Re: Converting from C style struct offset to C++ style member pointer

From:

Jorgen Grahn <grahn+nntp@snipabacken.se>

Newsgroups:

comp.lang.c++

Date:

23 Mar 2014 21:22:09 GMT

Message-ID:

<slrnliuk40.hu4.grahn+nntp@frailea.sa.invalid>

On Sun, 2014-03-23, Asger Joergensen wrote:

Hi Marcel
%
Marcel M?ller wrote:

When I get a struct offset from a C style API as int. I.e.:

struct S
{ int I1;
double D1;
int I2;
};

int offset = offsetof(S, I2);

Now in C++ code I would like to convert this offset to a
type safe C++ member
pointer of type int S::*. Older gcc versions simply accepted a cast, newer
version reject the cast.

I am curious, where do you use this offsetof
I mean in what situations are
you using it ?
I've been programming for 15+ years and it is the first time I see this
offsetof define, usually I just cast to the struct name and then address
it's members by their name, leaving the offset part to the compiler.

Would you be so kind and explain the need and give a couple of
small examples ?

Download the Linux kernel sources; it's used there. I can't remember
what for right now, but I think it was for container types or run-time
polymorphism, i.e. things supported without such hacks in C++.

/Jorgen

--
// Jorgen Grahn <grahn@ Oo o. . .
\X/ snipabacken.se> O o .