15 Preprocessing directives [cpp]

15.6 Macro replacement [cpp.replace]

15.6.3 The ## operator [cpp.concat]

A ## preprocessing token shall not occur at the beginning or at the end of a replacement list for either form of macro definition.

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

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.

[ 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

]

[ 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

]

[ 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.

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