Re: Nice way of allocating flexible struct.

From	Michael S <already5chosen@yahoo.com>
Newsgroups	comp.lang.c
Subject	Re: Nice way of allocating flexible struct.
Date	2025-10-08 12:53 +0300
Organization	A noiseless patient Spider
Message-ID	<20251008125301.000044e0@yahoo.com> (permalink)
References	<20251007233002.852@kylheku.com> <10c59nc$1frvi$1@raubtier-asyl.eternal-september.org> <10c5ah6$1frvi$2@raubtier-asyl.eternal-september.org>

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On Wed, 8 Oct 2025 11:23:13 +0200
Bonita Montero <Bonita.Montero@gmail.com> wrote:

> Am 08.10.2025 um 11:09 schrieb Bonita Montero:
> > Am 08.10.2025 um 08:35 schrieb Kaz Kylheku:  
> >> Jonas Lund of https://whizzter.woorlic.org/ mentioned this
> >> trick in a HackerNews comment:
> >>
> >> Given:
> >>
> >>    struct S {
> >>      // ...
> >>      T A[];
> >>    };
> >>
> >> Don't do this:
> >>
> >>    malloc(offsetof(S, A) + n * sizeof (T));
> >>
> >> But rather this:
> >>
> >>    malloc(offsetof(S, A[n]));
> >>
> >> It's easy to forget that the second argument of offsetof is a
> >> designator, not simply a member name.
> >>  
> > 
> > In a real language:
> > 
> > #include <iostream>
> > #include <optional>
> > #include <array>
> > 
> > using namespace std;
> > 
> > template<typename T, typename Derived>
> > struct flex_base
> > {
> >      T &operator []( size_t i )
> >      {
> >          return static_cast<Derived &>( *this ).m_arr[i];
> >      }
> >      virtual ~flex_base() {};
> > };
> > 
> > template<typename T, size_t N>
> > struct flex_array : flex_base<T, flex_array<T, N>>
> > {
> >      virtual ~flex_array() {};
> > private:
> >      template<typename T, typename Derived>
> >      friend struct flex_base;
> >      std::array<T, N> m_arr;
> > };
> > 
> > 
> > int main()
> > {
> >      auto &fb = *new flex_array<string, 100>();
> >      for( size_t i = 0; i != 100; ++i )
> >          fb[i] = "hello world";
> > }
> > 
> > Somewhat more complicated to declare, but much shorter and
> > more readable usage.
> > C really sucks.  
> 
> OMG, I was blind:
> 
> 	T * new T[N];

You don't understand the meaning of the word 'flexible'.
The whole point of it is that N is unknown at compile time.

Formally speaking, flexible array members are not supported in
inferior tongue that you call "real language" although they can be
emulated and in practice will work with any production-quality
compiler.
However, if I am not mistaken, it works just because implementors are
sane people, rather than because the language itself provides sane
guarantees.

