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Message-ID: <5784342.ZASKD2KPVS@tjmaciei-mobl5>
Date: Sun, 02 Nov 2025 16:56:02 -0800
From: Thiago Macieira <thiago@...ieira.org>
To: Arthur O'Dwyer <arthur.j.odwyer@...il.com>
Cc: Alejandro Colomar <alx@...nel.org>, Florian Weimer <fw@...eb.enyo.de>,
libc-alpha@...rceware.org, musl@...ts.openwall.com,
Jonathan Wakely <jwakely@...hat.com>
Subject: Re: realloci(): A realloc() variant that works in-place
On Sunday, 2 November 2025 15:55:53 Pacific Standard Time Arthur O'Dwyer wrote:
> But this whole (bunch of) thread(s) started because of Thiago's throwaway
> comment along the lines of "C++ doesn't care about realloc because realloc
> has a bad API," and I think this thread (these threads) are just driving
> that point home. If I wanted to reach a good allocator API, "I wouldn't
> start from here." I don't think one can design a good allocator API by
> making a ton of tiny patches on top of `malloc` and `realloc`. You have to
> design the API first, and then show how to implement `malloc` and `realloc`
> in terms of it. (Also, C++ couldn't use it without also redesigning
> `std::allocator`, which is almost just a thin wrapper around `malloc` and
> `free`. `std::allocator` doesn't even have a counterpart to `realloc` at
> the moment.)
Indeed, and that is something the C++ Committee will need to figure out. As it
stands, it won't be able to benefit from realloci. I have a few ideas, which we
can discuss back in std-proposals/discussion.
But QVector, QString and QByteArray will, because we *have* a different
allocator interface that would allow retrying with smaller values and fine-
tuning. In fact, QString and QByteArray already use realloc(), because we know
for sure the container's element type is most trivially relocatable: char and
char16_t. Other C++ libraries not using std::allocator will benefit from this
too. As will C libraries operating on arrays.
--
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
Principal Engineer - Intel Data Center - Platform & Sys. Eng.
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