15 Preprocessing directives [cpp]

15.1 Preamble [cpp.pre]

preprocessing-file:
	group
	module-file

module-file:
	pp-global-module-fragment pp-module group pp-private-module-fragment

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

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

group:
	group-part
	group group-part

group-part:
	control-line
	if-section
	text-line
	# conditionally-supported-directive

control-line:
	# include pp-tokens new-line
	pp-import
	# define  identifier replacement-list new-line
	# define  identifier lparen identifier-list ) replacement-list new-line
	# define  identifier lparen ... ) replacement-list new-line
	# define  identifier lparen identifier-list , ... ) replacement-list new-line
	# undef   identifier new-line
	# line    pp-tokens new-line
	# error   pp-tokens new-line
	# pragma  pp-tokens new-line
	# new-line

if-section:
	if-group elif-groups else-group endif-line

if-group:
	# if      constant-expression new-line group
	# ifdef   identifier new-line group
	# ifndef  identifier new-line group

elif-groups:
	elif-group
	elif-groups elif-group

elif-group:
	# elif    constant-expression new-line group

else-group:
	# else    new-line group

endif-line:
	# endif   new-line

text-line:
	pp-tokens new-line

conditionally-supported-directive:
	pp-tokens new-line

lparen:
	a ( character not immediately preceded by white-space

identifier-list:
	identifier
	identifier-list , identifier

replacement-list:
	pp-tokens

pp-tokens:
	preprocessing-token
	pp-tokens preprocessing-token

new-line:
	the new-line character
1
#
A 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 directive-introducing token, begins with the first character in the source file (optionally after white space containing no new-line characters) or follows white space containing at least one new-line character, and is
  • (1.1)
    a # preprocessing token, or
  • (1.2)
    an import preprocessing token immediately followed on the same logical line by a header-name, <, identifier, string-literal, or : preprocessing token, or
  • (1.3)
    a module preprocessing token immediately followed on the same logical line by an identifier, :, or ; preprocessing token, or
  • (1.4)
    an export preprocessing token immediately followed on the same logical line by one of the two preceding forms.
The last token in the sequence is the first token within the sequence that is immediately followed by whitespace containing a new-line character.141
[ 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
 ]
[ Example
: 
#                       // 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
 ]
2
#
A sequence of preprocessing tokens is only a text-line if it does not begin with a directive-introducing token.
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.
3
#
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.
4
#
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.
5
#
The only white-space 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 white-space characters in translation phase 3).
6
#
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.
7
#
The preprocessing tokens within a preprocessing directive are not subject to macro expansion unless otherwise stated.
[ Example
:
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
 ]
141)
Thus, preprocessing directives are commonly called “lines”.
These “lines” have no other syntactic significance, as all white space is equivalent except in certain situations during preprocessing (see the # character string literal creation operator in [cpp.stringize], for example).
⮥

15.2 Conditional inclusion [cpp.cond]

defined-macro-expression:
	defined identifier
	defined ( identifier )

h-preprocessing-token:
	any preprocessing-token other than >

h-pp-tokens:
	h-preprocessing-token
	h-pp-tokens h-preprocessing-token

header-name-tokens:
	string-literal
	< h-pp-tokens >

has-include-expression:
	_­_­has_­include ( header-name )
	_­_­has_­include ( header-name-tokens )

