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Asked: 2026-05-14T14:17:33+00:00

Help settle the debate that’s going on in the comments at this question about

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Help settle the debate that’s going on in the comments at this question about bool and 1:

Can a standards-conforming C++ preprocessor allow one to use #define to redefine a language keyword? If so, must a standards-conforming C++ preprocessor allow this?

If a C++ program redefines a language keyword, can that program itself be standards conforming?

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1 Answer

  1. Editorial Team

    In C++, the closest thing to forbidding #defineing a keyword is §17.4.3.1.1/2, which only disallows it in a translation unit that includes a standard library header:

    A translation unit that includes a header shall not contain any macros that define names declared or defined in that header. Nor shall such a translation unit define macros for names lexically identical to keywords.

    The second sentence of that paragraph has been changed in C++0x to outright forbid #defineing a keyword (C++0x FCD §17.6.3.3.1):

    A translation unit shall not #define or #undef names lexically identical to keywords.

    Edit: As pointed out by Ken Bloom in comments to his answer, the rules have not changed in C++0x; the text has just been rearranged to confuse people like me. 🙂

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