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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}
\nontermdef{control-line}\br
\terminal{\# include} pp-tokens new-line\br
pp-import\br
\terminal{\# embed \ } pp-tokens new-line\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}
\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 \opt{elif-groups}
\end{bnf}
\begin{bnf}
\nontermdef{elif-group}\br
\terminal{\# elif \ \ \ } constant-expression new-line \opt{group}\br
\terminal{\# elifdef } identifier new-line \opt{group}\br
\terminal{\# elifndef} identifier new-line \opt{group}
\end{bnf}
\begin{bnf}
\nontermdef{else-group}\br
\terminal{\# else \ \ } new-line \opt{group}
\end{bnf}
\begin{bnf}
\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 \opt{pp-tokens}
\end{bnf}
\begin{bnf}
\nontermdef{embed-parameter-seq}\br
embed-parameter \opt{embed-parameter-seq}
\end{bnf}
\begin{bnf}
\nontermdef{embed-parameter}\br
embed-standard-parameter\br
embed-prefixed-parameter
\end{bnf}
\begin{bnf}
\nontermdef{embed-standard-parameter}\br
\terminal{limit} \terminal{(} pp-balanced-token-seq \terminal{)}\br
\terminal{prefix} \terminal{(} \opt{pp-balanced-token-seq} \terminal{)}\br
\terminal{suffix} \terminal{(} \opt{pp-balanced-token-seq} \terminal{)}\br
\terminal{if_empty} \terminal{(} \opt{pp-balanced-token-seq} \terminal{)}
\end{bnf}
\begin{bnf}
\nontermdef{embed-prefixed-parameter}\br
identifier :: identifier\br
identifier :: identifier \terminal{(} \opt{pp-balanced-token-seq} \terminal{)}
\end{bnf}
\begin{bnf}
\nontermdef{pp-balanced-token-seq}\br
pp-balanced-token \opt{pp-balanced-token-seq}
\end{bnf}
\begin{bnf}
\nontermdef{pp-balanced-token}\br
\terminal{(} \opt{pp-balanced-token-seq} \terminal{)}\br
\terminal{[} \opt{pp-balanced-token-seq} \terminal{]}\br
\terminal{\{} \opt{pp-balanced-token-seq} \terminal{\}}\br
\textnormal{any} pp-token \textnormal{except:}\br
\bnfindent\textnormal{parenthesis (\unicode{0028}{left parenthesis} and \unicode{0029}{right parenthesis}),}\br
\bnfindent\textnormal{bracket (\unicode{005b}{left square bracket} and \unicode{005d}{right square bracket}), or}\br
\bnfindent\textnormal{brace (\unicode{007b}{left curly bracket} and \unicode{007d}{right curly bracket}).}
\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 preprocessing 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 source 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 source line by an
\grammarterm{identifier},
\tcode{:}, or
\tcode{;}
preprocessing token, or
\item
an \keyword{export} preprocessing token
immediately followed on the same logical source line by
one of the two preceding forms.
\end{itemize}
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.
\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.
\begin{example}
\begin{codeblock}
using module = int;
module i; // not a \grammarterm{text-line} and not a \grammarterm{control-line}
int foo() {
return i;
}
\end{codeblock}
The example is not a valid \grammarterm{preprocessing-file}.
\end{example}
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
If one of the \grammarterm{pp-tokens} of a \tcode{\#embed} directive
(before macro replacement) is the \grammarterm{identifier}
\tcode{limit}, \tcode{prefix}, \tcode{suffix}, or \tcode{if_empty}
and that \grammarterm{identifier} is defined
as a macro\iref{cpp.replace.general}, the program is ill-formed.
Any \grammarterm{embed-prefixed-parameter} is conditionally-supported,
with \impldef{supported forms of \#embed prefix parameters} 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 \opt{h-pp-tokens}
\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_embed}}%
\begin{bnf}
\nontermdef{has-embed-expression}\br
\terminal{\xname{has_embed}} \terminal{(} header-name \opt{pp-balanced-token-seq} \terminal{)}\br
\terminal{\xname{has_embed}} \terminal{(} header-name-tokens \opt{pp-balanced-token-seq} \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},
\grammarterm{has-include-expression}{s},
\grammarterm{has-attribute-expression}{s},
and/or \grammarterm{has-embed-expression}{s}
as unary operator expressions.
A \grammarterm{defined-macro-expression} shall not appear
within a \grammarterm{has-include-expression} or \grammarterm{has-embed-expression}.
\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 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} of a \tcode{\#include} directive,
except that no further macro expansion is performed.
If the preprocessing token sequence does not consist solely of a \grammarterm{header-name}
or cannot be combined\iref{cpp.include} into a single
\grammarterm{header-name} preprocessing token, 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
The parenthesized preprocessing token sequence of each contained
\grammarterm{has-embed-expression} is processed as if that
preprocessing token sequence were the \grammarterm{pp-tokens}
of a \tcode{\#embed} directive\iref{cpp.embed},
except that no further macro expansion is performed.