has-attribute-expression:
	_­_­has_­cpp_­attribute ( pp-tokens )
1
#
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 below142 and it may contain zero or more defined-macro-expressions and/or has-include-expressions and/or has-attribute-expressions as unary operator expressions.
2
#
A defined-macro-expression evaluates to 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 ([cpp.import]), for example because it has been the subject of a #define preprocessing directive without an intervening #undef directive with the same subject identifier), 0 if it is not.
3
#
The second form of 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.
4
#
The header or source file identified by the parenthesized preprocessing token sequence in each contained has-include-expression is searched for as if that preprocessing token sequence were the pp-tokens in a #include directive, except that no further macro expansion is performed.
If such a directive would not satisfy the syntactic requirements of a #include directive, the program is ill-formed.
The has-include-expression evaluates to 1 if the search for the source file succeeds, and to 0 if the search fails.
5
#
Each has-attribute-expression is replaced by a non-zero pp-number matching the form of an integer-literal if the implementation supports an attribute with the name specified by interpreting the pp-tokens, after macro expansion, as an attribute-token, and by 0 otherwise.
The program is ill-formed if the pp-tokens do not match the form of an attribute-token.
6
#
For an attribute specified in this document, the value of the has-attribute-expression is given by Table 18.
For other attributes recognized by the implementation, the value is implementation-defined.
[ Note
:
It is expected that the availability of an attribute can be detected by any non-zero result.
— end note
 ]
Table 18: _­_­has_­cpp_­attribute values   [tab:cpp.cond.ha]
Attribute
Value
carries_­dependency
200809L
deprecated
201309L
fallthrough
201603L
likely
201803L
maybe_­unused
201603L
no_­unique_­address
201803L
nodiscard
201907L
noreturn
200809L
unlikely
201803L
7
#
The #ifdef and #ifndef directives, and the defined conditional inclusion operator, shall treat _­_­has_­include and _­_­has_­cpp_­attribute as if they were the names of defined macros.
The identifiers _­_­has_­include and _­_­has_­cpp_­attribute shall not appear in any context not mentioned in this subclause.
8
#
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.
9
#
Preprocessing directives of the forms 
# if      constant-expression new-line group
# elif    constant-expression new-line group
check whether the controlling constant expression evaluates to nonzero.
10
#
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 defined unary operator), just as in normal text.
If the token defined is generated as a result of this replacement process or use of the defined unary operator does not match one of the two specified forms prior to macro replacement, the behavior is undefined.
11
#
After all replacements due to macro expansion and evaluations of defined-macro-expressions, has-include-expressions, and has-attribute-expressions have been performed, all remaining identifiers and keywords, except for true and false, are replaced with the pp-number 0, and then each preprocessing token is converted into a token.
[ Note
:
An alternative token is not an identifier, even when its spelling consists entirely of letters and underscores.
Therefore it is not subject to this replacement.
— end note
 ]
12
#
The resulting tokens comprise the controlling constant expression which is evaluated according to the rules of [expr.const] using arithmetic that has at least the ranges specified in [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, intmax_­t or uintmax_­t ([cstdint]).
[ Note
:
Thus on an implementation where std​::​numeric_­limits<int>​::​max() is 0x7FFF and std​::​numeric_­limits<unsigned int>​::​max() is 0xFFFF, the integer literal 0x8000 is signed and positive within a #if expression even though it is unsigned in translation phase 7.
— end note
 ]
This includes interpreting character-literals, which may involve converting escape sequences into execution character set members.
Whether the numeric value for these character-literals matches the value obtained when an identical character-literal occurs in an expression (other than within a #if or #elif directive) is implementation-defined.
[ Note
:
Thus, the constant expression in the following #if directive and if statement ([stmt.if]) is not guaranteed to evaluate to the same value in these two contexts: 
#if 'z' - 'a' == 25
if ('z' - 'a' == 25)
— end note
 ]
Also, whether a single-character character-literal may have a negative value is implementation-defined.
Each subexpression with type bool is subjected to integral promotion before processing continues.
13
#
Preprocessing directives of the forms 
# ifdef   identifier new-line group
# ifndef  identifier new-line group
check whether the identifier is or is not currently defined as a macro name.
Their conditions are equivalent to #if defined identifier and #if !defined identifier respectively.
14
#
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 #else directive, the group controlled by the #else is processed; lacking a #else directive, all the groups until the #endif are skipped.143
15
#
[ Example
:
This demonstrates a way to include a library optional facility only if it is available:
#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 example
 ]
16
#
[ Example
:
This demonstrates a way to use the attribute [[acme​::​deprecated]] only if it is available.
#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 example
 ]
142)
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.
⮥
143)
As indicated by the syntax, a preprocessing token shall not follow a #else or #endif directive before the terminating new-line character.
However, comments may appear anywhere in a source file, including within a preprocessing directive.
⮥

15.3 Source file inclusion [cpp.include]

1
#
A #include directive shall identify a header or source file that can be processed by the implementation.
2
#
A preprocessing directive of the form 
# include < h-char-sequence > new-line
searches a sequence of implementation-defined places for a header identified uniquely by the specified sequence between the < and > 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 implementation-defined.
3
#
A preprocessing directive of the form 
# include " q-char-sequence " new-line
causes the replacement of that directive by the entire contents of the source file identified by the specified sequence between the " delimiters.
The named source file is searched for in an implementation-defined manner.
If this search is not supported, or if the search fails, the directive is reprocessed as if it read 
# include < h-char-sequence > new-line
with the identical contained sequence (including > characters, if any) from the original directive.
4
#
A preprocessing directive of the form 
# include pp-tokens new-line
(that does not match one of the two previous forms) is permitted.
The preprocessing tokens after 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.144
The method by which a sequence of preprocessing tokens between a < and a > preprocessing token pair or a pair of " characters is combined into a single header name preprocessing token is implementation-defined.
5
#
The implementation shall provide unique mappings for sequences consisting of one or more nondigits or digits followed by a period (.) and a single nondigit.
The first character shall not be a digit.
The implementation may ignore distinctions of alphabetical case.
6
#
A #include preprocessing directive may appear in a source file that has been read because of a #include directive in another file, up to an implementation-defined nesting limit.
7
#
If the header identified by the header-name denotes an importable header ([module.import]), it is implementation-defined whether the #include preprocessing directive is instead replaced by an import directive ([cpp.import]) of the form 
import header-name ; new-line
8
#
[ Note
:
Although an implementation may provide a mechanism for making arbitrary source files available to the < > search, in general programmers should use the < > form for headers provided with the implementation, and the " " form for sources outside the control of the implementation.
For instance:
#include <stdio.h>
#include <unistd.h>
#include "usefullib.h"
#include "myprog.h"
— end note
 ]
9
#
[ Example
:
This illustrates macro-replaced #include directives:
#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 example
 ]
144)
Note that adjacent string-literals are not concatenated into a single string-literal (see the translation phases in [lex.phases]); thus, an expansion that results in two string-literals is an invalid directive.
⮥

