What are the differences between the C and C++ preprocessors?
c, c++, c-preprocessor, standards
Solution
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.
Problem
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.