All the tokens of the replacement list are the same

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All the tokens of the replacement list are the same. If there are any parameters, they are the same. Whitespace appears in the same places in both. It need not be exactly the same amount of whitespace, though. Remember that comments count as whitespace. These definitions are effectively the same: #define FOUR (2 + 2) #define FOUR (2 + 2) #define FOUR (2 /* two */ + 2) but these are not: #define FOUR (2 + 2) #define FOUR ( 2+2 ) #define FOUR (2 * 2) #define FOUR(score,and,seven,years,ago) (2 + 2) If a macro is redefined with a definition that is not effectively the same as the old one, the preprocessor issues a warning and changes the macro to use the new definition. If the new definition is effectively the same, the redefinition is silently ignored. This allows, for instance, two different headers to define a common macro. The preprocessor will only complain if the definitions do not match. 3.9 Directives Within Macro Arguments Occasionally it is convenient to use preprocessor directives within the arguments of a macro. The C and C ++ standards declare that behavior in these cases is undefined. Versions of CPP prior to 3.2 would reject such constructs with an error message. This was the only syntactic difference between normal functions and function-like macros, so it seemed attractive to remove this limitation, and people would often be surprised that they could not use macros in this way. Moreover, sometimes people would use conditional compilation in the argument list to a normal library function like ‘ printf ’, only to find that after a library upgrade ‘ printf ’ had changed to be a function-like macro, and their code would no longer compile. So from version 3.2 we changed CPP to successfully process arbitrary directives within macro arguments in exactly the same way as it would have processed the directive were the function-like macro invocation not present. If, within a macro invocation, that macro is redefined, then the new definition takes effect in time for argument pre-expansion, but the original definition is still used for argument replacement. Here is a pathological example:
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Chapter 3: Macros 34 #define f(x) x x f (1 #undef f #define f 2 f) which expands to 1 2 1 2 with the semantics described above. 3.10 Macro Pitfalls In this section we describe some special rules that apply to macros and macro expansion, and point out certain cases in which the rules have counter-intuitive consequences that you must watch out for. 3.10.1 Misnesting When a macro is called with arguments, the arguments are substituted into the macro body and the result is checked, together with the rest of the input file, for more macro calls. It is possible to piece together a macro call coming partially from the macro body and partially from the arguments. For example, #define twice(x) (2*(x)) #define call_with_1(x) x(1) call_with_1 (twice) 7→ twice(1) 7→ (2*(1)) Macro definitions do not have to have balanced parentheses. By writing an unbalanced open parenthesis in a macro body, it is possible to create a macro call that begins inside the macro body but ends outside of it. For example,
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