2 - Lexical conventions [lex]

-1- The text of the program is kept in units called source files in this International Standard. A source file together with all the headers (lib.headers) and source files included (cpp.include) via the preprocessing directive #include, less any source lines skipped by any of the conditional inclusion (cpp.cond) preprocessing directives, is called a translation unit. [Note: a C++ program need not all be translated at the same time. ]

-2- [Note: previously translated translation units and instantiation units can be preserved individually or in libraries. The separate translation units of a program communicate (basic.link) by (for example) calls to functions whose identifiers have external linkage, manipulation of objects whose identifiers have external linkage, or manipulation of data files. Translation units can be separately translated and then later linked to produce an executable program. (basic.link). ]

2.1 - Phases of translation [lex.phases]

-1- The precedence among the syntax rules of translation is specified by the following phases.*

[Footnote: Implementations must behave as if these separate phases occur, although in practice different phases might be folded together. --- end foonote]

1. Physical source file characters are mapped, in an implementation-defined manner, to the basic source character set (introducing new-line characters for end-of-line indicators) if necessary. Trigraph sequences (lex.trigraph) are replaced by corresponding single-character internal representations. Any source file character not in the basic source character set (lex.charset) is replaced by the universal-character-name that designates that character. (An implementation may use any internal encoding, so long as an actual extended character encountered in the source file, and the same extended character expressed in the source file as a universal-character-name (i.e. using the \uXXXX notation), are handled equivalently.)

2. Each instance of a new-line character and an immediately preceding backslash character is deleted, splicing physical source lines to form logical source lines. If, as a result, a character sequence that matches the syntax of a universal-character-name is produced, the behavior is undefined. If a source file that is not empty does not end in a new-line character, or ends in a new-line character immediately preceded by a backslash character, the behavior is undefined.

3. The source file is decomposed into preprocessing tokens (lex.pptoken) and sequences of white-space characters (including comments). A source file shall not end in a partial preprocessing token or partial comment*.

   [Footnote: A partial preprocessing token would arise from a source file ending in the first portion of a multi-character token that requires a terminating sequence of characters, such as a header-name that is missing the closing " or >. A partial comment would arise from a source file ending with an unclosed /* comment. --- end foonote]

   Each comment is replaced by one space character. New-line characters are retained. Whether each nonempty sequence of white-space characters other than new-line is retained or replaced by one space character is implementation-defined. The process of dividing a source file's characters into preprocessing tokens is context-dependent. [Example: see the handling of < within a #include preprocessing directive. ]

4. Preprocessing directives are executed and macro invocations are expanded. If a character sequence that matches the syntax of a universal-character-name is produced by token concatenation (cpp.concat), the behavior is undefined. A #include preprocessing directive causes the named header or source file to be processed from phase 1 through phase 4, recursively.

5. Each source character set member, escape sequence, or universal-character-name in character literals and string literals is converted to a member of the execution character set (lex.ccon, lex.string).

6. Adjacent ordinary string literal tokens are concatenated. Adjacent wide string literal tokens are concatenated.

7. White-space characters separating tokens are no longer significant. Each preprocessing token is converted into a token. (lex.token). The resulting tokens are syntactically and semantically analyzed and translated. [Note: Source files, translation units and translated translation units need not necessarily be stored as files, nor need there be any one-to-one correspondence between these entities and any external representation. The description is conceptual only, and does not specify any particular implementation. ]

8. Translated translation units and instantiation units are combined as follows: [Note: some or all of these may be supplied from a library. ] Each translated translation unit is examined to produce a list of required instantiations. [Note: this may include instantiations which have been explicitly requested (temp.explicit). ] The definitions of the required templates are located. It is implementation-defined whether the source of the translation units containing these definitions is required to be available. [Note: an implementation could encode sufficient information into the translated translation unit so as to ensure the source is not required here. ] All the required instantiations are performed to produce instantiation units. [Note: these are similar to translated translation units, but contain no references to uninstantiated templates and no template definitions. ] The program is ill-formed if any instantiation fails.

