Meeting Pearls 1/Meeting Pearls Vol 1 (1994).iso/installed_progs/text/faqs/g++-faq.plain

Internet Message Format | 1994-05-17 | 41.9 KB

Subject: FAQ for g++ and libg++, plain text version [Revised 15 May 1994]
Newsgroups: gnu.g++.help,comp.lang.c++,news.answers,comp.answers
From: [e-mail] (Joe Buck)
Date: Sun, 15 May 1994 13:00:35 GMT

Archive-name: g++-FAQ/plain
Last-modified: 15 May 1994
Frequency: bimonthly

[ this is the plain text version, the parent is the texinfo version ]


Preface
*******

   This is a list of frequently asked questions (FAQ) for g++ users;
thanks to all those who sent suggestions for improvements.  Thanks to
Marcus Speh for doing the index.

   Many FAQ's, including this one, are available on the archive site
rtfm.mit.edu, in the directory `pub/usenet/news.answers'.  This FAQ may
be found in the subdirectory g++-FAQ.

   Some information in this FAQ was developed for earlier versions of
the compiler and may now be obsolete.  Please send corrections.

   I'm looking for new questions (*with* answers), better answers, or
both.  You can mail comments, suggestions, flames, etc. to
[e-mail].  Please don't assume, though, that because my name
is on this thing that I am the world expert on g++/C++ and you should
mail all your tricky questions to me.  I'd like to be helpful but I'm
getting more of this than I can deal with lately.  Also please don't
assume that I am an official spokesman for g++, FSF, or Cygnus; I'm not.

   This FAQ is intended to supplement, not replace, Marshall Cline's
excellent FAQ for the C++ language and for the newsgroup comp.lang.c++.
Especially if g++ is the first C++ compiler you've ever used, the
question "How do I do <X> with g++?" is probably really "How do I do
<X> in C++?".  You can obtain the C++ FAQ by anonymous FTP from
sun.soe.clarkson.edu [128.153.12.3], in the file `~ftp/pub/C++/FAQ'.
(There is also a mail server for that FAQ, but it seems to be broken).


Obtaining Source Code
*********************


How do I get a copy of g++ for Unix?
====================================

   First, you may already have it if you have gcc for your platform;
g++ and gcc are combined now (as of gcc version 2.0).

   You can get g++ from a friend who has a copy, by anonymous FTP or
UUCP, or by ordering a tape or CD-ROM from the Free Software Foundation.

   The Free Software Foundation is a nonprofit organization that
distributes software and manuals to raise funds for more GNU
development.  Getting your copy from the FSF contributes directly to
paying staff to develop GNU software.  CD-ROMs cost $400 if an
organization is buying, or $100 if an individual is buying.  Tapes cost
around $200 depending on media type.  I recommend asking for version 2,
not version 1, of g++.

   For more information about ordering from the FSF, contact
[e-mail], phone [phone] or anonymous ftp file
`/pub/gnu/GNUinfo/ORDERS' from prep.ai.mit.edu or one of the sites
listed below.

   Here is a list of anonymous FTP archive sites for GNU software.

     ASIA: ftp.cs.titech.ac.jp, utsun.s.u-tokyo.ac.jp:/ftpsync/prep,
          cair.kaist.ac.kr:/pub/gnu
     AUSTRALIA: archie.oz.au:/gnu (archie.oz or archie.oz.au for ACSnet)
     AFRICA: ftp.sun.ac.za:/pub/gnu
     MIDDLE-EAST: ftp.technion.ac.il:/pub/unsupported/gnu
     EUROPE: irisa.irisa.fr:/pub/gnu, grasp1.univ-lyon1.fr:pub/gnu,
          ftp.mcc.ac.uk, unix.hensa.ac.uk:/pub/uunet/systems/gnu,
          src.doc.ic.ac.uk:/gnu, ftp.win.tue.nl, ugle.unit.no, ftp.denet.dk,
          ftp.informatik.rwth-aachen.de:/pub/gnu, ftp.informatik.tu-muenchen.de,
          ftp.eunet.ch, nic.switch.ch:/mirror/gnu, nic.funet.fi:/pub/gnu,
          isy.liu.se, ftp.stacken.kth.se, ftp.luth.se:/pub/unix/gnu, archive.eu.net
     CANADA: ftp.cs.ubc.ca:/mirror2/gnu
     USA: wuarchive.wustl.edu:/mirrors/gnu, labrea.stanford.edu,
          ftp.kpc.com:/pub/mirror/gnu, ftp.cs.widener.edu, uxc.cso.uiuc.edu,
          col.hp.com:/mirrors/gnu, ftp.cs.columbia.edu:/archives/gnu/prep,
          gatekeeper.dec.com:/pub/GNU, ftp.uu.net:/systems/gnu

   The "official site" is prep.ai.mit.edu, but your transfer will
probably go faster if you use one of the above machines.