15.4 Module directive [cpp.module]

pp-module:
	export module pp-tokens ; new-line
1
#
A pp-module shall not appear in a context where module or (if it is the first token of the pp-module) export is an identifier defined as an object-like macro.
2
#
Any preprocessing tokens after the module preprocessing token in the module directive are processed just as in normal text.
[ Note
:
Each identifier currently defined as a macro name is replaced by its replacement list of preprocessing tokens.
— end note
 ]
3
#
The module and export (if it exists) preprocessing tokens are replaced by the module-keyword and export-keyword preprocessing tokens respectively.
[ Note
:
This makes the line no longer a directive so it is not removed at the end of phase 4.
— end note
 ]

15.5 Header unit importation [cpp.import]

pp-import:
	export import header-name pp-tokens ; new-line
	export import header-name-tokens pp-tokens ; new-line
	export import pp-tokens ; new-line
1
#
A pp-import shall not appear in a context where import or (if it is the first token of the pp-import) export is an identifier defined as an object-like macro.
2
#
The preprocessing tokens after the import preprocessing token in the import 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).
An import directive matching the first two forms of a pp-import instructs the preprocessor to import macros from the header unit ([module.import]) denoted by the header-name.
The point of macro import for the first two forms of pp-import is immediately after the new-line terminating the pp-import.
The last form of pp-import is only considered if the first two forms did not match.
3
#
If a pp-import is produced by source file inclusion (including by the rewrite produced when a #include directive names an importable header) while processing the group of a module-file, the program is ill-formed.
4
#
In all three forms of pp-import, the import and export (if it exists) preprocessing tokens are replaced by the import-keyword and export-keyword preprocessing tokens respectively.
[ 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 pp-import, a header-name token is formed as if the header-name-tokens were the pp-tokens of a #include directive.
The header-name-tokens are replaced by the header-name token.
[ Note
:
This ensures that imports are treated consistently by the preprocessor and later phases of translation.
— end note
 ]
5
#
Each #define directive encountered when preprocessing each translation unit in a program results in a distinct macro definition.
[ Note
:
A predefined macro name ([cpp.predefined]) is not introduced by a #define directive.
Implementations providing mechanisms to predefine additional macros are encouraged to not treat them as being introduced by a #define directive.
— end note
 ]
Importing macros from a header unit makes macro definitions from a translation unit visible in other translation units.
Each macro definition has at most one point of definition in each translation unit and at most one point of undefinition, as follows:
  • (5.1)
    The point of definition of a macro definition within a translation unit is the point at which its #define directive occurs (in the translation unit containing the #define directive), or, if the macro name is not lexically identical to a keyword ([lex.key]) or to the identifiers module or import, the first point of macro import of a translation unit containing a point of definition for the macro definition, if any (in any other translation unit).
  • (5.2)
    The point of undefinition of a macro definition within a translation unit is the first point at which a #undef directive naming the macro occurs after its point of definition, or the first point of macro import of a translation unit containing a point of undefinition for the macro definition, whichever (if any) occurs first.
6
#
A macro directive is 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.
7
#
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 ([cpp.replace]).
[ Note
:
The relative order of pp-imports has no bearing on whether a particular macro definition is active.
— end note
 ]
8
#
[ Example
:

Importable header "a.h":

#define X 123   // #1
#define Y 45    // #2
#define Z a     // #3
#undef X        // point of undefinition of #1 in "a.h"

Importable header "b.h":

import "a.h";   // point of definition of #1, #2, and #3, point of undefinition of #1 in "b.h"
#define X 456   // OK, #1 is not active
#define Y 6     // error: #2 is active

Importable header "c.h":

#define Y 45    // #4
#define Z c     // #5

Importable header "d.h":

import "a.h";   // point of definition of #1, #2, and #3, point of undefinition of #1 in "d.h"
import "c.h";   // point of definition of #4 and #5 in "d.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 Z
— end example
 ]