9. All external object and function references are resolved. Library components are linked to satisfy external references to functions and objects not defined in the current translation. All such translator output is collected into a program image which contains information needed for execution in its execution environment.

-1- The basic source character set consists of 96 characters: the space character, the control characters representing horizontal tab, vertical tab, form feed, and new-line, plus the following 91 graphical characters:*

[Footnote: The glyphs for the members of the basic source character set are intended to identify characters from the subset of ISO/IEC 10646 which corresponds to the ASCII character set. However, because the mapping from source file characters to the source character set (described in translation phase 1) is specified as implementation-defined, an implementation is required to document how the basic source characters are represented in source files. --- end foonote]

a b c d e f g h i j k l m n o p q r s t u v w x y z
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
0 1 2 3 4 5 6 7 8 9
_ { } [ ] # ( ) < > % : ; . ? * + - / ^ & | ~ ! = , \ " '

-2- The universal-character-name construct provides a way to name other characters.

hex-quad:
	hexadecimal-digit hexadecimal-digit hexadecimal-digit hexadecimal-digit

universal-character-name:
	\u hex-quad
	\U hex-quad hex-quad

-3- The basic execution character set and the basic execution wide-character set shall each contain all the members of the basic source character set, plus control characters representing alert, backspace, and carriage return, plus a null character (respectively, null wide character), whose representation has all zero bits. For each basic execution character set, the values of the members shall be non-negative and distinct from one another. The execution character set and the execution wide-character set are supersets of the basic execution character set and the basic execution wide-character set, respectively. The values of the members of the execution character sets are implementation-defined, and any additional members are locale-specific.

2.3 - Trigraph sequences [lex.trigraph]

-1- Before any other processing takes place, each occurrence of one of the following sequences of three characters (``trigraph sequences'') is replaced by the single character indicated in Table ??.

-2- [Example:

??=define arraycheck(a,b) a??(b??) ??!??! b??(a??)

#define arraycheck(a,b) a[b] || b[a]

-3- No other trigraph sequence exists. Each ? that does not begin one of the trigraphs listed above is not changed.

2.4 - Preprocessing tokens [lex.pptoken]

preprocessing-token:
	header-name
	identifier
	pp-number
	character-literal
	string-literal
	preprocessing-op-or-punc
	each non-white-space character that cannot be one of the above

-1- Each preprocessing token that is converted to a token (lex.token) shall have the lexical form of a keyword, an identifier, a literal, an operator, or a punctuator.

-2- A preprocessing token is the minimal lexical element of the language in translation phases 3 through 6. The categories of preprocessing token are: header names, identifiers, preprocessing numbers, character literals, string literals, preprocessing-op-or-punc, and single non-white-space characters that do not lexically match the other preprocessing token categories. If a ' or a " character matches the last category, the behavior is undefined. Preprocessing tokens can be separated by white space; this consists of comments (lex.comment), or white-space characters (space, horizontal tab, new-line, vertical tab, and form-feed), or both. As described in clause cpp, in certain circumstances during translation phase 4, white space (or the absence thereof) serves as more than preprocessing token separation. White space 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, 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.

-4- [Example: The program fragment 1Ex is parsed as a preprocessing number token (one that is not a valid floating or integer literal token), even though a parse as the pair of preprocessing tokens 1 and Ex might produce a valid expression (for example, if Ex were a macro defined as +1). Similarly, the program fragment 1E1 is parsed as a preprocessing number (one that is a valid floating literal token), whether or not E is a macro name. ]

-5- [Example: The program fragment x+++++y is parsed as x ++ ++ + y, which, if x and y are of built-in types, violates a constraint on increment operators, even though the parse x ++ + ++ y might yield a correct expression. ]

2.5 - Alternative tokens [lex.digraph]

-1- Alternative token representations are provided for some operators and punctuators*.

