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%!TEX root = std.tex
\rSec0[cpp]{Preprocessing directives}%
\indextext{preprocessing directive|(}
\indextext{compiler control line|see{preprocessing directive}}%
\indextext{control line|see{preprocessing directive}}%
\indextext{directive, preprocessing|see{preprocessing directive}}
\gramSec[gram.cpp]{Preprocessing directives}
\rSec1[cpp.pre]{Preamble}
\begin{bnf}
\nontermdef{preprocessing-file}\br
\opt{group}\br
module-file
\end{bnf}
\begin{bnf}
\nontermdef{module-file}\br
\opt{pp-global-module-fragment} pp-module \opt{group} \opt{pp-private-module-fragment}
\end{bnf}
\begin{bnf}
\nontermdef{pp-global-module-fragment}\br
\keyword{module} \terminal{;} new-line \opt{group}
\end{bnf}
\begin{bnf}
\nontermdef{pp-private-module-fragment}\br
\keyword{module} \terminal{:} \keyword{private} \terminal{;} new-line \opt{group}
\end{bnf}
\begin{bnf}
\nontermdef{group}\br
group-part\br
group group-part
\end{bnf}
\begin{bnf}
\nontermdef{group-part}\br
control-line\br
if-section\br
text-line\br
\terminal{\#} conditionally-supported-directive
\end{bnf}
\begin{bnf}\obeyspaces
\nontermdef{control-line}\br
\terminal{\# include} pp-tokens new-line\br
pp-import\br
\terminal{\# define } identifier replacement-list new-line\br
\terminal{\# define } identifier lparen \opt{identifier-list} \terminal{)} replacement-list new-line\br
\terminal{\# define } identifier lparen \terminal{... )} replacement-list new-line\br
\terminal{\# define } identifier lparen identifier-list \terminal{, ... )} replacement-list new-line\br
\terminal{\# undef } identifier new-line\br
\terminal{\# line } pp-tokens new-line\br
\terminal{\# error } \opt{pp-tokens} new-line\br
\terminal{\# warning} \opt{pp-tokens} new-line\br
\terminal{\# pragma } \opt{pp-tokens} new-line\br
\terminal{\# }new-line
\end{bnf}
\begin{bnf}
\nontermdef{if-section}\br
if-group \opt{elif-groups} \opt{else-group} endif-line
\end{bnf}
\begin{bnf}\obeyspaces
\nontermdef{if-group}\br
\terminal{\# if } constant-expression new-line \opt{group}\br
\terminal{\# ifdef } identifier new-line \opt{group}\br
\terminal{\# ifndef } identifier new-line \opt{group}
\end{bnf}
\begin{bnf}
\nontermdef{elif-groups}\br
elif-group\br
elif-groups elif-group
\end{bnf}
\begin{bnf}\obeyspaces
\nontermdef{elif-group}\br
\terminal{\# elif } constant-expression new-line \opt{group}\br
\terminal{\# elifdef } constant-expression new-line \opt{group}\br
\terminal{\# elifndef} constant-expression new-line \opt{group}
\end{bnf}
\begin{bnf}\obeyspaces
\nontermdef{else-group}\br
\terminal{\# else } new-line \opt{group}
\end{bnf}
\begin{bnf}\obeyspaces
\nontermdef{endif-line}\br
\terminal{\# endif } new-line
\end{bnf}
\begin{bnf}
\nontermdef{text-line}\br
\opt{pp-tokens} new-line
\end{bnf}
\begin{bnf}
\nontermdef{conditionally-supported-directive}\br
pp-tokens new-line
\end{bnf}
\begin{bnf}
\nontermdef{lparen}\br
\descr{a \terminal{(} character not immediately preceded by whitespace}
\end{bnf}
\begin{bnf}
\nontermdef{identifier-list}\br
identifier\br
identifier-list \terminal{,} identifier
\end{bnf}
\begin{bnf}
\nontermdef{replacement-list}\br
\opt{pp-tokens}
\end{bnf}
\begin{bnf}
\nontermdef{pp-tokens}\br
preprocessing-token\br
pp-tokens preprocessing-token
\end{bnf}
\begin{bnf}
\nontermdef{new-line}\br
\descr{the new-line character}
\end{bnf}
\pnum
A \defn{preprocessing directive} consists of a sequence of preprocessing tokens
that satisfies the following constraints:
At the start of translation phase 4,
the first token in the sequence,
referred to as a \defnadj{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
\begin{itemize}
\item
a \tcode{\#} preprocessing token, or
\item
an \keyword{import} preprocessing token
immediately followed on the same logical line by a
\grammarterm{header-name},
\tcode{<},
\grammarterm{identifier},
\grammarterm{string-literal}, or
\tcode{:}
preprocessing token, or
\item
a \keyword{module} preprocessing token
immediately followed on the same logical line by an
\grammarterm{identifier},
\tcode{:}, or
\tcode{;}
preprocessing token, or
\item
an \keyword{export} preprocessing token
immediately followed on the same logical line by
one of the two preceding forms.
