Path: utzoo!attcan!uunet!ncrlnk!ncrcae!ece-csc!mcnc!rutgers!mailrus!tut.cis.ohio-state.edu!cs.utexas.edu!sm.unisys.com!ism780c!haddock!karl
From: karl@haddock.ima.isc.com (Karl Heuer)
Newsgroups: comp.lang.c
Subject: Re: Passing types to a subroutine
Message-ID: <10600@haddock.ima.isc.com>
Date: 11 Nov 88 21:30:47 GMT
References: <14461@mimsy.UUCP> <32119@bbn.COM>
Reply-To: karl@haddock.ima.isc.com (Karl Heuer)
Organization: Interactive Systems, Boston
Lines: 20

In article <32119@bbn.COM> mesard@bbn.com (Wayne Mesard) writes:
>Or, how 'bout using a good old union to get rid of the extra param:
>   union ptr { float *f; double *d; };
>   main() {
>       float f[10]; double d[10];
>       sub(0, f); sub(1, d);
>   }
>   void sub(t, p) enum whichtype t; union ptr p; { ... }

This is not correct code.  The second parameter is declared to be `union ptr',
but the type actually passed is one of its member types.  This will fail if,
for example, the implementation passes scalars on the stack and structs/unions
with some other mechanism.  To do this `right', you'd have to declare a union
object, copy the value `f' or `d' into its appropriate member, and then pass
the union as the actual argument.

(Also, since you've already gone to the trouble of declaring an enum, you
should probably say `TYPE_FLOAT' or `TYPE_DOUBLE' instead of `0' or `1'.)

Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint