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 (17.4.1.2) and source files included (16.2) via the preprocessing directive #include, less any source lines skipped by any of the conditional inclusion (16.1) 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 (3.5) 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. (3.5). ]
1 The precedence among the syntax rules of translation is specified by the following phases.13)
13) Implementations must behave as if these separate phases occur, although in practice different phases might be folded together. [back to text]14) 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. [back to text]
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:15)
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.
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.
15) 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. [back to text]
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 1.
| Table 1---trigraph sequences |
_ __________________________________________________________________
_ trigraph replacement trigraph replacement trigraph replacement
_ __________________________________________________________________
__________________________________________________________________
_ ??= # ??( [ ??< {
__________________________________________________________________
_ ??/ \ ??) ] ??> }
__________________________________________________________________
_ ??' ^ ??! | ??- ~
__________________________________________________________________
|
2 [Example:
becomes??=define arraycheck(a,b) a??(b??) ??!??! b??(a??)
---end example]#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.
1 Each preprocessing token that is converted to a token (2.6) 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 (2.7), or white-space characters (space, horizontal tab, new-line, vertical tab, and form-feed), or both. As described in clause 16, 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. ]
1 Alternative token representations are provided for some operators and punctuators16).
2 In all respects of the language, each alternative token behaves the same, respectively, as its primary token, except for its spelling17). The set of alternative tokens is defined in Table 2.
| Table 2---alternative tokens |
_ ______________________________________________________________
_ alternative primary alternative primary alternative primary
_ ______________________________________________________________
______________________________________________________________
_ <% { and && and_eq &=
______________________________________________________________
_ %> } bitor | or_eq |=
______________________________________________________________
_ <: [ or || xor_eq ^=
______________________________________________________________
_ :> ] xor ^ not !
______________________________________________________________
_ %: # compl ~ not_eq !=
______________________________________________________________
_ %:%: ## bitand &
______________________________________________________________
|
16) 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''. [back to text]17) Thus the ``stringized'' values (16.3.2) of [ and <: will be different, maintaining the source spelling, but the tokens can otherwise be freely interchanged. [back to text]
1 There are five kinds of tokens: identifiers, keywords, literals,18) operators, and other separators. Blanks, horizontal and vertical tabs, newlines, formfeeds, and comments (collectively, ``white space''), as described below, are ignored except as they serve to separate tokens. [Note: Some white space is required to separate otherwise adjacent identifiers, keywords, numeric literals, and alternative tokens containing alphabetic characters. ]
18) Literals include strings and character and numeric literals. [back to text]
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 formfeed 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. ]
1 Header name preprocessing tokens shall only appear within a #include preprocessing directive (16.2). 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 16.2.
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.19)
19) Thus, sequences of characters that resemble escape sequences cause undefined behavior. [back to text]
1 Preprocessing number tokens lexically include all integral literal tokens (2.13.1) and all floating literal tokens (2.13.3).
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, 2.1) to an integral literal token or a floating literal token.
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 E. Upper- and lower-case letters are different. All characters are significant.20)
2 In addition, some identifiers are reserved for use by C++ implementations and standard libraries (17.4.3.1.2) and shall not be used otherwise; no diagnostic is required.
20) 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. [back to text]
1 The identifiers shown in Table 3 are reserved for use as keywords (that is, they are unconditionally treated as keywords in phase 7):
| Table 3---keywords |
_ __________________________________________________________________________________ asm do if return typedef auto double inline short typeid bool dynamic_cast int signed typename break else long sizeof union case enum mutable static unsigned catch explicit namespace static_cast using char export new struct virtual class extern operator switch void const false private template volatile const_cast float protected this wchar_t continue for public throw while default friend register true _ delete goto reinterpret_cast try __________________________________________________________________________________ |
2 Furthermore, the alternative representations shown in Table 4 for certain operators and punctuators (2.5) are reserved and shall not be used otherwise:
| Table 4---alternative representations |
_ _______________________________________________________ and and_eq bitand bitor compl not _ not_eq or or_eq xor xor_eq _______________________________________________________ |
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.21)
21) The term ``literal'' generally designates, in this International Standard, those tokens that are called ``constants'' in ISO C. [back to text]
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.22) A hexadecimal integer literal (base sixteen) begins with 0x or 0X and consists of a sequence of hexadecimal digits, which include the decimal digits and the letters a through f and A through F with decimal values ten through fifteen. [Example: the number twelve can be written 12, 014, or 0XC. ]
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.
22) The digits 8 and 9 are not octal digits. [back to text]
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.23) The value of a wide-character literal containing a single c-char has value equal to the numerical value of the encoding of the c-char in the execution wide-character set. The value of a wide-character literal containing multiple c-chars is implementation-defined.
3 Certain nongraphic characters, the single quote ', the double quote ", the question mark ?, and the backslash \, can be represented according to Table 5.
| Table 5---escape sequences |
_ ______________________________ new-line NL (LF) \n horizontal tab HT \t vertical tab VT \v backspace BS \b carriage return CR \r form feed FF \f alert BEL \a backslash \ \\ question mark ? \? single quote ' \' double quote " \" octal number ooo \ooo _ hex number hhh \xhhh ______________________________ |
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 implementationdefined 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. ]
23) They are intended for character sets where a character does not fit into a single byte. [back to text]
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.
1 A string literal is a sequence of characters (as defined in 2.13.2) surrounded by double quotes, optionally beginning with the letter L, as in "..." or L"...". A string literal that does not begin with L is an ordinary string literal, also referred to as a narrow string literal. An ordinary string literal has type ``array of n const char'' and static storage duration (3.7), where n is the size of the string as defined below, and is initialized with the given characters. A string literal that begins with L, such as L"asdf", is a wide string literal. A wide string literal has type ``array of n const wchar_t'' and has static storage duration, where n is the size of the string as defined below, and is initialized with the given characters.
2 Whether all string literals are distinct (that is, are stored in nonoverlapping objects) is implementationdefined. The effect of attempting to modify a string literal is undefined.
3 In translation phase 6 (2.1), adjacent narrow string literals are concatenated and adjacent wide string literals are concatenated. If a narrow string literal token is adjacent to a wide string literal token, the behavior is undefined. Characters in concatenated strings are kept distinct. [Example:
contains the two characters '\xA' and 'B' after concatenation (and not the single hexadecimal character '\xAB'). ]"\xA" "B"
4 After any necessary concatenation, in translation phase 7 (2.1), '\0' is appended to every string literal so that programs that scan a string can find its end.
5 Escape sequences and universal-character-names in string literals have the same meaning as in character literals (2.13.2), except that the single quote ' is representable either by itself or by the escape sequence \', and the double quote " shall be preceded by a \. In a narrow string literal, a universal-character-name may map to more than one char element due to multibyte encoding. The size of a wide string literal is the total number of escape sequences, universal-character-names, and other characters, plus one for the terminating L'\0'. The size of a narrow string literal is the total number of escape sequences and other characters, plus at least one for the multibyte encoding of each universal-character-name, plus one for the terminating '\0'.
1 The Boolean literals are the keywords false and true. Such literals have type bool. They are not lvalues.