Bug#204580: marked as done (g++: unexpected output using variadic functions in classes)
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and subject line Bug#204580: g++: unexpected output using variadic functions in classes
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From: "Frank B. Brokken" <f.b.brokken@rc.rug.nl>
To: Debian Bug Tracking System <submit@bugs.debian.org>
Subject: g++: unexpected output using variadic functions in classes
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Package: g++
Version: 3:3.3-2
Severity: normal
-- System Information
Debian Release: 3.0
Architecture: i386
Kernel: Linux suffix 2.4.21 #1 Fri Aug 1 13:53:33 CEST 2003 i686
Locale: LANG=C, LC_CTYPE=C
Versions of packages g++ depends on:
ii cpp 3:3.3-2 The GNU C preprocessor.
ii g++-3.3 1:3.3.1-1 The GNU C++ compiler
ii gcc-3.3 1:3.3.1-1 The GNU C compiler
Hi,
The following little program produces unexpected output. As an
alternate compiler (as a check I used the Intel C++ compiler version 7.0) did
not produce the problem, I assume I ran into a little bug; hence the
bugreport. The problem is described in some detail beyond the program's
source:
=====================================================================
#include <iostream>
#include <cstdarg>
using namespace std;
class Foo
{
va_list list;
public:
Foo(int n, ...)
{
va_start(list, n);
}
void bar() const
{
va_list l(list);
cout << "Saw " << va_arg(l, int) << endl;
}
};
void two(int start, Foo foo)
{
for (int idx = 0; idx < 2; idx++)
foo.bar();
}
void three(int start, int end, Foo foo)
{
for (int idx = 0; idx < 2; idx++)
foo.bar();
}
int main(int argc, char **argv)
{
Foo foo(3, 15);
two(2, foo);
three(2, 2, foo); // unexpected output from here.
two(2, foo);
return 0;
}
==================================================================
Problem description:
The clas `Foo' has a constructor accepting variable numbers of arguments. It
initializes a `va_list' data member which is used to initialize a local
`va_list' variable inside its `bar()' function. The `bar()' function then
prints the first of the variable arguments, which is, in this program, always
an `int' value.
The function `two()' receives two value-type arguments: an `int' and a `Foo'
object: the `Foo' object is therefore copied internally to initialize the
parameter `foo'. The `int' argument is not used, and `foo.bar()' is called
twice.
The function `three()' receives three value-type arguments, two `int's and a
`Foo' object. Both `int' arguments are not used, and `foo.bar()' is called
twice, like in the function `two()'.
In `main()' a `Foo' object is constructed, receiving 2 arguments. The second
value, 15, is the `int' to be accessed by `Foo::bar()'.
At the first call of `two()', 15 is displayed, as expected. Then, unexpected
values are displayed by the call of `three()'. Therafter, the problem
persists: calling `two()' for the second time now also produces unexpected
values.
On my computer I get the following output:
Saw 15
Saw 15
Saw -1073743160
Saw -1073743160
Saw -1073743160
Saw -1073743160
As mentioned, when the program is compiled by, e.g., the Intel V 7.0 compiler
the problem does not appear. In that case, all displayed values are 15, as
expected.
Hopefully the problem can be solved. If I can assist, please let me know.
Kind regards,
Frank B. Brokken.
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To: 204580-done@bugs.debian.org
Subject: Re: Bug#204580: g++: unexpected output using variadic functions in classes
References: <[🔎] 200308081050.h78Ao69u021080@suffix.rc.rug.nl>
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<20030808143015.GA25957@rc.rug.nl>
From: Falk Hueffner <falk.hueffner@student.uni-tuebingen.de>
Date: 08 Aug 2003 16:44:51 +0200
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Submitter says he can't reproduce it with non-invalid code, so closing
--
Falk
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