2 Lexical conventions [lex]

2.2 Phases of translation [lex.phases]

The precedence among the syntax rules of translation is specified by the following phases.11

  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. The set of physical source file characters accepted is implementation-defined. 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 except where this replacement is reverted in a raw string literal.)

  2. Each instance of a backslash character (\) immediately followed by a new-line character is deleted, splicing physical source lines to form logical source lines. Only the last backslash on any physical source line shall be eligible for being part of such a splice. If, as a result, a character sequence that matches the syntax of a universal-character-name is produced, the behavior is undefined. A source file that is not empty and that does not end in a new-line character, or that ends in a new-line character immediately preceded by a backslash character before any such splicing takes place, shall be processed as if an additional new-line character were appended to the file.

  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 in a partial comment.12 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 unspecified. 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.  — end example ]

  4. Preprocessing directives are executed, macro invocations are expanded, and _Pragma unary operator expressions are executed. 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. All preprocessing directives are then deleted.

  5. Each source character set member in a character literal or a string literal, as well as each escape sequence and universal-character-name in a character literal or a non-raw string literal, is converted to the corresponding member of the execution character set ([lex.ccon], [lex.string]); if there is no corresponding member, it is converted to an implementation-defined member other than the null (wide) character.13

  6. Adjacent 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 as a translation unit. [ Note: The process of analyzing and translating the tokens may occasionally result in one token being replaced by a sequence of other tokens ([temp.names]). — end note ] [ 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.  — end note ]

  8. Translated translation units and instantiation units are combined as follows: [ Note: Some or all of these may be supplied from a library.  — end note ] 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]).  — end note ] 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.  — end note ] 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.  — end note ] The program is ill-formed if any instantiation fails.

  9. All external entity references are resolved. Library components are linked to satisfy external references to entities 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.

Implementations must behave as if these separate phases occur, although in practice different phases might be folded together.

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.

An implementation need not convert all non-corresponding source characters to the same execution character.