C++ preprocessor VARARGS - invalid syntax in a NodejS header file

Question

I'm experimenting a bit with trying to build NodeJS 24.11.1 with GCC 13.4.0 on an older OS X system, mostly as a can-it-be-done project.

Regardless of the reasons, among the hurdles that could be expected is a rather surprising one that seems to raise a valid general question about C++ syntax. FWIW, this happens when compiling deps/v8/src/baseline/baseline.cc.

In the deps/v8/src/codegen/interface-descriptors.h header file, there are these 2 macros, among others (I have stripped the embedded comments because otherwise I couldn't generate the preprocessed baseline.i file):

#define DEFINE_RESULT_AND_PARAMETERS(return_count, ...)   \
  static constexpr int kReturnCount = return_count;       \
  enum ParameterIndices {                                 \
    __dummy = -1, \
    ##__VA_ARGS__,                                        \
                                                          \
    kParameterCount,                                      \
    kContext = kParameterCount    \
  };

#define DEFINE_PARAMETERS(...) DEFINE_RESULT_AND_PARAMETERS(1, ##__VA_ARGS__)

class V8_EXPORT_PRIVATE VoidDescriptor
    : public StaticCallInterfaceDescriptor<VoidDescriptor> {
 public: 
  // The void descriptor could (and indeed probably should) also be NO_CONTEXT,
  // but this breaks some code assembler unittests.
  INTERNAL_DESCRIPTOR() 
  DEFINE_PARAMETERS()
  DEFINE_PARAMETER_TYPES()                                  
  DECLARE_DESCRIPTOR(VoidDescriptor)                        
    
  static constexpr auto registers();
};

Note how DEFINE_PARAMETERS is called without arguments, which expands to:

static constexpr int kReturnCount = 1; enum ParameterIndices { __dummy = -1,, kParameterCount, kContext = kParameterCount };

And a compiler error that seems completely justified:

../deps/v8/src/codegen/interface-descriptors.h:662:18: error: expected identifier before ',' token
  662 |     ##__VA_ARGS__,                                        \

The thing is, this particular header file code hasn't changed since Node 22.21.1 - and I have been able to do a normal build of that version with clang (16), on the same system. And with GCC 13 on a Linux host. Both of these compilers must have seen that same code - and yes, when I just take the failing compile command and execute it from the shell with clang++-16 instead of g++-13, I also get an error about that missing identifier.

What am I missing here?

##__VA_ARGS__ is a non-standard GCC extension that's supposed to suppress the preceding comma if the replacement list is empty. The standard defines __VA_OPT__ that can be used for that purpose. Not sure why that doesn't work in your case. — Igor Tandetnik
– Igor Tandetnik, Commented Nov 23 at 14:03
Ah, triaging the compiler invocation differences between the failing Mac build and the Linux build I indeed found that -std=c++20 vs. -std=gnu++20 indeed explains the failure. Not the 1st thing I would have thought of since this seems something where C and C++ should agree... — RJVB
– RJVB, Commented Nov 23 at 14:08
To be clear, I didn't try to use __VA_OPT__ - yet. Are you saying I should be able to replace all ##__VA_ARGS__ with __VA_OPT__? If I do that blindly I get an error that __VA_OPT__ should be followed by an open parenthesis. EDIT: never mind, I hadn't read the linked discussion (oops...). — RJVB
– RJVB, Commented Nov 23 at 14:20
__dummy = -1, ##__VA_ARGS__, -> __dummy = -1 __VA_OPT__ (,) __VA_ARGS__, — Jarod42
– Jarod42, Commented Nov 23 at 14:26

RJVB · Accepted Answer · 2025-11-23 14:36:32Z

3

To answer my own question, based on the comments above: the preprocessed syntax is indeed invalid according to the official standards. Hence the use of a GNU-extension preprocessor tag that requires -std=gnu++XY.

In my Mac build I added -std=c++20 myself, thinking it wouldn't hurt but in fact shooting myself in the foot.

answered Nov 23 at 14:36

RJVB

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