15.6 Macro replacement [cpp.replace]

1
#
Two replacement lists are identical if and only if the preprocessing tokens in both have the same number, ordering, spelling, and white-space separation, where all white-space separations are considered identical.
2
#
An identifier currently defined as an object-like macro (see below) may be redefined by another #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 function-like macro (see below) may be redefined by another #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.
3
#
[ Example
:
The following sequence is valid: 
#define OBJ_LIKE      (1-1)
#define OBJ_LIKE      /* white space */ (1-1) /* other */
#define FUNC_LIKE(a)   ( a )
#define FUNC_LIKE( a )(     /* note the white space */ \
                a /* other stuff on this line
                  */ )
But the following redefinitions are invalid: 
#define OBJ_LIKE    (0)         // different token sequence
#define OBJ_LIKE    (1 - 1)     // different white space
#define FUNC_LIKE(b) ( a )      // different parameter usage
#define FUNC_LIKE(b) ( b )      // different parameter spelling
— end example
 ]
4
#
There shall be white-space between the identifier and the replacement list in the definition of an object-like macro.
5
#
If the identifier-list in the macro definition does not end with an ellipsis, the number of arguments (including those arguments consisting of no preprocessing tokens) in an invocation of a function-like macro shall equal the number of parameters in the macro definition.
Otherwise, there shall be at least as many arguments in the invocation as there are parameters in the macro definition (excluding the ...).
There shall exist a ) preprocessing token that terminates the invocation.
6
#
The identifiers __VA_­ARGS__ and __VA_­OPT__ shall occur only in the replacement-list of a function-like macro that uses the ellipsis notation in the parameters.
7
#
A parameter identifier in a function-like macro shall be uniquely declared within its scope.
8
#
The identifier immediately following the define is called the macro name.
There is one name space for macro names.
Any white-space characters preceding or following the replacement list of preprocessing tokens are not considered part of the replacement list for either form of macro.
9
#
If a # preprocessing token, followed by an identifier, occurs lexically at the point at which a preprocessing directive could begin, the identifier is not subject to macro replacement.
10
#
A preprocessing directive of the form 
# define identifier replacement-list new-line
defines an object-like macro that causes each subsequent instance of the macro name145 to be replaced by the replacement list of preprocessing tokens that constitute the remainder of the directive.146
The replacement list is then rescanned for more macro names as specified below.
11
#
[ Example
:
The simplest use of this facility is to define a “manifest constant”, as in 
#define TABSIZE 100
int table[TABSIZE];
— end example
 ]
12
#
A preprocessing directive of the form 
# define identifier lparen identifier-list ) replacement-list new-line
# define identifier lparen ... ) replacement-list new-line
# define identifier lparen identifier-list , ... ) replacement-list new-line
defines a function-like macro with parameters, whose use is similar syntactically to a function call.
The parameters are specified by the optional list of identifiers, whose scope extends from their declaration in the identifier list until the new-line character that terminates the #define preprocessing directive.
Each subsequent instance of the function-like macro name followed by a ( as the next preprocessing token introduces the sequence of preprocessing tokens that is replaced by the replacement list in the definition (an invocation of the macro).
The replaced sequence of preprocessing tokens is terminated by the matching ) preprocessing token, skipping intervening matched pairs of left and right parenthesis preprocessing tokens.
Within the sequence of preprocessing tokens making up an invocation of a function-like macro, new-line is considered a normal white-space character.
13
#
The sequence of preprocessing tokens bounded by the outside-most matching parentheses forms the list of arguments for the function-like macro.
The individual arguments within the list are separated by comma preprocessing tokens, but comma preprocessing tokens between matching inner parentheses do not separate arguments.
If there are sequences of preprocessing tokens within the list of arguments that would otherwise act as preprocessing directives,147 the behavior is undefined.
14
#
[ Example
:
The following defines a function-like macro whose value is the maximum of its arguments.
It has the disadvantages of evaluating one or the other of its arguments a second time (including side effects) and generating more code than a function if invoked several times.
It also cannot have its address taken, as it has none.
#define max(a, b) ((a) > (b) ? (a) : (b))
The parentheses ensure that the arguments and the resulting expression are bound properly.
— end example
 ]
15
#
If there is a ... immediately preceding the ) in the function-like macro definition, then the trailing arguments (if any), including any separating comma preprocessing tokens, are merged to form a single item: the variable arguments.
The number of arguments so combined is such that, following merger, the number of arguments is either equal to or one more than the number of parameters in the macro definition (excluding the ...).
145)
Since, by macro-replacement time, all character-literals and string-literals are preprocessing tokens, not sequences possibly containing identifier-like subsequences (see [lex.phases], translation phases), they are never scanned for macro names or parameters.
⮥
146)
An alternative token is not an identifier, even when its spelling consists entirely of letters and underscores.
Therefore it is not possible to define a macro whose name is the same as that of an alternative token.
⮥
147)
A conditionally-supported-directive is a preprocessing directive regardless of whether the implementation supports it.
⮥

