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From: pete@kvvax4.UUCP (Peter J Story)
Newsgroups: net.lang.c
Subject: Re: Function returning pointer to function ? (80 lines)
Message-ID: <635@kvvax4.UUCP>
Date: Thu, 18-Jul-85 15:19:46 EDT
Article-I.D.: kvvax4.635
Posted: Thu Jul 18 15:19:46 1985
Date-Received: Sun, 21-Jul-85 01:11:50 EDT
References: <11582@brl-tgr.UUCP>
Organization: Kongsberg Vaapenfabrikk, Defence Division, Norway
Lines: 81

> Replies to me please, unless you feel the net needs a diversion
> from the debating on matters of style and efficiency.

It certainly does!  In any case that ARPA gap always defeats my mail.

> Is it legal for a function to return a pointer to a function?

Yes, here is some playing that I did, together with a similar problem
which I have never solved.  Maybe someone can help.

/* I had better apologise for the cryptic naming, but in the     */
/* interest of compactness...  The basic rule for declarations   */
/* is to try to figure out how you would access the basic object */
/* from an object of the type you are trying to declare, and     */
/* make the declaration look like that.  It often helps to build */
/* it up from smaller units, thus:                               */

int a = 6;
int fi() {return(a);}		    /* fn returning an int */
int *fpi() {return(&a);}	    /* fn returning ptr to int */
int (*pfi)() = fi;		    /* ptr to fn returning int */

/* Now the tricky bit.  Imagine I had a function which returned  */
/* a pointer to a function returning an int.  Then I could       */
/* assign its result to pfi, thus: pfi = gfpfi();                */
/* Put this in the declaration of pfi above, and you are there   */

int (*gfpfi())() {return(pfi);};    /* fn returning ptr to fn returning int */

/* a step further? */

int *(*pfpi)() = fpi;		    /* ptr to fn returning ptr to int */
int *(*gfpfpi())() {return(pfpi);}; /* fn rtning ptr to fn rtning ptr to int */

main()
{
   int x;
   x = (*pfi)();	printf("%d",x);
   x = *(*pfpi)();	printf("%d",x);
   x = (*gfpfi())();	printf("%d",x);
   x = *(*gfpfpi())();	printf("%d",x);
   printf("\n");
   myproblem();  /* see below !! */
}

/* ---------------------------------------------------------- */

/* Now my turn to ask a question.  Here are the declarations: */
 
char *f1()  { return("1"); }
char *f2()  { return("2"); }

/* decl array of ptrs to fns returning ptr to char */
char *(*funcs[2])() = {f1,f2};

/* decl ptr to array of ptrs to fns returning ptr to char */
char *(*(*pfuncs)[])() = funcs; 

/* "junk.c", line 47: warning: illegal pointer combination        */
/* I can't get that to compile without a warning but it works ok. */

/* It should be analogous to this, which also won't compile      */
/* without a warning.						 */
char cd[2] = {'c','d'};
char (*pcd)[] = cd;
/* "junk.c", line 55: warning: illegal pointer combination        */

/* What is it that the compiler (4.1 BSD) doesn't like?          */

myproblem()
{
   int i;
   for (i=0; i<2; i++) printf("%c",*(*funcs[i])());	/* 12 */
   for (i=0; i<2; i++) printf("%c",*(*(*pfuncs)[i])());	/* 12 */
   printf("%c",(*pcd)[0]);				/* c */
   printf("\n");
}
-- 
Pete Story	      {decvax,philabs}!mcvax!kvport!kvvax4!pete
A/S Kongsberg Vaapenfabrikk, PO Box 25, N3601 Kongsberg, Norway
Tel:  + 47 3 739644   Tlx:  71491 vaapn n