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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.

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  • That "extension" question is really the same as the main question! Commented Feb 23, 2011 at 0:02
  • Yes, but the question was originally driven by that "extension". Commented Feb 23, 2011 at 0:04
  • 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. Commented Sep 13, 2011 at 7:38
  • Since C++11 not ending a translation unit w/ a newline is no longer undefined behavior Commented Feb 12, 2017 at 6:01

3 Answers 3

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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 (two-letter alternative tokens) 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).

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2 Comments

other than trigraphs, in C99 digraphs (but %: and the "quadrigraph" %:%:) are just tokens and not modified further in the preprocessing phase. They are then interpreted by the "real" compiler phases.
Also about C++03 vs C89, the C++ preprocessor has a special treatment of boolean literals.
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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.

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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.

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Erm... you apparently forgot about C11... it's not only C++ that moved ahead. (Especially with regards to thread support and associated macros.)
@DevSolar thanks for the tip. Just to confirm: you mean that C11 added many more macros which are not in C++11 is that it? That's my impression on a quick look. Haven't looked into C++14, though, but there is stuff which won't be there for sure like __STDC_NO_VLA__
Well, both C11 and C++11 added threads, and the preprocessor macros necessary to handle them -- just different ones obviously. And I found it curious that you link C++11 documentation, but (outdated) C99 to compare. ;)
FYI: Syntax: C: # non-directive, C++: # conditionally-supported-directive.

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