Xref: utzoo comp.lang.c:13934 comp.lang.c++:1915
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From: turner@sdti.UUCP (Prescott K. Turner)
Newsgroups: comp.lang.c,comp.lang.c++
Subject: Re: Something new for C?
Summary: offsetof
Keywords: offset structures
Message-ID: <326@sdti.UUCP>
Date: 9 Nov 88 01:18:34 GMT
References: <73@dsoft.UUCP>
Reply-To: turner@sdti.UUCP (0006-Prescott K. Turner, Jr.)
Organization: Software Development Technologies, Sudbury MA
Lines: 27

In article <73@dsoft.UUCP> root@dsoft.UUCP (Super user) writes:
>Maybe I've missed something in C, but One of the things I've never found a way
>to do and have always wanted, was a precompiler command to allow me to use
>the offset of an item into a structure.

>   val = offset(test,that);
ANSI C introduces the "offsetof" macro to do just what you ask.
    val = offsetof(test,that);

C++ has a closely related feature called pointers to members.
    int test::*val = &test::that;
    ... 
    test x;
    x.*val = 2;
Here, val is not an int, but a more strongly typed thing which can be used
only to point to offsets within a "struct test" that have type int.
"offsetof" appears simpler and more flexible to me.  The only rationale I can
see for pointers to members is that they make pointers to member functions
possible.  And I wonder whether it might not have been better to find a way
to use "offsetof" with member functions as well.

Sorry, I don't know which C compilers already have "offsetof".
--
Prescott K. Turner, Jr.
Software Development Technologies, Inc.
375 Dutton Rd., Sudbury, MA 01776 USA        (508) 443-5779
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