15 Preprocessing directives [cpp]

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 called preprocessing, because conceptually they occur before translation of the resulting translation unit.

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

(2.1)
a # preprocessing token, or
(2.2)
an import preprocessing token immediately followed on the same logical source line by a header-name, <, identifier, or : preprocessing token, or
(2.3)
a module preprocessing token immediately followed on the same logical source line by an identifier, :, or ; preprocessing token, or
(2.4)
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.117

[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: # / preprocessing directive module ; / preprocessing directive export module leftpad; / preprocessing directive import <string>; / preprocessing directive export import "squee"; / preprocessing directive import rightpad; / preprocessing directive import :part; / preprocessing directive module / not a preprocessing directive ; / not a preprocessing directive export / not a preprocessing directive import / not a preprocessing directive foo; / not a preprocessing directive export / not a preprocessing directive import foo; / preprocessing directive (ill-formed at phase 7) import :: / not a preprocessing directive import -> / not a preprocessing directive — end example]

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

[Example 2: using module = int; module i; / not a text-line and not a control-line int foo() { return i; }

The example is not a valid preprocessing-file.

— end example]

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

A conditionally-supported-directive is conditionally-supported with implementation-defined semantics.

If one of the pp-tokens of a #embed directive (before macro replacement) is the identifier limit, prefix, suffix, or if_empty and that identifier is defined as a macro ([cpp.replace.general]), the program is ill-formed.

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 U+0020 space and U+0009 character tabulation (including spaces that have replaced comments or possibly other whitespace characters in translation phase 3).

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 EMPTY has been replaced.

— end example]

117)117)

Thus, preprocessing directives are commonly called “lines”.

These “lines” have no other syntactic significance, as all whitespace is equivalent except in certain situations during preprocessing (see the # character string literal creation operator in [cpp.stringize], for example).