\end{itemize}
The last token in the sequence is the first token within the sequence that
is immediately followed by whitespace containing a new-line character.
\begin{footnote}
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
\tcode{\#}
character string literal creation operator in~\ref{cpp.stringize}, for example).
\end{footnote}
\begin{note}
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}
\begin{example}
\begin{codeblock}
# // 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{codeblock}
\end{example}
\pnum
A sequence of preprocessing tokens is only a \grammarterm{text-line}
if it does not begin with a directive-introducing token.
A sequence of preprocessing tokens is only a \grammarterm{conditionally-supported-directive}
if it does not begin with any of the directive names
appearing after a \tcode{\#} in the syntax.
A \grammarterm{conditionally-supported-directive} is
conditionally-supported with
\impldef{additional supported forms of preprocessing directive}
semantics.
\pnum
At the start of phase 4 of translation,
the \grammarterm{group} of a \grammarterm{pp-global-module-fragment} shall
contain neither a \grammarterm{text-line} nor a \grammarterm{pp-import}.
\pnum
When in a group that is skipped\iref{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.
\pnum
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).
\pnum
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
\defn{preprocessing},
because conceptually they occur
before translation of the resulting translation unit.
\pnum
The preprocessing tokens within a preprocessing directive
are not subject to macro expansion unless otherwise stated.
\begin{example}
In:
\begin{codeblock}
#define EMPTY
EMPTY # include <file.h>
\end{codeblock}
the sequence of preprocessing tokens on the second line is \textit{not}
a preprocessing directive, because it does not begin with a \tcode{\#} at the start of
translation phase 4, even though it will do so after the macro \tcode{EMPTY}
has been replaced.
\end{example}
\rSec1[cpp.cond]{Conditional inclusion}%
\indextext{preprocessing directive!conditional inclusion}%
\indextext{inclusion!conditional|see{preprocessing directive, conditional inclusion}}
\indextext{\idxcode{defined}}%
\begin{bnf}
\nontermdef{defined-macro-expression}\br
\terminal{defined} identifier\br
\terminal{defined (} identifier \terminal{)}
\end{bnf}
\begin{bnf}
\nontermdef{h-preprocessing-token}\br
\textnormal{any \grammarterm{preprocessing-token} other than \terminal{>}}
\end{bnf}
\begin{bnf}
\nontermdef{h-pp-tokens}\br
h-preprocessing-token\br
h-pp-tokens h-preprocessing-token
\end{bnf}
\begin{bnf}
\nontermdef{header-name-tokens}\br
string-literal\br
\terminal{<} h-pp-tokens \terminal{>}
\end{bnf}
\indextext{\idxxname{has_include}}%
\begin{bnf}
\nontermdef{has-include-expression}\br
\terminal{\xname{has_include}} \terminal{(} header-name \terminal{)}\br
\terminal{\xname{has_include}} \terminal{(} header-name-tokens \terminal{)}
\end{bnf}
\indextext{\idxxname{has_cpp_attribute}}%
\begin{bnf}
\nontermdef{has-attribute-expression}\br
\terminal{\xname{has_cpp_attribute} (} pp-tokens \terminal{)}
\end{bnf}
\pnum
The expression that controls conditional inclusion
shall be an integral constant expression except that
identifiers
(including those lexically identical to keywords)
are interpreted as described below
\begin{footnote}
Because the controlling constant expression is evaluated
during translation phase 4,
all identifiers either are or are not macro names ---
there simply are no keywords, enumeration constants, etc.