[Footnote: These include ``digraphs'' and additional reserved words. The term ``digraph'' (token consisting of two characters) is not perfectly descriptive, since one of the alternative preprocessing-tokens is %:%: and of course several primary tokens contain two characters. Nonetheless, those alternative tokens that aren't lexical keywords are colloquially known as ``digraphs''. --- end foonote]

-2- In all respects of the language, each alternative token behaves the same, respectively, as its primary token, except for its spelling*.

[Footnote: Thus the ``stringized'' values (cpp.stringize) of [ and <: will be different, maintaining the source spelling, but the tokens can otherwise be freely interchanged. --- end foonote]

2.6 - Tokens [lex.token]

token:
	identifier
	keyword
	literal
	operator
	punctuator

-1- There are five kinds of tokens: identifiers, keywords, literals,*

[Footnote: Literals include strings and character and numeric literals. --- end foonote]

-1- The characters /* start a comment, which terminates with the characters */. These comments do not nest. The characters // start a comment, which terminates with the next new-line character. If there is a form-feed or a vertical-tab character in such a comment, only white-space characters shall appear between it and the new-line that terminates the comment; no diagnostic is required. [Note: The comment characters //, /*, and */ have no special meaning within a // comment and are treated just like other characters. Similarly, the comment characters // and /* have no special meaning within a /* comment. ]

2.8 - Header names [lex.header]

header-name:
	<h-char-sequence>
	"q-char-sequence"

h-char-sequence:
	h-char
	h-char-sequence h-char

h-char:
	any member of the source character set except
		new-line and >

q-char-sequence:
	q-char
	q-char-sequence q-char

q-char:
	any member of the source character set except
		new-line and "

-1- Header name preprocessing tokens shall only appear within a #include preprocessing directive (cpp.include). The sequences in both forms of header-names are mapped in an implementation-defined manner to headers or to external source file names as specified in cpp.include.

-2- If either of the characters ' or \, or either of the character sequences /* or // appears in a q-char-sequence or a h-char-sequence, or the character " appears in a h-char-sequence, the behavior is undefined.*

[Footnote: Thus, sequences of characters that resemble escape sequences cause undefined behavior. --- end foonote]

pp-number:
	digit
	. digit
	pp-number digit
	pp-number nondigit
	pp-number e sign
	pp-number E sign
	pp-number .

-1- Preprocessing number tokens lexically include all integral literal tokens (lex.icon) and all floating literal tokens (lex.fcon).

-2- A preprocessing number does not have a type or a value; it acquires both after a successful conversion (as part of translation phase 7, lex.phases) to an integral literal token or a floating literal token.

2.10 - Identifiers [lex.name]

identifier:
	nondigit
	identifier nondigit
	identifier digit

nondigit: one of
	universal-character-name
	_ a b c d e f g h i j k l m
	  n o p q r s t u v w x y z
	  A B C D E F G H I J K L M
	  N O P Q R S T U V W X Y Z

digit: one of
	0 1 2 3 4 5 6 7 8 9

-1- An identifier is an arbitrarily long sequence of letters and digits. Each universal-character-name in an identifier shall designate a character whose encoding in ISO 10646 falls into one of the ranges specified in Annex extendid. Upper- and lower-case letters are different. All characters are significant.*

[Footnote: On systems in which linkers cannot accept extended characters, an encoding of the universal-character-name may be used in forming valid external identifiers. For example, some otherwise unused character or sequence of characters may be used to encode the \u in a universal-character-name. Extended characters may produce a long external identifier, but C++ does not place a translation limit on significant characters for external identifiers. In C++, upper- and lower-case letters are considered different for all identifiers, including external identifiers. --- end foonote]

-2- In addition, some identifiers are reserved for use by C++ implementations and standard libraries (lib.global.names) and shall not be used otherwise; no diagnostic is required.

