Xref: utzoo comp.lang.c:13953 comp.lang.c++:1921
Path: utzoo!utgpu!watmath!clyde!bellcore!faline!sword!gamma!ulysses!andante!alice!ark
From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c,comp.lang.c++
Subject: Re: Something new for C?
Keywords: offset of vars within structures
Message-ID: <8402@alice.UUCP>
Date: 8 Nov 88 05:48:01 GMT
References: <73@dsoft.UUCP>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 48

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. for example:


> struct test {
>    long this;
>    int  that;
>    char those[8];
> };
 
>    val = offset(test,that);

C++ has it -- it's called a `pointer to member'.

For example:

	struct test {
		long here;	// `this' is a keyword in C++
		int that;
		char those[8];
	};

Now, you can declare `testp' to be a pointer to an (unspecified)
int element of an (unspecified) test structure:

	int test::*testp;

Of course in this example, there's only one member testp could
possibly point to; let's make it point there:

	testp = &test::that;

Now, let's declare a `test' object:

	test t;

Finally, we'll set the field of t addressed by testp to 7:

	t.*testp = 7;

To learn many more details about member pointers, see the paper
by Lippman and Stroustrup in the proceedings of the 1988 USENIX
C++ conference.
-- 
				--Andrew Koenig
				  ark@europa.att.com