The \grammarterm{has-embed-expression} evaluates to:
\begin{itemize}
\item
\mname{STDC_EMBED_FOUND} if the search for the resource succeeds,
all the given \grammarterm{embed-parameter}s in the \grammarterm{embed-parameter-seq}
are supported, and the resource is not empty.
\item
Otherwise, \mname{STDC_EMBED_EMPTY} if the search for the resource succeeds,
all the given \grammarterm{embed-parameter}s in the \grammarterm{embed-parameter-seq}
are supported, and the resource is empty.
\item
Otherwise, \mname{STDC_EMBED_NOT_FOUND}.
\end{itemize}
\begin{note}
An unrecognized \grammarterm{embed-parameter} in an \grammarterm{has-embed-expression}
is not ill-formed and is instead treated as not supported.
\end{note}
\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,
it is \impldef{value of \grammarterm{has-attribute-expression}
for standard attributes}
whether the value of the \grammarterm{has-attribute-expression}
is \tcode{0} or 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{deprecated} & \tcode{201309L} \\
\tcode{fallthrough} & \tcode{201603L} \\
\tcode{indeterminate} & \tcode{202403L} \\
\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}, \xname{has_embed}, and \xname{has_cpp_attribute}
as if they were the names of defined macros.
The identifiers \xname{has_include}, \xname{has_embed}, 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}
\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 replacement of macros in the preprocessing tokens following the sequence
\tcode{\xname{has_embed} (} and before a matching \tcode{)}
(possibly produced by macro expansion)
encounters a preprocessing token that is one of the \grammarterm{identifier}s
\tcode{limit}, \tcode{prefix}, \tcode{suffix}, or \tcode{if_empty}
and that \grammarterm{identifier} is defined
as a macro\iref{cpp.replace.general}, the program is ill-formed.
If the preprocessing 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,
\grammarterm{has-embed-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}
\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 \defnadj{header}{search} for a sequence of characters
searches a sequence of places for a header
identified uniquely by that sequence of characters.
How the places are determined or the header identified
is \impldef{determination of places and identification of headers during header search}.
\pnum
A \defnadj{source file}{search} for a sequence of characters
attempts to identify a source file that is named by the sequence of characters.
The named source file is searched for
in an \impldef{search for source files during source file search} manner.
If the implementation does not support a source file search
for that sequence of characters, or if the search fails,
the result of the source file search
is the result of a header search for the same sequence of characters.
\pnum
A preprocessing directive of the form
\begin{ncsimplebnf}
\terminal{\# include} header-name new-line
\end{ncsimplebnf}
causes the replacement of that directive by the entire contents
of the header or source file identified by \grammarterm{header-name}.
\pnum
If the \grammarterm{header-name} is of the form
\begin{ncsimplebnf}
\terminal{<} h-char-sequence \terminal{>}
\end{ncsimplebnf}
a header is identified by a header search for the sequence of characters
of the \grammarterm{h-char-sequence}.
\pnum
If the \grammarterm{header-name} is of the form
\begin{ncsimplebnf}
\terminal{"} q-char-sequence \terminal{"}
\end{ncsimplebnf}
the source file or header is identified by a source file search
for the sequence of characters of the \grammarterm{q-char-sequence}.
\pnum
If a header search fails, or if a source file search or header search
identifies a header or source file that cannot be processed by the implementation,
the program is ill-formed.
\begin{note}
If the header or source file cannot be processed,
the program is ill-formed even when evaluating \xname{has_include}.
\end{note}
\pnum
A preprocessing directive of the form
\begin{ncsimplebnf}
\terminal{\# include} pp-tokens new-line
\end{ncsimplebnf}
(that does not match the previous form) 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).
Then, an attempt is made to form a \grammarterm{header-name}
preprocessing token\iref{lex.header} from the whitespace and the characters
of the spellings of the resulting sequence of preprocessing tokens;
the treatment of whitespace
is \impldef{treatment of whitespace when processing a \tcode{\#include} directive}.
If the attempt succeeds, the directive with the so-formed \grammarterm{header-name}
is processed as specified for the previous form.
Otherwise, the behavior is undefined.
\begin{note}
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{note}
\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}
\keyword{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.embed]{Resource inclusion}
\indextext{preprocessing directive!embed a resource}
\indextext{\idxcode{\#embed}}%
\rSec2[cpp.embed.gen]{General}
\pnum
A \defnadj{bracket resource}{search} for a sequence of characters
searches a sequence of places for a resource identified uniquely
by that sequence of characters.
How the places are determined or the resource identified
is \impldef{determination of places and identification of resources during bracket resource search}.
