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Linker error message is a triumph of misdirection

From: Edward Welbourne
Subject: Linker error message is a triumph of misdirection
Date: Tue, 07 Sep 2004 15:41:34 +0200

Previously submitted to gccbug, who point out that there is nothing
the compiler can do about it; this is a linker issue; in their system
it was [Bug c++/17338].  The g++-bug report follows.  Basic problem is
that the linker's complaint doesn't address the right problem.
Priority is modest.  ld --version says

GNU ld version 20031029 Debian GNU/Linux
Copyright 2002 Free Software Foundation, Inc.
This program is free software; you may redistribute it under the terms of
the GNU General Public License.  This program has absolutely no warranty.

        Feed g++ the following erroneous code:

class Base { public: virtual void Method(void) /* = 0 */; }; // missing text
class Derived : public Base { public: virtual void Method(void) {} };
int main(void)
        Derived thing;
        Base *ptr = &thing;
        return 0;

        You will get an error message which makes no mention of the actual
        problem (no definition of virtual function in base class, nor does the
        base class overtly = 0 it):

g++ -Wall    virtual.cpp   -o virtual
/usr/local/home/eddy/.sys/tmp/ccAtrV2C.o(.gnu.linkonce.t._ZN4BaseC2Ev+0x8): In 
function `Base::Base[not-in-charge]()':
: undefined reference to `vtable for Base'
undefined reference to `typeinfo for Base'
collect2: ld returned 1 exit status

The complaint directs me to the (implicit) constructor; I have seen it
attribute this to a line in a header file from a third-party library.
I have seen this (on larger and more complex code) without the message
about typeinfo for Base being absent; only the vtable's absence was
reported.  Some users of g++ might not know what a vtable is.

System: Linux whorl 2.4.26-1-686-smp #1 SMP Sat May 1 19:17:11 EST 2004 i686 
Architecture: i686
host: i486-pc-linux-gnu
build: i486-pc-linux-gnu
target: i486-pc-linux-gnu
configured with: ../src/configure -v 
--enable-languages=c,c++,java,f77,pascal,objc,ada,treelang --prefix=/usr 
--mandir=/usr/share/man --infodir=/usr/share/info 
--with-gxx-include-dir=/usr/include/c++/3.3 --enable-shared --with-system-zlib 
--enable-nls --without-included-gettext --enable-__cxa_atexit 
--enable-clocale=gnu --enable-debug --enable-java-gc=boehm 
--enable-java-awt=xlib --enable-objc-gc i486-linux

           Summary: Linker error message is a triumph of misdirection
           Product: gcc
           Version: 3.3.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: eddy at opera dot com
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i486-pc-linux-gnu
  GCC host triplet: i486-pc-linux-gnu
GCC target triplet: i486-pc-linux-gnu


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