Is a C++ preprocessor identical to a C preprocessor?

27
votes

I am wondering how different the preprocessors for C++ and C are.

The reason for the question is this question on a preprocessor-specific question where the paragraph of the standard that addresses the question has a different wording (and a different paragraph number) and also are difference concerning the true and false keywords in C++.

So, are there more differences or is this the only difference.

An extension of the question would be when is a source file emitted differently by a C++ preprocessor and a C preprocessor.

3
c++cstandardsc-preprocessor
That "extension" question is really the same as the main question! - Oliver Charlesworth
Yes, but the question was originally driven by that "extension". - René Nyffenegger
Although similar to true and false, there are also the named operators: and, and_eq, bitand, bitor, compl, not, not_eq, or, or_eq, xor and xor_eq. - rioki
Since C++11 not ending a translation unit w/ a newline is no longer undefined behavior - Shafik Yaghmour

3 Answers

19
votes

The C++03 preprocessor is (at least intended to be) similar to the C preprocessor before C99. Although the wording and paragraph numbers are slightly different, the only technical differences I'm aware of between the two are that the C++ preprocessor handles digraphs and universal character names, which are not present in C.

As of C99, the C preprocessor added some new capabilities (e.g., variadic macros) that do not exist in the current version of C++. I don't remember for sure, but don't believe that digraphs were added.

I believe C++0x will bring the two in line again (at least that's the intent). Again, the paragraph numbers and wording won't be identical, but I believe the intent is that they should work the same (other than retaining the differences mentioned above).

4
votes

They are supposed to be the same: C++98 and C++03 should match C90, and C++0x should match C99. There may be bugs in the wording, though.

4
votes

Predefined macros differ between the preprocessors, mostly for obvious language feature differences. E.g. compare:

In particular:

  • C requires you not to define __cplusplus, C++ uses it to represent the version
  • C uses __STDC__ to represent the version, C++ says is implementation defined and uses __cplusplus instead
  • C has __STDC_IEC_559__ and __STDC_IEC_559_COMPLEX__ to indicate floating point characteristics, C++ does not and seems replace that with the per type std::numeric_limits<float>::is_iec559 constants
  • C does not have the macros prefixed with __STDCPP: _STDCPP_STRICT_POINTER_SAFETY__ and __STDCPP_THREADS__

As mentioned by DevSolar, C11 added many more defines which are not part of C++11.