Thread View: gnu.g++.bug
2 messages
2 total messages
Started by rjc
Fri, 17 Mar 1989 21:50
G++ 1.34.1 named return values (no fix)
Author: rjc
Date: Fri, 17 Mar 1989 21:50
Date: Fri, 17 Mar 1989 21:50
94 lines
2435 bytes
2435 bytes
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
This is what happens when I sit on my keyboard: AISBDFAORI:KHGEDSYTFBFISKVDJ MN
Next week: what happens when I throw my keyboard out the window!
G++ 1.34.1 named return values (and related 1.34.1 problems)
Author: Doug Lea
Date: Sat, 18 Mar 1989 13:39
Date: Sat, 18 Mar 1989 13:39
96 lines
3193 bytes
3193 bytes
I think Michael's documentation was wrong (or at least misleading). If
you have a named return value, you should *only* return via `return'
or by falling-off-the-edge. (Otherwise, in your example, it says to
make tmp out of itself using A(A&), which is not very sensible.) See
below. [Anyone wanting further rationale for this construct should see
my recent comp.lang.c++ postings.]
A few other notes about my experience with 1.34.1+ (1.34.1 plus Michael's
2 posted patches) on a vax750, 4.3BSD:
The -felide-constructors switch seems buggy. All libg++ problems I've
been able to diagnose when testing this switch appear due to the kind
of local-variable-trashing behavior shown below. Note, however, that
constructs both of the form `var a = fun()' and `var a(fun())' now, in
1.34.1, produce the same results (no temps!) with or without this switch.
Using the -fsave-memoized switch also seems buggy. Several libg++
tests (test5, intList.cc) fail to compile with this switch.
By the way, in the example, if you #include <stream.h>, you ought to
use `cout.form(...)' (or `cout << form(...)') instead of `printf' to
ensure proper flushing of output on program termination (on most
machines, this apparently only matters if you are redirecting output).
-Doug
------ file A.cc (a revision of rjc's example)
#include <stream.h>
struct A {
int i;
A(int x) { cout.form("A(%d)\n", x); i = x; };
A(A& x) { cout.form("A(A& x), x.i == %d\n", x.i); i = x.i; };
~A(void) { cout.form("~A(%d)\n", i); };
void operator += (A& x) { cout.form("+=(%d, %d)\n", i, x.i); i += x.i; };
};
A foo () return tmp(4); // essentially equivalent to `tmp=4',
// but explicitly use a constr. for clarity!
{
cout.form("foo(void), tmp.i == %d\n", tmp.i);
return; // *not* `return tmp'
}
main()
{
A bar = 5;
bar += foo();
cout.form("main(), bar.i == %d\n", bar.i);
A baz = foo();
cout.form("main(), baz.i == %d\n", baz.i);
A baz_con(foo());
cout.form("main(), baz_con.i == %d\n", baz_con.i);
}
-------- A correct compile/run without -felide-constructors
R>g++ -g -O -finline-functions A.cc
R>a.out
A(5)
A(4)
foo(void), tmp.i == 4
+=(5, 4)
~A(4)
main(), bar.i == 9
A(4)
foo(void), tmp.i == 4
main(), baz.i == 4
A(4)
foo(void), tmp.i == 4
main(), baz_con.i == 4
~A(4)
~A(4)
~A(9)
----------- with -felide-constructors
R>g++ -g -O -finline-functions -felide-constructors A.cc
R>a.out
A(5)
A(4)
foo(void), tmp.i == 4
+=(4, 4) # `bar' is trashed!
main(), bar.i == 8
A(4)
foo(void), tmp.i == 4
main(), baz.i == 4
A(4)
foo(void), tmp.i == 4
main(), baz_con.i == 4
~A(4)
~A(4)
~A(8)
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