What are the differences between the C and C++ preprocessors? [duplicate]

Question

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• Is a C++ preprocessor identical to a C preprocessor? 3 answers

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.

Answer 1

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.