6.10.3.1 — Argument substitution

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6.10.3.1 — Argument substitution

Constraints

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

Semantics

• (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.145)
• (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.
  #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.
• (1) After the arguments for the invocation of a function-like macro have been identified, argument substitution takes place. A parameter in the replacement list, unless preceded by a # or ## preprocessing token or followed by a ## preprocessing token (see below), is replaced by the corresponding argument after all macros contained therein have been expanded. Before being substituted, each argument’s preprocessing tokens are completely macro replaced as if they formed the rest of the preprocessing file; no other preprocessing tokens are available.
• (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.
  6.10.3.2 The # operator

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