\pnum
A \defnadj{quote resource}{search} for a sequence of characters
attempts to identify a resource that is named by the sequence of characters.
The named resource is searched for
in an \impldef{search for resources during quote resource search} manner.
If the implementation does not support a quote resource search
for that sequence of characters, or if the search fails,
the result of the quote resource search
is the result of a bracket resource search for the same sequence of characters.
\pnum
A preprocessing directive of the form
\begin{ncsimplebnf}
\terminal{\# embed} header-name \opt{pp-tokens} new-line
\end{ncsimplebnf}
causes the replacement of that directive
by preprocessing tokens derived from data
in the resource identified by \grammarterm{header-name},
as specified below.
\pnum
If the \grammarterm{header-name} is of the form
\begin{ncsimplebnf}
\terminal{<} h-char-sequence \terminal{>}
\end{ncsimplebnf}
the resource is identified by a bracket resource search
for the sequence of characters of the \grammarterm{h-char-sequence}.
\pnum
If the \grammarterm{header-name} is of the form
\begin{ncsimplebnf}
\terminal{"} q-char-sequence \terminal{"}
\end{ncsimplebnf}
the resource is identified by a quote resource search
for the sequence of characters of the \grammarterm{q-char-sequence}.
\pnum
If a bracket resource search fails,
or if a quote or bracket resource search identifies a resource
that cannot be processed by the implementation, the program is ill-formed.
\begin{note}
If the resource cannot be processed, the program is ill-formed
even when processing \tcode{\#embed} with \tcode{limit(0)}\iref{cpp.embed.param.limit}
or evaluating \xname{has_embed}.
\end{note}
\pnum
\recommended A mechanism similar to, but distinct from, the
\impldef{sequence of places searched for a header}
search paths used for \tcode{\#include}\iref{cpp.include}
is encouraged.
\pnum
Either form of the \tcode{\#embed} directive processes the
\grammarterm{pp-tokens}, if present, just as in normal text.
The \grammarterm{pp-tokens} shall then have the form
\grammarterm{embed-parameter-seq}.
\pnum
A resource is a source of data accessible from the translation environment.
A resource has an \defn{implementation-resource-width}, which is the
\impldef{size in bits of a resource}
size in bits of the resource.
If the implementation-resource-width is not an integral multiple of
\libmacro{CHAR_BIT}, the program is ill-formed.
Let \defn{implementation-resource-count} be
implementation-resource-width divided by \libmacro{CHAR_BIT}.
Every resource also has a \defn{resource-count}, which is
\begin{itemize}
\item
the value as computed from the optionally-provided \tcode{limit}
\grammarterm{embed-parameter}\iref{cpp.embed.param.limit}, if present;
\item
otherwise, the implementation-resource-count.
\end{itemize}
A resource is empty if the resource-count is zero.
\pnum
\begin{example}
\begin{codeblock}
// ill-formed if the implementation-resource-width is 6 bits
#embed "6_bits.bin"
\end{codeblock}
\end{example}
\pnum
The \tcode{\#embed} directive is replaced by a comma-separated list of integer
literals of type \tcode{int}, unless otherwise modified by embed
parameters\iref{cpp.embed.param}.
\pnum
The integer literals in the comma-separated list correspond to
resource-count consecutive calls to \tcode{std::fgetc}\iref{cstdio.syn}
from the resource, as a binary file.
If any call to \tcode{std::fgetc} returns \tcode{EOF}, the program is
ill-formed.
\pnum
\recommended The value of each integer literal should closely represent
the bit stream of the resource unmodified.
This can require an implementation to consider potential differences between
translation and execution environments, as well as any other applicable
sources of mismatch.
\begin{example}
\begin{codeblock}
#include <cstring>
#include <cstddef>
#include <fstream>
#include <vector>
#include <cassert>
int main() {
// If the file is the same as the resource in the translation environment, no assert in this program should fail.
constexpr unsigned char d[] = {
#embed <data.dat>
};
const std::vector<unsigned char> vec_d = {
#embed <data.dat>
};
constexpr std::size_t expected_size = sizeof(d);
// same file in execution environment as was embedded
std::ifstream f_source("data.dat", std::ios::binary | std::ios::in);
unsigned char runtime_d[expected_size];
char* ifstream_ptr = reinterpret_cast<char*>(runtime_d);
assert(!f_source.read(ifstream_ptr, expected_size));
std::size_t ifstream_size = f_source.gcount();
assert (ifstream_size == expected_size);
int is_same = std::memcmp(&d[0], ifstream_ptr, ifstream_size);
assert(is_same == 0);
int is_same_vec = std::memcmp(vec_d.data(), ifstream_ptr, ifstream_size);
assert(is_same_vec == 0);
}
\end{codeblock}
\end{example}
\begin{example}
\begin{codeblock}
int i = {
#embed "i.dat"