Message-ID: <fad7228e-a920-4ebb-973c-a48cc5066b71@gmail.com>
Date: Mon, 8 Dec 2025 09:33:10 +0100
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Subject: Re: Make copyObject work in C++
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>
References: 
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 <1bec56e5-aef9-4d3b-b4ee-723c6575a44e@eisentraut.org>
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From: David Geier <geidav.pg@gmail.com>
In-Reply-To: <1bec56e5-aef9-4d3b-b4ee-723c6575a44e@eisentraut.org>
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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