\end{footnote}
and it may contain zero or more \grammarterm{defined-macro-expression}{s} and/or
\grammarterm{has-include-expression}{s} and/or
\grammarterm{has-attribute-expression}{s} as unary operator expressions.
\pnum
A \grammarterm{defined-macro-expression} evaluates to \tcode{1}
if the identifier is currently defined
as a macro name
(that is, if it is predefined
or if it has one or more active macro definitions\iref{cpp.import},
for example because
it has been the subject of a
\tcode{\#define}
preprocessing directive
without an intervening
\tcode{\#undef}
directive with the same subject identifier), \tcode{0} if it is not.
\pnum
The second form of \grammarterm{has-include-expression}
is considered only if the first form does not match,
in which case the preprocessing tokens are processed just as in normal text.
\pnum
The header or source file identified by
the parenthesized preprocessing token sequence
in each contained \grammarterm{has-include-expression}
is searched for as if that preprocessing token sequence
were the \grammarterm{pp-tokens} in a \tcode{\#include} directive,
except that no further macro expansion is performed.
If such a directive would not satisfy the syntactic requirements
of a \tcode{\#include} directive, the program is ill-formed.
The \grammarterm{has-include-expression} evaluates
to \tcode{1} if the search for the source file succeeds, and
to \tcode{0} if the search fails.
\pnum
Each \grammarterm{has-attribute-expression} is replaced by
a non-zero \grammarterm{pp-number}
matching the form of an \grammarterm{integer-literal}
if the implementation supports an attribute
with the name specified by interpreting
the \grammarterm{pp-tokens}, after macro expansion,
as an \grammarterm{attribute-token},
and by \tcode{0} otherwise.
The program is ill-formed if the \grammarterm{pp-tokens}
do not match the form of an \grammarterm{attribute-token}.
\pnum
For an attribute specified in this document,
the value of the \grammarterm{has-attribute-expression}
is given by \tref{cpp.cond.ha}.
For other attributes recognized by the implementation,
the value is
\impldef{value of \grammarterm{has-attribute-expression}
for non-standard attributes}.
\begin{note}
It is expected
that the availability of an attribute can be detected by any non-zero result.
\end{note}
\begin{floattable}{\xname{has_cpp_attribute} values}{cpp.cond.ha}
{ll}
\topline
\lhdr{Attribute} & \rhdr{Value} \\ \rowsep
\tcode{assume} & \tcode{202207L} \\
\tcode{carries_dependency} & \tcode{200809L} \\
\tcode{deprecated} & \tcode{201309L} \\
\tcode{fallthrough} & \tcode{201603L} \\
\tcode{likely} & \tcode{201803L} \\
\tcode{maybe_unused} & \tcode{201603L} \\
\tcode{no_unique_address} & \tcode{201803L} \\
\tcode{nodiscard} & \tcode{201907L} \\
\tcode{noreturn} & \tcode{200809L} \\
\tcode{unlikely} & \tcode{201803L} \\
\end{floattable}
\pnum
The
\tcode{\#ifdef}, \tcode{\#ifndef}, \tcode{\#elifdef}, and \tcode{\#elifndef}
directives, and
the \tcode{defined} conditional inclusion operator,
shall treat \xname{has_include} and \xname{has_cpp_attribute}
as if they were the names of defined macros.
The identifiers \xname{has_include} and \xname{has_cpp_attribute}
shall not appear in any context not mentioned in this subclause.