15.6.1 Argument substitution [cpp.subst]

va-opt-replacement:
	__VA_­OPT__ ( pp-tokens )
1
#
After the arguments for the invocation of a function-like macro have been identified, argument substitution takes place.
For each parameter in the replacement list that is neither preceded by a # or ## preprocessing token nor followed by a ## preprocessing token, the preprocessing tokens naming the parameter are replaced by a token sequence determined as follows:
  • (1.1)
    If the parameter is of the form va-opt-replacement, the replacement preprocessing tokens are the preprocessing token sequence for the corresponding argument, as specified below.
  • (1.2)
    Otherwise, the replacement preprocessing tokens are the preprocessing tokens of corresponding argument after all macros contained therein have been expanded. The argument's preprocessing tokens are completely macro replaced before being substituted as if they formed the rest of the preprocessing file with no other preprocessing tokens being available.
[ Example
: 
#define LPAREN() (
#define G(Q) 42
#define F(R, X, ...)  __VA_OPT__(G R X) )
int x = F(LPAREN(), 0, <:-);    // replaced by int x = 42;
— end example
 ]
2
#
An identifier __VA_­ARGS__ that occurs in the replacement list shall be treated as if it were a parameter, and the variable arguments shall form the preprocessing tokens used to replace it.
3
#
[ Example
: 
#define debug(...) fprintf(stderr, __VA_ARGS__)
#define showlist(...) puts(#__VA_ARGS__)
#define report(test, ...) ((test) ? puts(#test) : printf(__VA_ARGS__))
debug("Flag");
debug("X = %d\n", x);
showlist(The first, second, and third items.);
report(x>y, "x is %d but y is %d", x, y);
results in 
fprintf(stderr, "Flag");
fprintf(stderr, "X = %d\n", x);
puts("The first, second, and third items.");
((x>y) ? puts("x>y") : printf("x is %d but y is %d", x, y));
— end example
 ]
4
#
The identifier __VA_­OPT__ shall always occur as part of the preprocessing token sequence va-opt-replacement; its closing ) is determined by skipping intervening pairs of matching left and right parentheses in its pp-tokens.
The pp-tokens of a va-opt-replacement shall not contain __VA_­OPT__.
If the pp-tokens would be ill-formed as the replacement list of the current function-like macro, the program is ill-formed.
A va-opt-replacement is treated as if it were a parameter, and the preprocessing token sequence for the corresponding argument is defined as follows.
If the substitution of __VA_­ARGS__ as neither an operand of # nor ## consists of no preprocessing tokens, the argument consists of a single placemarker preprocessing token ([cpp.concat], [cpp.rescan]).
Otherwise, the argument consists of the results of the expansion of the contained pp-tokens as the replacement list of the current function-like macro before removal of placemarker tokens, rescanning, and further replacement.
[ Note
:
The placemarker tokens are removed before stringization ([cpp.stringize]), and can be removed by rescanning and further replacement ([cpp.rescan]).
— end note
 ]
[ Example
: 
#define F(...)           f(0 __VA_OPT__(,) __VA_ARGS__)
#define G(X, ...)        f(0, X __VA_OPT__(,) __VA_ARGS__)
#define SDEF(sname, ...) S sname __VA_OPT__(= { __VA_ARGS__ })
#define EMP

F(a, b, c)          // replaced by f(0, a, b, c)
F()                 // replaced by f(0)
F(EMP)              // replaced by f(0)

G(a, b, c)          // replaced by f(0, a, b, c)
G(a, )              // replaced by f(0, a)
G(a)                // replaced by f(0, a)

SDEF(foo);          // replaced by S foo;
SDEF(bar, 1, 2);    // replaced by S bar = { 1, 2 };

#define H1(X, ...) X __VA_OPT__(##) __VA_ARGS__ // error: ## may not appear at
                                                // the beginning of a replacement list ([cpp.concat])

#define H2(X, Y, ...) __VA_OPT__(X ## Y,) __VA_ARGS__
H2(a, b, c, d)      // replaced by ab, c, d

#define H3(X, ...) #__VA_OPT__(X##X X##X)
H3(, 0)             // replaced by ""

#define H4(X, ...) __VA_OPT__(a X ## X) ## b
H4(, 1)             // replaced by a b

#define H5A(...) __VA_OPT__()/**/__VA_OPT__()
#define H5B(X) a ## X ## b
#define H5C(X) H5B(X)
H5C(H5A())          // replaced by ab
— end example
 ]

15.6.2 The # operator [cpp.stringize]

1
#
Each # preprocessing token in the replacement list for a function-like macro shall be followed by a parameter as the next preprocessing token in the replacement list.
2
#
If, in the replacement list, a parameter is immediately preceded by a # preprocessing token, both are replaced by a single character string literal preprocessing token that contains the spelling of the preprocessing token sequence for the corresponding argument (excluding placemarker tokens).
Let the stringizing argument be the preprocessing token sequence for the corresponding argument with placemarker tokens removed.
Each occurrence of white space between the stringizing argument's preprocessing tokens becomes a single space character in the character string literal.
White space before the first preprocessing token and after the last preprocessing token comprising the stringizing argument is deleted.
Otherwise, the original spelling of each preprocessing token in the stringizing argument is retained in the character string literal, except for special handling for producing the spelling of string-literals and character-literals: a \ character is inserted before each " and \ character of a character-literal or string-literal (including the delimiting " characters).
If the replacement that results is not a valid character string literal, the behavior is undefined.
The character string literal corresponding to an empty stringizing argument is "".
The order of evaluation of # and ## operators is unspecified.

