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Re: Allocating memory styles

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jt@toerring.de (Jens Thoms Toerring) writes:

> janus <emekamicro@gmail.com> wrote:
>> I am going through a book and I stumbled on this class of memory allocation:
>
>> void* p = calloc(1, class->size);
>
>> *(const struct Class**) P = class;
>
>> The last line is what is hard for me to understand. Why casting to
>> pointer-to-pointer, and then dereference it immediately?  My
>> understanding is that struct Class is the first element in P struct,
>> so by casting P struct by struct class, one immediately has access
>> to the first element of the struct. This makes sense to me. But why
>> using pointer-to-pointer?
>
>> I was thinking that the same thing might be achieved by this:
>
>> (const struct Class*) P = class;
>
> No, with that you'd have an rvalue on the left hand side -
> the result of a cast is a value, the value of what's stored
> in 'p', but with a different type attached to it, and not a
> (new) variable that you can assign to (try it out and see
> what the compiler is going to tell you).
>
> But the original code is rather strange. 'class' obviously
> must be a pointer (to some kind of structure), otherwise
> 'class->size' would be a syntax errer. And 'p' seems to be
> intended to be used as another pointer to such a structure,
> or the second line makes no sense.

My guess: P is supposed to point to an "object" of the class described
by the structure pointed to by the class variable.  The very first
member of such an object is a pointer to it's class description.  That's
my guess at least.  I've seen that sort of code about dozen times
because there was a time when every one write their own object system in
C.

<snip>
-- 
Ben.

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Allocating memory styles janus <emekamicro@gmail.com> - 2015-07-20 05:36 -0700
  Re: Allocating memory styles Noob <root@127.0.0.1> - 2015-07-20 14:53 +0200
    Re: Allocating memory styles janus <emekamicro@gmail.com> - 2015-07-20 07:23 -0700
      Re: Allocating memory styles Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-20 16:40 +0100
        Re: Allocating memory styles janus <emekamicro@gmail.com> - 2015-07-20 12:19 -0700
  Re: Allocating memory styles Richard Heathfield <rjh@cpax.org.uk> - 2015-07-20 14:07 +0100
  Re: Allocating memory styles Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-20 14:18 +0100
  Re: Allocating memory styles jt@toerring.de (Jens Thoms Toerring) - 2015-07-20 13:47 +0000
    Re: Allocating memory styles Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-20 15:21 +0100
  Re: Allocating memory styles Johann Klammer <klammerj@NOSPAM.a1.net> - 2015-07-20 16:27 +0200
  Re: Allocating memory styles luser droog <luser.droog@gmail.com> - 2015-07-20 12:34 -0700
    Re: Allocating memory styles Keith Thompson <kst-u@mib.org> - 2015-07-20 14:00 -0700
      Re: Allocating memory styles luser droog <luser.droog@gmail.com> - 2015-07-20 20:29 -0700
        Re: Allocating memory styles raltbos@xs4all.nl (Richard Bos) - 2015-07-22 15:21 +0000
    Re: Allocating memory styles janus <emekamicro@gmail.com> - 2015-07-20 14:13 -0700
  Re: Allocating memory styles "Chris M. Thomasson" <nospam@nospam.nospam> - 2015-07-20 14:54 -0700
    Re: Allocating memory styles "Chris M. Thomasson" <nospam@nospam.nospam> - 2015-07-20 14:57 -0700

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