   Most GNU utilities are compressed with "gzip", the GNU compression
utility.  All GNU archive sites should have a copy of this program,
which you will need to uncompress the distributions.

   UUNET customers can get GNU sources from UUNET via UUCP.  UUCP-only
sites can get GNU sources by "anonymous UUCP" from site "osu-cis" at
Ohio State University.  You pay for the long-distance call to OSU; the
price isn't too bad on weekends at 9600 bps.  Send mail to
[e-mail] or osu-cis!uucp for more information.

   OSU lines are often busy.  If you're in the USA, and are willing to
spend more money, you can get sources via UUCP from UUNET using their
900 number: 1-900-GOT-SRCS (900 numbers don't work internationally).
You will be billed $0.50/minute by your phone company.

   Don't forget to retrieve libg++ as well!


Getting gcc/g++ for the HP Precision Architecture
=================================================

   If you use the HP Precision Architecture (HP-9000/7xx and
HP-9000/8xx) and you want to use debugging, you'll need to grab a
special version of GAS from the University of Utah, site
jaguar.cs.utah.edu.  Look in the `/dist' directory for gas-2.2.u7.  A
non-standard debug format is used, since until recently HP considered
their debug format a trade secret.  Thanks to the work of lots of good
folks both inside and outside HP, the company has seen the error of its
ways and has now released the required information.  The team at the
University of Utah now has code that understands the native HP format.
For now, the GNU debugger, GDB, understands the debug format produced
by this version of GAS, but not the format produced by HP's compilers.
However, another there is another version of GDB avaiable from this site
that understands the debug format produced by the HP C compiler (but no
version that understands both, unfortunately).

   Some enhancements for the HP that haven't been integrated back into
the official GCC are available from the same site in version
gcc-2.5.8.u8.  The site also has GDB (4.12.u3), GAS (2.2.u7), and
libg++ (2.5.3.u3).  The libg++ version is actually the same as the FSF
2.5.3.

   I recommend that HP users use the Utah versions of the tools (see
above).  At present, the Utah compiler, assembler, binutils, and
debugger *must* be used as a complete set; they won't work with the
generic FSF versions.  This will change in the future once the Utah
changes have been merged back into the main FSF distributions.

   HP GNU users can also find useful stuff on the site geod.emr.ca in
the `/pub/GNU-HP' directory.


Getting gcc/g++ binaries for Solaris 2.x
========================================

   "Sun took the C compiler out of Solaris 2.x.  Am I stuck?"

   No; prep.ai.mit.edu and its mirror sites provide GCC binaries for
Solaris.  As a rule, these binaries are not updated as often as the
sources are, so if you want the very latest version of gcc/g++, you may
need to grab and install binaries for an older version and use it to
bootstrap the latest version from source.

   The latest gcc binaries on prep.ai.mit.edu and its mirror sites are
for version 2.5.6 for Solaris on the Sparc, and version 2.4.5 for
Solaris on Intel 386/486 machines.  There are also binaries for "gzip",
the GNU compression utility, which you'll need for uncompressing the
binary distribution.


How do I get a copy of g++ for (some other platform)?
=====================================================

   The standard gcc/g++ distribution includes VMS support.  Since the
FSF people don't use VMS, it's likely to be somewhat less solid than
the Unix version.  Precompiled copies of g++ and libg++ in
VMS-installable form are available by FTP from mango.rsmas.miami.edu.
See also the site ftp.stacken.kth.se (in Sweden), directory
/pub/GNU-VMS/contrib, which has gcc-2.5.8 and libg++-2.5.3.

   DJ Delorie has ported gcc/g++ to MS-DOS; this port is popularly
known as "DJGPP" (the P's stand for "plus").  It can be found on many
FTP archive sites; its "home" is on oak.oakland.edu, directory
`~ftp/pub/msdos/djgpp'.

   The latest version of DJGPP is 1.11.maint2.  This version runs under
Windows 3.x.  It is a port of gcc 2.5.7.

   FSF sells floppies with DJGPP on them; see above for ordering
software from the FSF.

   For information on Amiga ports of gcc/g++, retrieve the file
`/pub/gnu/MicrosPorts/Amiga' from prep.ai.mit.edu, or write to Markus
M. Wild <[e-mail]>, who I hope won't be too upset that
I mentioned his name here.

   A port of gcc to the Atari ST can be found on the site
"atari.archive.umich.edu", under `/atari/Gnustuff/Tos', along with many
other GNU programs.  This version is usually the same as the latest FSF
release.  See the "Software FAQ" for the Usenet group
"comp.sys.atari.st" for more information.

