Something new for C?

[Andrew Koenig]

In article <73 at dsoft.UUCP>, root at 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 at europa.att.com