2.11 - Keywords [lex.key]

-1- The identifiers shown in Table ?? are reserved for use as keywords (that is, they are unconditionally treated as keywords in phase 7):

-2- Furthermore, the alternative representations shown in Table ?? for certain operators and punctuators (lex.digraph) are reserved and shall not be used otherwise:

2.12 - Operators and punctuators [lex.operators]

-1- The lexical representation of C++ programs includes a number of preprocessing tokens which are used in the syntax of the preprocessor or are converted into tokens for operators and punctuators:

preprocessing-op-or-punc: one of
	{	}	[	]	#	##	(	)
	<:	:>	<%	%>	%:	%:%:	;	:	...
	new	delete	?	::	.	.*
	+	-	*	/	%	^	&	|	~
	!	=	<	>	+=	-=	*=	/=	%=
	^=	&=	|=	<<	>>	>>=	<<=	==	!=
	<=	>=	&&	||	++	--	,	->*	->
	and	and_eq	bitand	bitor	compl	not	not_eq
	or	or_eq	xor	xor_eq

-1- There are several kinds of literals.*

[Footnote: The term ``literal'' generally designates, in this International Standard, those tokens that are called ``constants'' in ISO C. --- end foonote]

literal:
	integer-literal
	character-literal
	floating-literal
	string-literal
	boolean-literal

integer-literal:
	decimal-literal integer-suffixopt
	octal-literal integer-suffixopt
	hexadecimal-literal integer-suffixopt

decimal-literal:
	nonzero-digit
	decimal-literal digit

octal-literal:
	0
	octal-literal octal-digit

hexadecimal-literal:
	0x hexadecimal-digit
	0X hexadecimal-digit
	hexadecimal-literal hexadecimal-digit

nonzero-digit: one of
	1  2  3  4  5  6  7  8  9

octal-digit: one of
	0  1  2  3  4  5  6  7

hexadecimal-digit: one of
	0  1  2  3  4  5  6  7  8  9
	a  b  c  d  e  f
	A  B  C  D  E  F

integer-suffix:
	unsigned-suffix long-suffixopt
	long-suffix unsigned-suffixopt

unsigned-suffix: one of
	u  U

long-suffix: one of
	l  L

-1- An integer literal is a sequence of digits that has no period or exponent part. An integer literal may have a prefix that specifies its base and a suffix that specifies its type. The lexically first digit of the sequence of digits is the most significant. A decimal integer literal (base ten) begins with a digit other than 0 and consists of a sequence of decimal digits. An octal integer literal (base eight) begins with the digit 0 and consists of a sequence of octal digits.*

[Footnote: The digits 8 and 9 are not octal digits. --- end foonote]

-2- The type of an integer literal depends on its form, value, and suffix. If it is decimal and has no suffix, it has the first of these types in which its value can be represented: int, long int; if the value cannot be represented as a long int, the behavior is undefined. If it is octal or hexadecimal and has no suffix, it has the first of these types in which its value can be represented: int, unsigned int, long int, unsigned long int. If it is suffixed by u or U, its type is the first of these types in which its value can be represented: unsigned int, unsigned long int. If it is suffixed by l or L, its type is the first of these types in which its value can be represented: long int, unsigned long int. If it is suffixed by ul, lu, uL, Lu, Ul, lU, UL, or LU, its type is unsigned long int.

-3- A program is ill-formed if one of its translation units contains an integer literal that cannot be represented by any of the allowed types.

2.13.2 - Character literals [lex.ccon]

character-literal:
	'c-char-sequence'
	L'c-char-sequence'

c-char-sequence:
	c-char
	c-char-sequence c-char

c-char:
	any member of the source character set except
		the single-quote  ', backslash  \, or new-line character
	escape-sequence
	universal-character-name

simple-escape-sequence: one of
	\'  \"  \?  \\
	\a  \b  \f  \n  \r  \t  \v

octal-escape-sequence:
	\ octal-digit
	\ octal-digit octal-digit
	\ octal-digit octal-digit octal-digit

hexadecimal-escape-sequence:
	\x hexadecimal-digit
	hexadecimal-escape-sequence hexadecimal-digit

-1- A character literal is one or more characters enclosed in single quotes, as in 'x', optionally preceded by the letter L, as in L'x'. A character literal that does not begin with L is an ordinary character literal, also referred to as a narrow-character literal. An ordinary character literal that contains a single c-char has type char, with value equal to the numerical value of the encoding of the c-char in the execution character set. An ordinary character literal that contains more than one c-char is a multicharacter literal. A multicharacter literal has type int and implementation-defined value.