\pnum
Each preprocessing token that remains (in the list of preprocessing tokens that
will become the controlling expression)
after all macro replacements have occurred
shall be in the lexical form of a token\iref{lex.token}.
\pnum
Preprocessing directives of the forms
\begin{ncsimplebnf}\obeyspaces
\indextext{\idxcode{\#if}}%
\terminal{\# if } constant-expression new-line \opt{group}\br
\indextext{\idxcode{\#elif}}%
\terminal{\# elif } constant-expression new-line \opt{group}
\end{ncsimplebnf}
check whether the controlling constant expression evaluates to nonzero.
\pnum
Prior to evaluation,
macro invocations in the list of preprocessing tokens
that will become the controlling constant expression
are replaced
(except for those macro names modified by the
\tcode{defined}
unary operator),
just as in normal text.
If the token
\tcode{defined}
is generated as a result of this replacement process
or use of the
\tcode{defined}
unary operator does not match one of the two specified forms
prior to macro replacement,
the behavior is undefined.
\pnum
After all replacements due to macro expansion and
evaluations of
\grammarterm{defined-macro-expression}s,
\grammarterm{has-include-expression}s, and
\grammarterm{has-attribute-expression}s
have been performed,
all remaining identifiers and keywords,
except for
\tcode{true}
and
\tcode{false},
are replaced with the \grammarterm{pp-number}
\tcode{0},
and then each preprocessing token is converted into a token.
\begin{note}
An alternative
token\iref{lex.digraph} is not an identifier,
even when its spelling consists entirely of letters and underscores.
Therefore it is not subject to this replacement.
\end{note}
\pnum
The resulting tokens comprise the controlling constant expression
which is evaluated according to the rules of~\ref{expr.const}
using arithmetic that has at least the ranges specified
in~\ref{support.limits}. For the purposes of this token conversion and evaluation
all signed and unsigned integer types
act as if they have the same representation as, respectively,
\tcode{intmax_t} or \tcode{uintmax_t}\iref{cstdint.syn}.
\begin{note}
Thus on an
implementation where \tcode{std::numeric_limits<int>::max()} is \tcode{0x7FFF}
and \tcode{std::numeric_limits<unsigned int>::max()} is \tcode{0xFFFF},
the integer literal \tcode{0x8000} is signed and positive within a \tcode{\#if}
expression even though it is unsigned in translation phase
7\iref{lex.phases}.
\end{note}
This includes interpreting \grammarterm{character-literal}s
according to the rules in \ref{lex.ccon}.
\begin{note}
The associated character encodings of literals are the same
in \tcode{\#if} and \tcode{\#elif} directives and in any expression.
\end{note}
Each subexpression with type
\tcode{bool}
is subjected to integral promotion before processing continues.
\pnum
Preprocessing directives of the forms
\begin{ncsimplebnf}\obeyspaces
\terminal{\# ifdef } identifier new-line \opt{group}\br
\indextext{\idxcode{\#ifdef}}%
\terminal{\# ifndef } identifier new-line \opt{group}\br
\indextext{\idxcode{\#ifndef}}%
\terminal{\# elifdef } identifier new-line \opt{group}\br
\indextext{\idxcode{\#elifdef}}%
\terminal{\# elifndef} identifier new-line \opt{group}
\indextext{\idxcode{\#elifndef}}%
\end{ncsimplebnf}
check whether the identifier is or is not currently defined as a macro name.
Their conditions are equivalent to
\tcode{\#if} \tcode{defined} \grammarterm{identifier},
\tcode{\#if} \tcode{!defined} \grammarterm{identifier},
\tcode{\#elif} \tcode{defined} \grammarterm{identifier}, and
\tcode{\#elif} \tcode{!defined} \grammarterm{identifier},
respectively.
\pnum
Each directive's condition is checked in order.
If it evaluates to false (zero),
the group that it controls is skipped:
directives are processed only through the name that determines
the directive in order to keep track of the level
of nested conditionals;
the rest of the directives' preprocessing tokens are ignored,
as are the other preprocessing tokens in the group.
