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Editorial Team
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Asked: 2026-05-26T12:22:59+00:00

In a #define one can use A ## B to concatenate preprocessor variables and

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In a #define one can use A ## B to concatenate preprocessor variables and defines to an identifier.

#define ADD_UNDERSCORE(X) X##_
/* ADD_UNDERSCORE(n) -> n_ */

Is there an equivalent leftside of the #define? E.g.

#define A a
#define B b
#define A##B(X) X
/* ab(n) -> n */
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1 Answer

  1. Editorial Team

    No. In a macro definition, the first token after the define has to be an identifier ((draft) ISO/IEC 9899;1999, 6.10, page 149).

    There is no other preprocessing of the #define directives which could make an identifier out of something else. In particular, the standard specifies (6.10.3):

    The preprocessing tokens within a preprocessing directive are not subject to macro
    expansion unless otherwise stated.

    And it isn’t stated “otherwise” for the macro name.

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