-2- A character literal that begins with the letter L, such as L'x', is a wide-character literal. A wide-character literal has type wchar_t.*

[Footnote: They are intended for character sets where a character does not fit into a single byte. --- end foonote]

-3- Certain nongraphic characters, the single quote ', the double quote " , the question mark ?, and the backslash \, can be represented according to Table ??.

The double quote " and the question mark ?, can be represented as themselves or by the escape sequences \" and \? respectively, but the single quote ' and the backslash \ shall be represented by the escape sequences \' and \\ respectively. If the character following a backslash is not one of those specified, the behavior is undefined. An escape sequence specifies a single character.

-4- The escape \ooo consists of the backslash followed by one, two, or three octal digits that are taken to specify the value of the desired character. The escape \xhhh consists of the backslash followed by x followed by one or more hexadecimal digits that are taken to specify the value of the desired character. There is no limit to the number of digits in a hexadecimal sequence. A sequence of octal or hexadecimal digits is terminated by the first character that is not an octal digit or a hexadecimal digit, respectively. The value of a character literal is implementation-defined if it falls outside of the implementation-defined range defined for char (for ordinary literals) or wchar_t (for wide literals).

-5- A universal-character-name is translated to the encoding, in the execution character set, of the character named. If there is no such encoding, the universal-character-name is translated to an implementation-defined encoding. [Note: in translation phase 1, a universal-character-name is introduced whenever an actual extended character is encountered in the source text. Therefore, all extended characters are described in terms of universal-character-names. However, the actual compiler implementation may use its own native character set, so long as the same results are obtained. ]

2.13.3 - Floating literals [lex.fcon]

floating-literal:
	fractional-constant exponent-partopt floating-suffixopt
	digit-sequence exponent-part floating-suffixopt

fractional-constant:
	digit-sequenceopt . digit-sequence
	digit-sequence .

exponent-part:
	e signopt digit-sequence
	E signopt digit-sequence

sign: one of
	+  -

digit-sequence:
	digit
	digit-sequence digit

floating-suffix: one of
	f  l  F  L

-1- A floating literal consists of an integer part, a decimal point, a fraction part, an e or E, an optionally signed integer exponent, and an optional type suffix. The integer and fraction parts both consist of a sequence of decimal (base ten) digits. Either the integer part or the fraction part (not both) can be omitted; either the decimal point or the letter e (or E) and the exponent (not both) can be omitted. The integer part, the optional decimal point and the optional fraction part form the significant part of the floating literal. The exponent, if present, indicates the power of 10 by which the significant part is to be scaled. If the scaled value is in the range of representable values for its type, the result is the scaled value if representable, else the larger or smaller representable value nearest the scaled value, chosen in an implementation-defined manner. The type of a floating literal is double unless explicitly specified by a suffix. The suffixes f and F specify float, the suffixes l and L specify long double. If the scaled value is not in the range of representable values for its type, the program is ill-formed.

2.13.4 - String literals [lex.string]

string-literal:
	"s-char-sequenceopt"
	L"s-char-sequenceopt"

s-char-sequence:
	s-char
	s-char-sequence s-char

s-char:
	any member of the source character set except
		the double-quote ", backslash \, or new-line character
	escape-sequence
	universal-character-name

boolean-literal:
	false
	true

-1- The Boolean literals are the keywords false and true. Such literals have type bool. They are not lvalues.