15.6.3 The ## operator [cpp.concat]

1
#
A ## preprocessing token shall not occur at the beginning or at the end of a replacement list for either form of macro definition.
2
#
If, in the replacement list of a function-like macro, a parameter is immediately preceded or followed by a ## preprocessing token, the parameter is replaced by the corresponding argument's preprocessing token sequence; however, if an argument consists of no preprocessing tokens, the parameter is replaced by a placemarker preprocessing token instead.148
3
#
For both object-like and function-like macro invocations, before the replacement list is reexamined for more macro names to replace, each instance of a ## preprocessing token in the replacement list (not from an argument) is deleted and the preceding preprocessing token is concatenated with the following preprocessing token.
Placemarker preprocessing tokens are handled specially: concatenation of two placemarkers results in a single placemarker preprocessing token, and concatenation of a placemarker with a non-placemarker preprocessing token results in the non-placemarker preprocessing token.
If the result is not a valid preprocessing token, the behavior is undefined.
The resulting token is available for further macro replacement.
The order of evaluation of ## operators is unspecified.
4
#
[ Example
:
The sequence 
#define str(s)      # s
#define xstr(s)     str(s)
#define debug(s, t) printf("x" # s "= %d, x" # t "= %s", \
               x ## s, x ## t)
#define INCFILE(n)  vers ## n
#define glue(a, b)  a ## b
#define xglue(a, b) glue(a, b)
#define HIGHLOW     "hello"
#define LOW         LOW ", world"

debug(1, 2);
fputs(str(strncmp("abc\0d", "abc", '\4')        // this goes away
    == 0) str(: @\n), s);
#include xstr(INCFILE(2).h)
glue(HIGH, LOW);
xglue(HIGH, LOW)
results in 
printf("x" "1" "= %d, x" "2" "= %s", x1, x2);
fputs("strncmp(\"abc\\0d\", \"abc\", '\\4') == 0" ": @\n", s);
#include "vers2.h"      (after macro replacement, before file access)
"hello";
"hello" ", world"
or, after concatenation of the character string literals, 
printf("x1= %d, x2= %s", x1, x2);
fputs("strncmp(\"abc\\0d\", \"abc\", '\\4') == 0: @\n", s);
#include "vers2.h"      (after macro replacement, before file access)
"hello";
"hello, world"
Space around the # and ## tokens in the macro definition is optional.
— end example
 ]
5
#
[ Example
:
In the following fragment:
#define hash_hash # ## #
#define mkstr(a) # a
#define in_between(a) mkstr(a)
#define join(c, d) in_between(c hash_hash d)
char p[] = join(x, y);          // equivalent to char p[] = "x ## y";
The expansion produces, at various stages:
join(x, y)
in_between(x hash_hash y)
in_between(x ## y)
mkstr(x ## y)
"x ## y"
In other words, expanding hash_­hash produces a new token, consisting of two adjacent sharp signs, but this new token is not the ## operator.
— end example
 ]
6
#
[ Example
:
To illustrate the rules for placemarker preprocessing tokens, the sequence 
#define t(x,y,z) x ## y ## z
int j[] = { t(1,2,3), t(,4,5), t(6,,7), t(8,9,),
  t(10,,), t(,11,), t(,,12), t(,,) };
results in 
int j[] = { 123, 45, 67, 89,
  10, 11, 12, };
— end example
 ]
148)
Placemarker preprocessing tokens do not appear in the syntax because they are temporary entities that exist only within translation phase 4.
⮥

15.6.4 Rescanning and further replacement [cpp.rescan]

1
#
After all parameters in the replacement list have been substituted and # and ## processing has taken place, all placemarker preprocessing tokens are removed.
Then the resulting preprocessing token sequence is rescanned, along with all subsequent preprocessing tokens of the source file, for more macro names to replace.
2
#
[ Example
:
The sequence 
#define x       3
#define f(a)    f(x * (a))
#undef  x
#define x       2
#define g       f
#define z       z[0]
#define h       g(~
#define m(a)    a(w)
#define w       0,1
#define t(a)    a
#define p()     int
#define q(x)    x
#define r(x,y)  x ## y
#define str(x)  # x

f(y+1) + f(f(z)) % t(t(g)(0) + t)(1);
g(x+(3,4)-w) | h 5) & m
    (f)^m(m);
p() i[q()] = { q(1), r(2,3), r(4,), r(,5), r(,) };
char c[2][6] = { str(hello), str() };
results in 
f(2 * (y+1)) + f(2 * (f(2 * (z[0])))) % f(2 * (0)) + t(1);
f(2 * (2+(3,4)-0,1)) | f(2 * (~ 5)) & f(2 * (0,1))^m(0,1);
int i[] = { 1, 23, 4, 5, };
char c[2][6] = { "hello", "" };
— end example
 ]
3
#
If the name of the macro being replaced is found during this scan of the replacement list (not including the rest of the source file's preprocessing tokens), it is not replaced.
Furthermore, if any nested replacements encounter the name of the macro being replaced, it is not replaced.
These nonreplaced macro name preprocessing tokens are no longer available for further replacement even if they are later (re)examined in contexts in which that macro name preprocessing token would otherwise have been replaced.
4
#
The resulting completely macro-replaced preprocessing token sequence is not processed as a preprocessing directive even if it resembles one, but all pragma unary operator expressions within it are then processed as specified in [cpp.pragma.op] below.

