[cpp.pre] (original) (raw)

15 Preprocessing directives [cpp]

15.1 Preamble [cpp.pre]

pp-private-module-fragment:
module : private ; new-line group

lparen:
a ( character not immediately preceded by whitespace

pp-balanced-token:
( pp-balanced-token-seq )
[ pp-balanced-token-seq ]
{ pp-balanced-token-seq }
any pp-token except:
parenthesis (U+0028 left parenthesis and U+0029 right parenthesis),
bracket (U+005b left square bracket and U+005d right square bracket), or
brace (U+007b left curly bracket and U+007d right curly bracket).

new-line:
the new-line character

A preprocessing directive consists of a sequence of preprocessing tokens that satisfies the following constraints: At the start of translation phase 4, the first preprocessing token in the sequence, referred to as a directive-introducing token, begins with the first character in the source file (optionally after whitespace containing no new-line characters) or follows whitespace containing at least one new-line character, and is

• a # preprocessing token, or
• an import preprocessing token immediately followed on the same logical source line by a,<,identifier,string-literal, or:preprocessing token, or
• a module preprocessing token immediately followed on the same logical source line by anidentifier,:, or;preprocessing token, or
• an export preprocessing token immediately followed on the same logical source line by one of the two preceding forms.

The last preprocessing token in the sequence is the first preprocessing token within the sequence that is immediately followed by whitespace containing a new-line character.122

[Note 1:

A new-line character ends the preprocessing directive even if it occurs within what would otherwise be an invocation of a function-like macro.

— _end note_]

[Example 1: # module ; export module leftpad; import <string>; export import "squee"; import rightpad; import :part; module; export importfoo; export import foo; import :: import -> — _end example_]

A sequence of preprocessing tokens is only a text-lineif it does not begin with a directive-introducing token.

[Example 2: using module = int;module i; int foo() { return i;}

The example is not a valid preprocessing-file.

— _end example_]

A sequence of preprocessing tokens is only a conditionally-supported-directiveif it does not begin with any of the directive names appearing after a # in the syntax.

A conditionally-supported-directive is conditionally-supported withimplementation-defined semantics.

Any embed-prefixed-parameter is conditionally-supported, with implementation-defined semantics.

At the start of phase 4 of translation, the group of a pp-global-module-fragment shall contain neither a text-line nor a pp-import.

When in a group that is skipped ([cpp.cond]), the directive syntax is relaxed to allow any sequence of preprocessing tokens to occur between the directive name and the following new-line character.

The only whitespace characters that shall appear between preprocessing tokens within a preprocessing directive (from just after the directive-introducing token through just before the terminating new-line character) are space and horizontal-tab (including spaces that have replaced comments or possibly other whitespace characters in translation phase 3).

The implementation can process and skip sections of source files conditionally, include other source files, import macros from header units, and replace macros.

These capabilities are calledpreprocessing, because conceptually they occur before translation of the resulting translation unit.

The preprocessing tokens within a preprocessing directive are not subject to macro expansion unless otherwise stated.

[Example 3:

In:#define EMPTY EMPTY # include <file.h> the sequence of preprocessing tokens on the second line is _not_a preprocessing directive, because it does not begin with a # at the start of translation phase 4, even though it will do so after the macro EMPTYhas been replaced.

— _end example_]