Only the first group
whose control condition evaluates to true (nonzero) is processed;
any following groups are skipped and their controlling directives
are processed as if they were in a group that is skipped.
If none of the conditions evaluates to true,
and there is a
\tcode{\#else}
\indextext{\idxcode{\#else}}%
directive,
the group controlled by the
\tcode{\#else}
is processed; lacking a
\tcode{\#else}
directive, all the groups until the
\tcode{\#endif}
\indextext{\idxcode{\#endif}}%
are skipped.%
\begin{footnote}
As indicated by the syntax,
a preprocessing token cannot follow a
\tcode{\#else}
or
\tcode{\#endif}
directive before the terminating new-line character.
However,
comments can appear anywhere in a source file,
including within a preprocessing directive.
\end{footnote}
\pnum
\begin{example}
This demonstrates a way to include a library \tcode{optional} facility
only if it is available:
\begin{codeblock}
#if __has_include(<optional>)
# include <optional>
# if __cpp_lib_optional >= 201603
# define have_optional 1
# endif
#elif __has_include(<experimental/optional>)
# include <experimental/optional>
# if __cpp_lib_experimental_optional >= 201411
# define have_optional 1
# define experimental_optional 1
# endif
#endif
#ifndef have_optional
# define have_optional 0
#endif
\end{codeblock}
\end{example}
\pnum
\begin{example}
This demonstrates a way to use the attribute \tcode{[[acme::deprecated]]}
only if it is available.
\begin{codeblock}
#if __has_cpp_attribute(acme::deprecated)
# define ATTR_DEPRECATED(msg) [[acme::deprecated(msg)]]
#else
# define ATTR_DEPRECATED(msg) [[deprecated(msg)]]
#endif
ATTR_DEPRECATED("This function is deprecated") void anvil();
\end{codeblock}
\end{example}
\rSec1[cpp.include]{Source file inclusion}
\indextext{preprocessing directive!header inclusion}
\indextext{preprocessing directive!source-file inclusion}
\indextext{inclusion!source file|see{preprocessing directive, source-file inclusion}}%
\indextext{\idxcode{\#include}}%
\pnum
A
\tcode{\#include}
directive shall identify a header or source file
that can be processed by the implementation.
\pnum
A preprocessing directive of the form
\begin{ncsimplebnf}
\terminal{\# include <} h-char-sequence \terminal{>} new-line
\end{ncsimplebnf}
searches a sequence of
\impldef{sequence of places searched for a header}
places
for a header identified uniquely by the specified sequence
between the
\tcode{<}
and
\tcode{>}
delimiters,
and causes the replacement of that
directive by the entire contents of the header.
How the places are specified
or the header identified
is \impldef{search locations for \tcode{<>} header}.
\pnum
A preprocessing directive of the form
\begin{ncsimplebnf}
\terminal{\# include "} q-char-sequence \terminal{"} new-line
\end{ncsimplebnf}
causes the replacement of that
directive by the entire contents of the
source file identified by the specified sequence between the
\tcode{"}
delimiters.
The named source file is searched for in an
\impldef{manner of search for included source file}
manner.
If this search is not supported,
or if the search fails,
the directive is reprocessed as if it read
\begin{ncsimplebnf}
\terminal{\# include <} h-char-sequence \terminal{>} new-line
\end{ncsimplebnf}
with the identical contained sequence (including
\tcode{>}
characters, if any) from the original directive.
\pnum
A preprocessing directive of the form
\begin{ncsimplebnf}
\terminal{\# include} pp-tokens new-line
\end{ncsimplebnf}
(that does not match one of the two previous forms) is permitted.
The preprocessing tokens after
\tcode{include}
in the directive are processed just as in normal text
(i.e., each identifier currently defined as a macro name is replaced by its
replacement list of preprocessing tokens).
If the directive resulting after all replacements does not match
one of the two previous forms, the behavior is
undefined.
\begin{footnote}
Note that adjacent \grammarterm{string-literal}s are not concatenated into
a single \grammarterm{string-literal}
(see the translation phases in~\ref{lex.phases});
thus, an expansion that results in two \grammarterm{string-literal}s is an
invalid directive.
