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Re: Make copyObject work in C++

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Jelte Fennema-Nio <postgres@jeltef.nl>
Cc: Peter Eisentraut <peter@eisentraut.org>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Thomas Munro <thomas.munro@gmail.com>
Date: 2026-01-25T16:50:28Z
Lists: pgsql-hackers 
Hi,

On 2026-01-25 13:42:51 +0100, Jelte Fennema-Nio wrote:
> This reverts to using positional initializers in PG_MODULE_MAGIC_DATA so
> that its possible to write C++ extensions in standard C++11. Sadly that
> means that using designated initializers in C++20 is still not allowed
> in PG_MODULE_MAGIC_EXT because mixing designated an positional
> initializers is a C only feature. This restriction for C++ extensions is
> now documented and tested.

I'm pretty sceptical this is the right direction. We were going for designated
initializers for a reason, namely that we expect more arguments to be added
over time and perhaps eventually also to remove some. And this will just lead
to that being harder because we have to worry about C++ extensions.


But I'm also confused as to why it's needed - there's nomixing of designated
and non-designated initializers that I can see? If you use
PG_MODULE_MAGIC_EXT(.name = "whatnot"), it evaluates down to

extern __attribute__((visibility("default"))) const Pg_magic_struct *Pg_magic_func(void); const Pg_magic_struct * Pg_magic_func(void) { static const Pg_magic_struct Pg_magic_data = { .len = sizeof(Pg_magic_struct), .abi_fields = { 190000 / 100, 100, 32, 64, true, "PostgreSQL", }, .name="whatnot"}; return &Pg_magic_data; } extern int no_such_variable;

And indeed, contra to what you reported upthread, I can't get clang to report
a warning about that.  I do obviously see warnings about the wrong order if I
pass the arguments in the wrong order, but that's a lot less problematic. And
omitted args don't trigger warnings, as you noted.


Greetings,

Andres Freund

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Enable test_cplusplusext with MSVC

  2. Disable some C++ warnings in MSVC

  3. meson: Make room for C++-only warning flags for MSVC

  4. Make fixed-length list building macros work in C++

  5. Make unconstify and unvolatize use StaticAssertVariableIsOfTypeMacro

  6. Use typeof everywhere instead of compiler specific spellings

  7. Test List macros in C++ extensions

  8. Test most StaticAssert macros in C++ extensions

  9. Revert "Change copyObject() to use typeof_unqual"

  10. Fix for C++ compatibility

  11. tests: Add a test C++ extension module