15.6.5 Scope of macro definitions [cpp.scope]

1
#
A macro definition lasts (independent of block structure) until a corresponding #undef directive is encountered or (if none is encountered) until the end of the translation unit.
Macro definitions have no significance after translation phase 4.
2
#
A preprocessing directive of the form 
# undef identifier new-line
causes the specified identifier no longer to be defined as a macro name.
It is ignored if the specified identifier is not currently defined as a macro name.

15.7 Line control [cpp.line]

1
#
The string-literal of a #line directive, if present, shall be a character string literal.
2
#
The line number of the current source line is one greater than the number of new-line characters read or introduced in translation phase 1 while processing the source file to the current token.
3
#
A preprocessing directive of the form 
# line digit-sequence new-line
causes the implementation to behave as if the following sequence of source lines begins with a source line that has a line number as specified by the digit sequence (interpreted as a decimal integer).
If the digit sequence specifies zero or a number greater than 2147483647, the behavior is undefined.
4
#
A preprocessing directive of the form 
# line digit-sequence " s-char-sequence " new-line
sets the presumed line number similarly and changes the presumed name of the source file to be the contents of the character string literal.
5
#
A preprocessing directive of the form 
# line pp-tokens new-line
(that does not match one of the two previous forms) is permitted.
The preprocessing tokens after line on the directive are processed just as in normal text (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; otherwise, the result is processed as appropriate.

15.8 Error directive [cpp.error]

1
#
A preprocessing directive of the form 
# error pp-tokens new-line
causes the implementation to produce a diagnostic message that includes the specified sequence of preprocessing tokens, and renders the program ill-formed.

15.9 Pragma directive [cpp.pragma]

1
#
A preprocessing directive of the form 
# pragma pp-tokens new-line
causes the implementation to behave in an implementation-defined manner.
The behavior might cause translation to fail or cause the translator or the resulting program to behave in a non-conforming manner.
Any pragma that is not recognized by the implementation is ignored.

15.10 Null directive [cpp.null]

1
#
A preprocessing directive of the form 
# new-line
has no effect.

15.11 Predefined macro names [cpp.predefined]

1
#
The following macro names shall be defined by the implementation:
  • (1.1)
    _­_­cplusplus
    The integer literal 202002L.
    [ Note
    :
    It is intended that future versions of this International Standard will replace the value of this macro with a greater value.
    — end note
     ]
  • (1.2)
    __DATE__
    The date of translation of the source file: a character string literal of the form "Mmm dd yyyy", where the names of the months are the same as those generated by the asctime function, and the first character of dd is a space character if the value is less than 10.
    If the date of translation is not available, an implementation-defined valid date shall be supplied.
  • (1.3)
    __FILE__
    The presumed name of the current source file (a character string literal).149
  • (1.4)
    __LINE__
    The presumed line number (within the current source file) of the current source line (an integer literal).150
  • (1.5)
    __STDC_­HOSTED__
    The integer literal 1 if the implementation is a hosted implementation or the integer literal 0 if it is not.
  • (1.6)
    __STDCPP_­DEFAULT_­NEW_­ALIGNMENT__
    An integer literal of type std​::​size_­t whose value is the alignment guaranteed by a call to operator new(std​::​size_­t) or operator new[](std​::​size_­t).
    [ Note
    :
    Larger alignments will be passed to operator new(std​::​size_­t, std​::​align_­val_­t), etc.
    — end note
     ]
  • (1.7)
    __TIME__
    The time of translation of the source file: a character string literal of the form "hh:mm:ss" as in the time generated by the asctime function.
    If the time of translation is not available, an implementation-defined valid time shall be supplied.
  • (1.8)
    The names listed in Table 19.