\end{footnote}
The method by which a sequence of preprocessing tokens between a
\tcode{<}
and a
\tcode{>}
preprocessing token pair or a pair of
\tcode{"}
characters is combined into a single header name
preprocessing token is \impldef{search locations for \tcode{""""} header}.
\pnum
The implementation shall provide unique mappings for
sequences consisting of one or more
\grammarterm{nondigit}{s} or \grammarterm{digit}{s}\iref{lex.name}
followed by a period
(\tcode{.})
and a single
\grammarterm{nondigit}.
The first character shall not be a \grammarterm{digit}.
The implementation may ignore distinctions of alphabetical case.
\pnum
A
\tcode{\#include}
preprocessing directive may appear
in a source file that has been read because of a
\tcode{\#include}
directive in another file,
up to an \impldef{nesting limit for \tcode{\#include} directives} nesting limit.
\pnum
If the header identified by the \grammarterm{header-name}
denotes an importable header\iref{module.import},
it is
\impldef{whether source file inclusion of importable header
is replaced with \tcode{import} directive}
whether the \tcode{\#include} preprocessing directive
is instead replaced by an \tcode{import} directive\iref{cpp.import} of the form
\begin{ncbnf}
\terminal{import} header-name \terminal{;} new-line
\end{ncbnf}
\pnum
\begin{note}
An implementation can provide a mechanism for making arbitrary
source files available to the \tcode{< >} search.
However, using the \tcode{< >} form for headers provided
with the implementation and the \tcode{" "} form for sources
outside the control of the implementation
achieves wider portability. For instance:
\begin{codeblock}
#include <stdio.h>
#include <unistd.h>
#include "usefullib.h"
#include "myprog.h"
\end{codeblock}
\end{note}
\pnum
\begin{example}
This illustrates macro-replaced
\tcode{\#include}
directives:
\begin{codeblock}
#if VERSION == 1
#define INCFILE "vers1.h"
#elif VERSION == 2
#define INCFILE "vers2.h" // and so on
#else
#define INCFILE "versN.h"
#endif
#include INCFILE
\end{codeblock}
\end{example}
\rSec1[cpp.module]{Module directive}
\indextext{preprocessing directive!module}%
\begin{bnf}
\nontermdef{pp-module}\br
\opt{\keyword{export}} \keyword{module} \opt{pp-tokens} \terminal{;} new-line
\end{bnf}
\pnum
A \grammarterm{pp-module} shall not
appear in a context where \tcode{module}
or (if it is the first token of the \grammarterm{pp-module}) \tcode{export}
is an identifier defined as an object-like macro.
\pnum
Any preprocessing tokens after the \tcode{module} preprocessing token
in the \tcode{module} directive are processed just as in normal text.
\begin{note}
Each identifier currently defined as a macro name
is replaced by its replacement list of preprocessing tokens.
\end{note}
\pnum
The \tcode{module} and \tcode{export} (if it exists) preprocessing tokens
are replaced by the \grammarterm{module-keyword} and
\grammarterm{export-keyword} preprocessing tokens respectively.
\begin{note}
This makes the line no longer a directive
so it is not removed at the end of phase 4.
\end{note}
\rSec1[cpp.import]{Header unit importation}
\indextext{header unit!preprocessing}%
\indextext{preprocessing directive!import}%
\indextext{macro!import|(}%
\begin{bnf}
\nontermdef{pp-import}\br
\opt{\keyword{export}} \keyword{import} header-name \opt{pp-tokens} \terminal{;} new-line\br
\opt{\keyword{export}} \keyword{import} header-name-tokens \opt{pp-tokens} \terminal{;} new-line\br
\opt{\keyword{export}} \keyword{import} pp-tokens \terminal{;} new-line
\end{bnf}
\pnum
A \grammarterm{pp-import} shall not
appear in a context where \tcode{import}
or (if it is the first token of the \grammarterm{pp-import}) \tcode{export}
is an identifier defined as an object-like macro.
