Path: utzoo!attcan!uunet!husc6!mailrus!cwjcc!hal!nic.MR.NET!tank!mimsy!sjr
From: sjr@mimsy.UUCP (Stephen J. Roznowski)
Newsgroups: comp.lang.c
Subject: Re: Passing types to a subroutine
Message-ID: <14549@mimsy.UUCP>
Date: 14 Nov 88 23:18:54 GMT
References: <14461@mimsy.UUCP> <32119@bbn.COM> <10600@haddock.ima.isc.com>
Reply-To: sjr@mimsy.umd.edu.UUCP (Stephen J. Roznowski)
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 34

In article <10600@haddock.ima.isc.com> karl@haddock.ima.isc.com (Karl Heuer) writes:
>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.

I have a problem with using unions. (actually two problems.)

One is since I do not know the type until run time, I need
to create a union that contains EVERY possible type.  Now
what happens if I want to pass an array that is inside of an
array of structures? (for example)

Second, it would not be unreasonable to have someone
call this routine with a 10,000 element array of 128 bit
double floating point values.  I do not want to have to copy
this data at any time, and total storage (in memory) needs
to be kept as low as possible. [Of course these subroutines
need to be as fast as possible]

Stephen
-- 
Stephen J. Roznowski                 sjr@mimsy.umd.edu