Re: Make copyObject work in C++

Peter Eisentraut <peter@eisentraut.org>

From: Peter Eisentraut <peter@eisentraut.org>

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

Date: 2025-12-08T07:57:17Z

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

Attachments

0001-Change-copyObject-to-use-typeof_unqual.patch (text/plain) patch 0001

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?