\pnum
The preprocessing tokens after the \tcode{import} preprocessing token
in the \tcode{import} \grammarterm{control-line}
are processed just as in normal text
(i.e., each identifier currently defined as a macro name
is replaced by its replacement list of preprocessing tokens).
\begin{note}
An \tcode{import} directive
matching the first two forms of a \grammarterm{pp-import}
instructs the preprocessor to import macros
from the header unit\iref{module.import}
denoted by the \grammarterm{header-name},
as described below.
\end{note}
\indextext{point of!macro import|see{macro, point of import}}%
The \defnx{point of macro import}{macro!point of import} for the
first two forms of \grammarterm{pp-import} is
immediately after the \grammarterm{new-line} terminating
the \grammarterm{pp-import}.
The last form of \grammarterm{pp-import} is only considered
if the first two forms did not match, and
does not have a point of macro import.
\pnum
If a \grammarterm{pp-import} is produced by source file inclusion
(including by the rewrite produced
when a \tcode{\#include} directive names an importable header)
while processing the \grammarterm{group} of a \grammarterm{module-file},
the program is ill-formed.
\pnum
In all three forms of \grammarterm{pp-import},
the \tcode{import} and \tcode{export} (if it exists) preprocessing tokens
are replaced by the \grammarterm{import-keyword} and
\grammarterm{export-keyword} preprocessing tokens respectively.
\begin{note}
This makes the line no longer a directive
so it is not removed at the end of phase 4.
\end{note}
Additionally, in the second form of \grammarterm{pp-import},
a \grammarterm{header-name} token is formed as if
the \grammarterm{header-name-tokens}
were the \grammarterm{pp-tokens} of a \tcode{\#include} directive.
The \grammarterm{header-name-tokens} are replaced by
the \grammarterm{header-name} token.
\begin{note}
This ensures that imports are treated consistently by
the preprocessor and later phases of translation.
\end{note}
\pnum
Each \tcode{\#define} directive encountered when preprocessing
each translation unit in a program results in a distinct
\defnx{macro definition}{macro!definition}.
\begin{note}
A predefined macro name\iref{cpp.predefined}
is not introduced by a \tcode{\#define} directive.
Implementations providing mechanisms to predefine additional macros
are encouraged to not treat them
as being introduced by a \tcode{\#define} directive.
\end{note}
Each macro definition has at most one point of definition in
each translation unit and at most one point of undefinition, as follows:
\begin{itemize}
\item
\indextext{point of!macro definition|see{macro, point of definition}}%
The \defnx{point of definition}{macro!point of definition}
of a macro definition within a translation unit $T$ is
\begin{itemize}
\item
if the \tcode{\#define} directive of the macro definition occurs within $T$,
the point at which that directive occurs, or otherwise,
\item
if the macro name is not lexically identical to a keyword\iref{lex.key}
or to the \grammarterm{identifier}{s} \tcode{module} or \tcode{import},
the first point of macro import in $T$ of a header unit
containing a point of definition for the macro definition, if any.
\end{itemize}
In the latter case, the macro is said
to be \defnx{imported}{macro!import} from the header unit.
\item
\indextext{point of!macro undefinition|see{macro, point of undefinition}}%
The \defnx{point of undefinition}{macro!point of undefinition}
of a macro definition within a translation unit
is the first point at which a \tcode{\#undef} directive naming the macro occurs
after its point of definition, or the first point
of macro import of a header unit containing a point of undefinition for the
macro definition, whichever (if any) occurs first.
\end{itemize}
\pnum
\indextext{active macro directive|see{macro, active}}%
A macro directive is \defnx{active}{macro!active} at a source location
if it has a point of definition in that translation unit preceding the location,
and does not have a point of undefinition in that translation unit preceding
the location.
