Path: utzoo!telly!attcan!uunet!bu-cs!bloom-beacon!oberon!ucla-cs!rjc
From: rjc@maui.cs.ucla.edu
Newsgroups: gnu.g++.bug
Subject: G++ 1.34.1 named return values (no fix)
Message-ID: <21944@shemp.CS.UCLA.EDU>
Date: 17 Mar 89 21:50:55 GMT
Sender: news@CS.UCLA.EDU
Reply-To: rjc@CS.UCLA.EDU ()
Distribution: gnu
Organization: UCLA Computer Science Department
Lines: 94

rjc@cs.ucla.edu
Vax 8350, Ultrix 2.2-1 System #4
G++ 1.34.1 (plus patched cplus-decl.c) Gcc 1.34

>[ Patch from Michael for cplus-decl.c ... ]
>  
>yahi% 
>================================================================
>Here is the test program which now compiles:
>
>struct A {
>	int i;
>	A(int x) { printf("A(%d)\n", x); i = x; };
>	A(A& x) { printf("A(A& x), x.i == %d\n", x.i); i = x.i; };
>	~A(void) { printf("~A(%d)\n", i); };
>	void operator += (A& x) { printf("+=(%d, %d)\n", i, x.i); i += x.i; };
>};
>
>A foo (void) return tmp = 4;
>{
>	printf("foo(void), tmp.i == %d\n", tmp.i);
>	return tmp;
>}
>
>main()
>{
>	A bar = 5;
>	bar += foo();
>	printf("main(), bar.i == %d\n", bar.i);
>}
>================================================================
>And here are the results:
>
>yahi% a.out
>A(5)
>A(4)			<- ******** ~A() never called
>foo(void), tmp.i == 4
>A(A& x), x.i == 4	<- ******** Shouldn't be called
>+=(5, 4)
>~A(4)
>main(), bar.i == 9
>~A(9)
>yahi% 
>================================================================
>
>Michael

Michael,

Yes, the patch allows the test program to compile.  Unfortunately, the
code that is produced is badly broken.  You can see in the above output
that A(A&) is called on return from foo().  This is not supposed to
happen with a named return (as I understand it).  This is not too bad,
just the same old inefficiency that we have been suffering with up to
now.  The *big* problem is that ~A() is not called for the `tmp' in foo(),
but only for the copy produced by the call to A(A&) on return from foo().

I an not familiar with the interals of g++, so I cannot provide a fix.

Thanks,

rob collins
(rjc@cs.ucla.edu)




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rjc@cs.ucla.edu
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