6.10.3.3 The ## operator

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1938 A ## preprocessing token shall not occur at the beginning or at the end of a replacement list for either form of macro definition.

1939 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;

1940 however, if an argument consists of no preprocessing tokens, the parameter is replaced by a placemarker preprocessing token instead.¹⁴⁸⁾

1941 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.

1942 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.

1943 If the result is not a valid preprocessing token, the behavior is undefined.

1944 The resulting token is available for further macro replacement.

1945 The order of evaluation of ## operators is unspecified.

1946 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.

1947 148) Placemarker preprocessing tokens do not appear in the syntax because they are temporary entities that exist only within translation phase 4.