+0020 space, U+0009 character tabulation, new-line, U+000b line tabulation, and U+000c form feed), or both.
As described in [cpp], in certain circumstances during translation phase 4, whitespace (or the absence thereof) serves as more than preprocessing token separation.
Whitespace can appear within a preprocessing token only as part of a header name or between the quotation characters in a character literal or string literal.
3
#
If the input stream has been parsed into preprocessing tokens up to a given character:
  • (3.1)
    If the next character begins a sequence of characters that could be the prefix and initial double quote of a raw string literal, such as R", the next preprocessing token shall be a raw string literal.
    Between the initial and final double quote characters of the raw string, any transformations performed in phase 2 (line splicing) are reverted; this reversion shall apply before any d-char, r-char, or delimiting parenthesis is identified.
    The raw string literal is defined as the shortest sequence of characters that matches the raw-string pattern
    encoding-prefix R raw-string
  • (3.2)
    Otherwise, if the next three characters are <​::​ and the subsequent character is neither : nor >, the < is treated as a preprocessing token by itself and not as the first character of the alternative token <:.
  • (3.3)
    Otherwise, the next preprocessing token is the longest sequence of characters that could constitute a preprocessing token, even if that would cause further lexical analysis to fail, except that a header-name ([lex.header]) is only formed
[Example 1: #define R "x" const char* s = R"y"; // ill-formed raw string, not "x" "y" — end example]
4
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The import-keyword is produced by processing an import directive ([cpp.import]), the module-keyword is produced by preprocessing a module directive ([cpp.module]), and the export-keyword is produced by preprocessing either of the previous two directives.
[Note 1: 
None has any observable spelling.
— end note]
5
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[Example 2: 
The program fragment 0xe+foo is parsed as a preprocessing number token (one that is not a valid integer-literal or floating-point-literal token), even though a parse as three preprocessing tokens 0xe, +, and foo can produce a valid expression (for example, if foo is a macro defined as 1).
Similarly, the program fragment 1E1 is parsed as a preprocessing number (one that is a valid floating-point-literal token), whether or not E is a macro name.
— end example]
6
#
[Example 3: 
The program fragment x+++++y is parsed as x ++ ++ + y, which, if x and y have integral types, violates a constraint on increment operators, even though the parse x ++ + ++ y can yield a correct expression.
— end example]