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Nice way of allocating flexible struct. Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-08 06:35 +0000
  Re: Nice way of allocating flexible struct. pozz <pozzugno@gmail.com> - 2025-10-08 09:09 +0200
    Re: Nice way of allocating flexible struct. richard@cogsci.ed.ac.uk (Richard Tobin) - 2025-10-08 12:01 +0000
    Re: Nice way of allocating flexible struct. Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-08 15:23 +0000
      Re: Nice way of allocating flexible struct. Michael S <already5chosen@yahoo.com> - 2025-10-08 19:04 +0300
        Re: Nice way of allocating flexible struct. Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-08 20:05 +0000
    Re: Nice way of allocating flexible struct. Michael S <already5chosen@yahoo.com> - 2025-10-08 18:52 +0300
    Re: Nice way of allocating flexible struct. Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-12-14 23:55 -0800
  Re: Nice way of allocating flexible struct. Bonita Montero <Bonita.Montero@gmail.com> - 2025-10-08 11:09 +0200
    Re: Nice way of allocating flexible struct. Bonita Montero <Bonita.Montero@gmail.com> - 2025-10-08 11:23 +0200
      Re: Nice way of allocating flexible struct. Michael S <already5chosen@yahoo.com> - 2025-10-08 12:53 +0300
        Re: Nice way of allocating flexible struct. Bonita Montero <Bonita.Montero@gmail.com> - 2025-10-08 12:09 +0200
          Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-08 15:59 +0200
            Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-08 12:29 -0500
              Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-08 21:04 +0200
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-08 22:49 -0500
                Re: Nice way of allocating flexible struct. bart <bc@freeuk.com> - 2025-10-10 01:13 +0100
                Re: Nice way of allocating flexible struct. Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-10 01:54 +0000
                Re: Nice way of allocating flexible struct. "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-10-09 19:43 -0700
                Re: Nice way of allocating flexible struct. bart <bc@freeuk.com> - 2025-10-10 11:25 +0100
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-09 22:50 -0500
                Re: Nice way of allocating flexible struct. Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-10-09 20:59 -0700
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-10 01:27 -0500
                Re: Nice way of allocating flexible struct. David Brown <david.brown@hesbynett.no> - 2025-10-10 12:06 +0200
                Re: Nice way of allocating flexible struct. Michael S <already5chosen@yahoo.com> - 2025-10-10 17:28 +0300
                Re: Nice way of allocating flexible struct. David Brown <david.brown@hesbynett.no> - 2025-10-10 17:47 +0200
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-10 16:32 -0500
                Re: Nice way of allocating flexible struct. Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-11 00:02 +0000
                Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-13 06:20 +0200
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-10 15:01 -0500
                Re: Nice way of allocating flexible struct. Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-10 23:45 +0000
                Re: Nice way of allocating flexible struct. Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-01-06 18:24 -0800
                Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-14 06:29 +0200
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-14 20:13 -0500
                Re: Nice way of allocating flexible struct. bart <bc@freeuk.com> - 2025-10-15 11:26 +0100
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-15 13:00 -0500
                Re: Nice way of allocating flexible struct. bart <bc@freeuk.com> - 2025-10-17 22:07 +0100
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-17 17:44 -0500
                Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-20 10:02 +0200
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-20 04:42 -0500
                Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-21 03:40 +0200
                Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-16 06:45 +0200
                Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-16 06:37 +0200
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-16 04:43 -0500
                Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-20 09:58 +0200
                Re: Nice way of allocating flexible struct. rbowman <bowman@montana.com> - 2025-10-20 18:36 +0000
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-20 16:44 -0500
                Re: Nice way of allocating flexible struct. rbowman <bowman@montana.com> - 2025-10-21 01:33 +0000
                Language-design, tradeoffs (was Re: Nice way of allocating flexible struct.) Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-21 04:19 +0200
                Re: Language-design, tradeoffs (was Re: Nice way of allocating flexible struct.) BGB <cr88192@gmail.com> - 2025-10-21 04:27 -0500
                Re: Language-design, tradeoffs (was Re: Nice way of allocating flexible struct.) Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-22 02:30 +0200
                Re: Language-design, tradeoffs (was Re: Nice way of allocating flexible struct.) BGB <cr88192@gmail.com> - 2025-10-22 02:10 -0500
                Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-21 03:51 +0200
                Re: Nice way of allocating flexible struct. Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-10-20 19:21 -0700
                Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-21 04:53 +0200
                Re: Nice way of allocating flexible struct. rbowman <bowman@montana.com> - 2025-10-21 18:21 +0000
                Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-21 13:42 -0500
                Re: Nice way of allocating flexible struct. James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-10-21 06:17 -0400
                Re: Nice way of allocating flexible struct. rbowman <bowman@montana.com> - 2025-10-21 18:41 +0000
                Re: Nice way of allocating flexible struct. rbowman <bowman@montana.com> - 2025-10-21 18:12 +0000
            Re: Nice way of allocating flexible struct. Bonita Montero <Bonita.Montero@gmail.com> - 2025-10-08 19:36 +0200
              Re: Nice way of allocating flexible struct. Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-08 19:51 +0200
    Re: Nice way of allocating flexible struct. Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-08 15:29 +0000
  Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-08 11:33 -0500
    Re: Nice way of allocating flexible struct. Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-10-08 14:57 -0700
      Re: Nice way of allocating flexible struct. Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-09 01:39 +0000
        Re: Nice way of allocating flexible struct. BGB <cr88192@gmail.com> - 2025-10-08 22:25 -0500
          Re: Nice way of allocating flexible struct. Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-10-09 19:50 -0700
            Re: Nice way of allocating flexible struct. Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-10 04:20 +0000
      Re: Nice way of allocating flexible struct. Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-12-15 11:24 -0800
  Re: Nice way of allocating flexible struct. "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-10-08 13:35 -0700
    Re: Nice way of allocating flexible struct. "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-10-08 13:36 -0700
  Re: Nice way of allocating flexible struct. Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-12-14 22:48 -0800

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