Subj : Re: Memory Pointers and Free
To   : borland.public.cpp.borlandcpp
From : Alistair
Date : Sun Jul 25 2004 02:47 pm

Interesting, a simple mistake of forgetting to write the casting from void *
to s_a * on the memory allocation in this quickly written example and I have
caused a raft of discussion on whether it would compile.  Including the
cast, it does compile and works perfectly fine in C++, with the compiler I
have (vc++6, vc++7 (.net) and borland c++ builder 4,5 + 6).

I thought it would cause a memory leak, but I wasnt too sure, thanks for the
help!

Oh, and I have never really done C, this is an interesting difference and
explains some of the code I have seen some people write in C++.

Many Thanks

Alistair


"Wayne A. King" <waking@idirect.com> wrote in message
news:41036c9b.6109911@newsgroups.borland.com...
> On Sun, 25 Jul 2004 01:27:49 +0100, =?ISO-8859-15?Q?Jo=E3o?= Paredes
> <joaommp@myrealbox.com> wrote:
>
> >> s_a *a = malloc(sizeof(s_a));
> >> a->somedata = malloc(512);
>
> >with the declaration for the structure and variable you posted, I don't
> >believe that would even compile.
>
> <chuckle> Interesting example of the differences between C and C++.
> You're right, it wouldn't compile. But for different reasons in the two
> languages.
>
> >If you create a type with "struct something
> >{...};" you would have to declare variables with "struct something
myvar;".
>
> True for C (C90), but not for C++. In the former, this is an error:
>  s_a *a = malloc(sizeof(s_a));
> It must be this for C90:
> struct s_a *a = malloc(sizeof(struct s_a));
>
> However, in C++ the original post is OK as the "struct" keyword
> may be omitted. However, in C++ the original example will fail
> for another reason - which doesn't apply to C90 - the void*
> returned by malloc must be typecast in C++ but may be
> omitted in C90. So C++ would look like this:
> s_a *a = (s_a*)malloc(sizeof(s_a));
>
> Note that C++ will also accept the explicit "struct" keyword,
> and C will accept the typecast. So a form which would pass
> both C and C++ would be like this:
> struct s_a *a = (struct s_a*)malloc(sizeof(struct s_a));
>
>
> --
> Wayne A. King
> (waking@idirect.com, Wayne_A_King@compuserve.com)

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