Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!rutgers!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: <21850@shemp.CS.UCLA.EDU>
Date: 16 Mar 89 18:31:35 GMT
Sender: news@CS.UCLA.EDU
Reply-To: rjc@CS.UCLA.EDU ()
Distribution: gnu
Organization: UCLA Computer Science Department
Lines: 138

G++ 1.34.1 with GCC 1.34
Vax 8350, Ultrix V2.2-1 System #4
config.g++ vax

I tried using named return values.  It didn't seem to work as advertised.
Is it broken, or was the advertisement wrong?

>From ucla-cs!oberon!bloom-beacon!tut.cis.ohio-state.edu!YAHI.STANFORD.EDU!mdt Thu Mar 16 09:53:32 PST 1989
>[...]
>
>GNU C++ 1.34.1 also has three new experimental features.
>
>1. Named return values.  Example:
>
>struct A { ... A (); A (A&); ... };
>
>// old way
>A foo ()
>{
>  A tmp;	// calls A() constructor
>
>  ... things which modify `tmp' ...
>  return tmp;	// calls A(A&) constructor
>}
>
>// new way
>A foo () return tmp;	// calls A () constructor on entry
>{
>  ... things which modify `tmp' ...
>  return tmp;	// call to A (A&) constructor not needed
>}
>[...]

Following this description, we are told that tmp is not declared:

bug.cc:
----------------------------------------------------------
/* test named return values
 * Rob Collins (rjc@cs.ucla.edu)
 */

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);
}
----------------------------------------------------------

Script started on Thu Mar 16 10:27:42 1989
% g++ -v bug.cc
g++ version 1.34.1
 /usr/local/lib/gcc-cpp -+ -v -undef -D__GNU__ -D__GNUG__ -Dvax -Dunix -D__vax__ -D__unix__ bug.cc /tmp/cc007922.cpp
GNU CPP version 1.34
 /usr/local/lib/gcc-c++ /tmp/cc007922.cpp -quiet -dumpbase bug.cc -noreg -version -o /tmp/cc007922.s
GNU C++ version 1.34.1 (vax) compiled by GNU C version 1.34.
In function foo ():
bug.cc:15: `tmp' was not declared (first use this function)
bug.cc:15: (Each undeclared identifier is reported only once
bug.cc:15: for each function it appears in.)
bug.cc:16: `tmp' was not declared (first use this function)
% ^D
script done on Thu Mar 16 10:27:51 1989

Adding a declaration of tmp inside the function foo results in successful
compilation:

bug.cc:
----------------------------------------------------------
/* test named return values
 * Rob Collins (rjc@cs.ucla.edu)
 */

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;
{
	A tmp = 1;
	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);
}
----------------------------------------------------------

Script started on Thu Mar 16 10:28:45 1989
% g++ -v bug.cc
g++ version 1.34.1
 /usr/local/lib/gcc-cpp -+ -v -undef -D__GNU__ -D__GNUG__ -Dvax -Dunix -D__vax__ -D__unix__ bug.cc /tmp/cc007930.cpp
GNU CPP version 1.34
 /usr/local/lib/gcc-c++ /tmp/cc007930.cpp -quiet -dumpbase bug.cc -noreg -version -o /tmp/cc007930.s
GNU C++ version 1.34.1 (vax) compiled by GNU C version 1.34.
 gas /tmp/cc007930.s -o bug.o
 /usr/local/lib/gcc-ld++ -C /usr/local/lib/crt0+.o bug.o -lg++ /usr/local/lib/gcc-gnulib -lc
% a.out
A(5)
A(1)
foo(void), tmp.i == 1
A(A& x), x.i == 1
~A(1)
+=(5, 1)
~A(1)
main(), bar.i == 6
~A(6)
% ^D
script done on Thu Mar 16 10:29:12 1989

In foo(), the return value constructor is not called.  In addition, tmp
is copied on return from foo().
-------------------------------------------------------------------------------
rjc@cs.ucla.edu
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