    The macros defined in Table 19 shall be defined to the corresponding integer literal.
    [ Note
    :
    Future versions of this International Standard might replace the values of these macros with greater values.
    — end note
     ]
Table 19: Feature-test macros   [tab:cpp.predefined.ft]
Macro name
Value
_­_­cpp_­aggregate_­bases
201603L
_­_­cpp_­aggregate_­nsdmi
201304L
_­_­cpp_­aggregate_­paren_­init
201902L
_­_­cpp_­alias_­templates
200704L
_­_­cpp_­aligned_­new
201606L
_­_­cpp_­attributes
200809L
_­_­cpp_­binary_­literals
201304L
_­_­cpp_­capture_­star_­this
201603L
_­_­cpp_­char8_­t
201811L
_­_­cpp_­concepts
201907L
_­_­cpp_­conditional_­explicit
201806L
_­_­cpp_­constexpr
201907L
_­_­cpp_­constexpr_­dynamic_­alloc
201907L
_­_­cpp_­constexpr_­in_­decltype
201711L
_­_­cpp_­consteval
201811L
_­_­cpp_­constinit
201907L
_­_­cpp_­decltype
200707L
_­_­cpp_­decltype_­auto
201304L
_­_­cpp_­deduction_­guides
201907L
_­_­cpp_­delegating_­constructors
200604L
_­_­cpp_­designated_­initializers
201707L
_­_­cpp_­enumerator_­attributes
201411L
_­_­cpp_­fold_­expressions
201603L
_­_­cpp_­generic_­lambdas
201707L
_­_­cpp_­guaranteed_­copy_­elision
201606L
_­_­cpp_­hex_­float
201603L
_­_­cpp_­if_­constexpr
201606L
_­_­cpp_­impl_­coroutine
201902L
_­_­cpp_­impl_­destroying_­delete
201806L
_­_­cpp_­impl_­three_­way_­comparison
201907L
_­_­cpp_­inheriting_­constructors
201511L
_­_­cpp_­init_­captures
201803L
_­_­cpp_­initializer_­lists
200806L
_­_­cpp_­inline_­variables
201606L
_­_­cpp_­lambdas
200907L
_­_­cpp_­modules
201907L
_­_­cpp_­namespace_­attributes
201411L
_­_­cpp_­noexcept_­function_­type
201510L
_­_­cpp_­nontype_­template_­args
201911L
_­_­cpp_­nontype_­template_­parameter_­auto
201606L
_­_­cpp_­nsdmi
200809L
_­_­cpp_­range_­based_­for
201603L
_­_­cpp_­raw_­strings
200710L
_­_­cpp_­ref_­qualifiers
200710L
_­_­cpp_­return_­type_­deduction
201304L
_­_­cpp_­rvalue_­references
200610L
_­_­cpp_­sized_­deallocation
201309L
_­_­cpp_­static_­assert
201411L
_­_­cpp_­structured_­bindings
201606L
_­_­cpp_­template_­template_­args
201611L
_­_­cpp_­threadsafe_­static_­init
200806L
_­_­cpp_­unicode_­characters
200704L
_­_­cpp_­unicode_­literals
200710L
_­_­cpp_­user_­defined_­literals
200809L
_­_­cpp_­using_­enum
201907L
_­_­cpp_­variable_­templates
201304L
_­_­cpp_­variadic_­templates
200704L
_­_­cpp_­variadic_­using
201611L
2
#
The following macro names are conditionally defined by the implementation:
  • (2.1)
    __STDC__
    Whether __STDC__ is predefined and if so, what its value is, are implementation-defined.
  • (2.2)
    __STDC_­MB_­MIGHT_­NEQ_­WC__
    The integer literal 1, intended to indicate that, in the encoding for wchar_­t, a member of the basic character set need not have a code value equal to its value when used as the lone character in an ordinary character literal.
  • (2.3)
    __STDC_­VERSION__
    Whether __STDC_­VERSION__ is predefined and if so, what its value is, are implementation-defined.
  • (2.4)
    __STDC_­ISO_­10646__
    An integer literal of the form yyyymmL (for example, 199712L).
    If this symbol is defined, then every character in the Unicode required set, when stored in an object of type wchar_­t, has the same value as the code point of that character.
    The Unicode required set consists of all the characters that are defined by ISO/IEC 10646, along with all amendments and technical corrigenda as of the specified year and month.
  • (2.5)
    __STDCPP_­STRICT_­POINTER_­SAFETY__
    Defined, and has the value integer literal 1, if and only if the implementation has strict pointer safety.
  • (2.6)
    __STDCPP_­THREADS__
    Defined, and has the value integer literal 1, if and only if a program can have more than one thread of execution.
3
#
The values of the predefined macros (except for __FILE__ and __LINE__) remain constant throughout the translation unit.
4
#
If any of the pre-defined macro names in this subclause, or the identifier defined, is the subject of a #define or a #undef preprocessing directive, the behavior is undefined.
Any other predefined macro names shall begin with a leading underscore followed by an uppercase letter or a second underscore.
149)
The presumed source file name can be changed by the #line directive.
⮥
150)
The presumed line number can be changed by the #line directive.
⮥

15.12 Pragma operator [cpp.pragma.op]

1
#
A unary operator expression of the form: 
_Pragma ( string-literal )
is processed as follows: The string-literal is destringized by deleting the L prefix, if present, deleting the leading and trailing double-quotes, replacing each escape sequence \" by a double-quote, and replacing each escape sequence \\ by a single backslash.
The resulting sequence of characters is processed through translation phase 3 to produce preprocessing tokens that are executed as if they were the pp-tokens in a pragma directive.
The original four preprocessing tokens in the unary operator expression are removed.
2
#
[ Example
: 
#pragma listing on "..\listing.dir"
can also be expressed as: 
_Pragma ( "listing on \"..\\listing.dir\"" )
The latter form is processed in the same way whether it appears literally as shown, or results from macro replacement, as in: 
#define LISTING(x) PRAGMA(listing on #x)
#define PRAGMA(x) _Pragma(#x)

LISTING( ..\listing.dir )
— end example
 ]