\pnum
If a macro would be replaced or redefined, and multiple macro definitions
are active for that macro name, the active macro definitions shall all be
valid redefinitions of the same macro\iref{cpp.replace}.
\begin{note}
The relative order of \grammarterm{pp-import}{s} has no bearing on whether a
particular macro definition is active.
\end{note}
\pnum
\begin{example}
\begin{codeblocktu}{Importable header \tcode{"a.h"}}
#define X 123 // \#1
#define Y 45 // \#2
#define Z a // \#3
#undef X // point of undefinition of \#1 in \tcode{"a.h"}
\end{codeblocktu}
\begin{codeblocktu}{Importable header \tcode{"b.h"}}
import "a.h"; // point of definition of \#1, \#2, and \#3, point of undefinition of \#1 in \tcode{"b.h"}
#define X 456 // OK, \#1 is not active
#define Y 6 // error: \#2 is active
\end{codeblocktu}
\begin{codeblocktu}{Importable header \tcode{"c.h"}}
#define Y 45 // \#4
#define Z c // \#5
\end{codeblocktu}
\begin{codeblocktu}{Importable header \tcode{"d.h"}}
import "c.h"; // point of definition of \#4 and \#5 in \tcode{"d.h"}
\end{codeblocktu}
\begin{codeblocktu}{Importable header \tcode{"e.h"}}
import "a.h"; // point of definition of \#1, \#2, and \#3, point of undefinition of \#1 in \tcode{"e.h"}
import "d.h"; // point of definition of \#4 and \#5 in \tcode{"e.h"}
int a = Y; // OK, active macro definitions \#2 and \#4 are valid redefinitions
int c = Z; // error: active macro definitions \#3 and \#5 are not valid redefinitions of \tcode{Z}
\end{codeblocktu}
\begin{codeblocktu}{Module unit \tcode{f}}
export module f;
export import "a.h";
int a = Y; // OK
\end{codeblocktu}
\begin{codeblocktu}{Translation unit \tcode{\#1}}
import f;
int x = Y; // error: \tcode{Y} is neither a defined macro nor a declared name
\end{codeblocktu}
\end{example}
\indextext{macro!import|)}
\rSec1[cpp.replace]{Macro replacement}%
\rSec2[cpp.replace.general]{General}%
\indextext{macro!replacement|(}%
\indextext{replacement!macro|see{macro, replacement}}%
\indextext{preprocessing directive!macro replacement|see{macro, replacement}}
\pnum
\indextext{macro!replacement list}%
Two replacement lists are identical if and only if
the preprocessing tokens in both have
the same number, ordering, spelling, and whitespace separation,
where all whitespace separations are considered identical.
\pnum
An identifier currently defined as an
\indextext{macro!object-like}%
object-like macro (see below) may be redefined by another
\tcode{\#define}
preprocessing directive provided that the second definition is an
object-like macro definition and the two replacement lists
are identical, otherwise the program is ill-formed.
Likewise, an identifier currently defined as a
\indextext{macro!function-like}%
function-like macro (see below) may be redefined by another
\tcode{\#define}
preprocessing directive provided that the second definition is a
function-like macro definition that has the same number and spelling
of parameters,
and the two replacement lists are identical,
otherwise the program is ill-formed.
\pnum
\begin{example}
The following sequence is valid:
\begin{codeblock}
#define OBJ_LIKE (1-1)
#define OBJ_LIKE @\tcode{/* whitespace */ (1-1) /* other */}@
#define FUNC_LIKE(a) ( a )
#define FUNC_LIKE( a )( @\tcode{/* note the whitespace */ \textbackslash}@
a @\tcode{/* other stuff on this line}@
@\tcode{*/}@ )
\end{codeblock}
But the following redefinitions are invalid:
\begin{codeblock}
#define OBJ_LIKE (0) // different token sequence
#define OBJ_LIKE (1 - 1) // different whitespace
#define FUNC_LIKE(b) ( a ) // different parameter usage
#define FUNC_LIKE(b) ( b ) // different parameter spelling
\end{codeblock}
\end{example}
\pnum
\indextext{macro!replacement list}%