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Are there any differences in behaviour between the C and C++ preprocessors?

They are defined by different passages of standards text (section 6.10 of the C standard and section 16 of the C++ standard).

My motivation for asking this is that a proposal for making the single quote a digit separator that was recently accepted into C++14 extends the C++ preprocessor grammar to accomodate this change (specifically, it extends the definition of a pp-number), and I'm wondering whether this introduces an incompatibility between the C and C++ preprocessors, and if so, whether it's the first feature to do so.

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  • In GCC both g++ (actually cc1plus) and gcc (actually cc1) use the same libcpp/ internal library for the preprocessing. Commented Feb 2, 2014 at 20:13
  • This answer gives some examples of differences that already exist: stackoverflow.com/questions/5085533/… Commented Feb 2, 2014 at 20:14
  • You could always look at variadic macros. C had them in C99. C++ had to wait until C++11. Commented Feb 2, 2014 at 20:26

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There are some differences between the C and C++ processors. Ignoring the differences in predefined macros and available system headers, some differences that come to mind in the current versions of C and of C++ are:

  • and and friends are operators, not identifiers, in C++. This means #define and && is valid in C, but not in C++, and means #if 1 and 2 is valid in C++, but not in C (unless and is suitably defined as a macro).
  • false and true are allowed in C++ #if expressions, but replaced by 0 (like all identifiers) in C. This means #if true/C++/#else/C/#endif expands to either C++, or C, depending on the language. Unlike and and friends, though, these are not operators, so may be redefined by #define in either language.
  • ::, .*, and ->* are tokens in C++. As a result of that, the ## operator can be used to form them in C++, but not in C.
  • Raw string literals are available in C++, but not in C. As a result, given a macro foo, R"x("foo")x" expands the macro in C, but not in C++.
  • Hexadecimal floating-point constants are available in C, but not in C++. As a result, given a macro foo, 0x1p+foo expands the macro in C++, but not in C.
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9 Comments

@JensGustedt + is only allowed as part of a pp-number if it follows e or E (C++), or e, E, p or P (C). If it were unconditionally allowed, 1+2 would be invalid. (0xE+0xF is already invalid for the same reason, and that is already somewhat of a problem.)
@hvd: false and true are keywords in C++ and not in C.
In C, defining a keyword as a macro name has undefined behavior (N1570 6.4.1p2; it's a "shall" outside a constraint). So a conforming C or C++ preprocessor could just treat keywords as ordinary identifiers (I'm less sure about the C++ rules).
@hvd: Keywords and operators are not exclusive categories. sizeof is a keyword that's used as an operator. and, or, et al are listed separately from the keywords, but they're "reserved and shall not be used otherwise"; I'm not sure what distinction the Standard is making between these alternative representations and actual keywords.
@KeithThompson It says "The above tokens (case sensitive) are reserved (in translation phases 7 and 8) for use as keywords, and shall not be used otherwise.", which does allow use otherwise in other translation phases, unless I'm misreading.
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