Re: Make copyObject work in C++

David Geier <geidav.pg@gmail.com>

From: David Geier <geidav.pg@gmail.com>

To: Peter Eisentraut <peter@eisentraut.org>, Jelte Fennema-Nio <postgres@jeltef.nl>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Thomas Munro <thomas.munro@gmail.com>

Date: 2025-12-08T08:33:10Z

Lists: pgsql-hackers

Commits

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

Enable test_cplusplusext with MSVC
- c05ad248f99c 19 (unreleased) landed
Disable some C++ warnings in MSVC
- 82a7cbea747c 19 (unreleased) landed
meson: Make room for C++-only warning flags for MSVC
- 4c83f1253593 19 (unreleased) landed
Make fixed-length list building macros work in C++
- f8e7ca328510 19 (unreleased) landed
Make unconstify and unvolatize use StaticAssertVariableIsOfTypeMacro
- 258248d0bdbf 19 (unreleased) landed
Use typeof everywhere instead of compiler specific spellings
- e2308350c9b7 19 (unreleased) landed
Test List macros in C++ extensions
- 3d28ecb5ac76 19 (unreleased) landed
Test most StaticAssert macros in C++ extensions
- 451650eaacd5 19 (unreleased) landed
Fix for C++ compatibility
- b4555cb070f1 19 (unreleased) landed
tests: Add a test C++ extension module
- 476b35d4e311 19 (unreleased) landed

On 08.12.2025 08:57, Peter Eisentraut wrote:
> On 05.12.25 15:46, Jelte Fennema-Nio wrote:
>> Calling copyObject fails in C++ with an error like in most setups:
>>
>> error: use of undeclared identifier 'typeof'; did you mean 'typeid'
>>
>> This is due to the C compiler supporting used to compile postgres
>> supporting typeof, but that function actually not being present in the
>> C++ compiler. This fixes that by using decltype instead of typeof when
>> including the header in C++.
>>
>> Realized because of Thomas' not about how much of our headers should
>> work in C++, and remembering I hit this specific problem myself.
>>
>> Another approach would be to force the value of HAVE_TYPEOF to 0 if
>> __cplusplus.
> 
> In the long run, I would like to change copyObject() to use
> typeof_unqual instead, because that handles qualifiers more correctly.
> (Currently, copyObject() of a const-qualified pointer results in a
> const-qualified pointer, which is nonsensical because the reason you
> made the copy is that you can modify it.)  See attached patch for an
> example.  Does C++ have something that is semantically similar to that?

Since C++11 there's std::remove_const which can be used as
std::remove_const<decltype(type)>::type.

I'm not aware of anything pre C++11, except for rolling your own variant
of std::remove_const via template specialization.

--
David Geier