   There are two different ports of gcc to OS/2, the so-called EMX
port, and a port called "gcc/2".  The EMX port's C library attempts to
provide a Unix-like environment; gcc/2 uses a rather buggy port of the
BSD libc.  For more information ask around on
"comp.os.os2.programmer.misc".  gcc/2, together with other GNUware for
OS/2, can be obtained by FTP from

     ftp-os2.cdrom.com(192.153.46.2) in /pub/os2/2_x/unix/gnu
     ftp-os2.nmsu.edu (128.123.35.151) in /pub/os2/2_x/unix/gnu
     luga.latrobe.edu.au (131.172.2.2) in /pub/os2/2_x/unix/gnu

   The current maintainer of the gcc/2 port is Colin Jensen (Michael
Johnson did the original port).  His address is [e-mail].

   Eberhard Mattes did the EMX port.  His address is
[e-mail].

   Because the legal policies of Apple threaten the long-term goals of
FSF, as well as the concept of free software, no support will be lent to
efforts to port GNU software to Macintosh or other Apple hardware.


But I can only find g++-1.42!
=============================

   "I keep hearing people talking about g++ 2.5.8 (or some other number
starting with 2), but the latest version I can find is g++ 1.42.  Where
is it?"

   As of gcc 2.0, C, C++, and Objective-C as well are all combined into
a single distribution called gcc.  If you get gcc you already have g++.
The standard installation procedure for any gcc version 2 compiler will
install the C++ compiler as well.

   One could argue that we shouldn't even refer to "g++-2.x.y" but it's
a convention.  It means "the C++ compiler included with gcc-2.x.y."


What is the latest version of gcc, g++, and libg++?
===================================================

   The latest "2.x" version of gcc/g++ is 2.5.8, released January 23,
1994.  The latest version of libg++ is 2.5.3, released December 21,
1993.  Don't use 2.5.x, with x less than 5, for C++ code; there were
some serious bugs that didn't have easy workarounds.

   For some non-Unix platforms, the latest port of gcc may be an earlier
version (2.4.5, say).  You'll need to use a version of libg++ that has
the same first two digits as the compiler version, e.g. use libg++ 2.4
with gcc version 2.4.5.

   The latest "1.x" version of gcc is 1.42, and the latest "1.x"
version of g++ is 1.42.0.  While gcc 1.42 is quite usable for C
programs, I recommend against using g++ 1.x except in special
circumstances.


Installation Issues and Problems
********************************


I can't build g++ 1.x.y with gcc-2.x.y!
=======================================

   "I obtained gcc-2.x.y and g++ 1.x.y and I'm trying to build it, but
I'm having major problems.  What's going on?"

   If you wish to build g++-1.42, you must obtain gcc-1.42 first.  The
installation instructions for g++ version 1 leave a lot to be desired,
unfortunately, and I would recommend that, unless you have a special
reason for needing the 1.x compiler, that C++ users use the latest
g++-2.x version, as it is the version that is being actively maintained.

   There is no template support in g++-1.x, and it is generally much
further away from the ANSI draft standard than g++-2.x is.


OK, I've obtained gcc; what else do I need?
===========================================

   First off, you'll want libg++ as you can do almost nothing without it
(unless you replace it with some other class library).

   Second, depending on your platform, you may need "gas", the GNU
assembler, or the GNU linker (see next question).

   Finally, while it is not required, you'll almost certainly want the
GNU debugger, gdb.  The latest version is 4.12, released Feb. 3, 1994.
Other debuggers (like dbx, for example) will normally not be able to
understand at least some of the debug information produced by g++.


Should I use the GNU linker, or should I use "collect"?
=======================================================

   First off, for novices: special measures must be taken with C++ to
arrange for the calling of constructors for global or static objects
before the execution of your program, and for the calling of
destructors at the end.  (Exception: System VR3 and System VR4 linkers
support user-defined segments; g++ on these systems requires neither
the GNU linker nor collect.  So if you have such a system, the answer
is that you don't need either one).

   If you have experience with AT&T's "cfront", this function is
performed there by programs named "patch" or "munch".  With GNU C++, it
is performed either by the GNU linker or by a program known as
"collect".  The collect program is part of the gcc-2.x distribution;
you can obtain the GNU linker separately as part of the "binutils"
package.  The latest version of binutils is 2.3, released Nov. 11, 1993.

   (To be technical, it's "collect2"; there were originally several
alternative versions of collect, and this is the one that survived).

   There are advantages and disadvantages to either choice.

   Advantages of the GNU linker:

   It's faster than using collect - collect basically runs the standard
Unix linker on your program twice, inserting some extra code after the
first pass to call the constructors.  This is a sizable time penalty
for large programs.  The GNU linker does not require this extra pass.

   GNU ld reports undefined symbols using their true names, not the
mangled names.

   If there are undefined symbols, GNU ld reports which object file(s)
refer to the undefined symbol(s).

   As of binutils version 2.2, on systems that use the so-called "a.out"
debug format (e.g. Suns running SunOS 4.x), the GNU linker compresses
the debug symbol table considerably, which in at least some cases may
make up, in disk space, for its inability to use shared libraries.

   Advantages of collect:

   If your native linker supports shared libraries, you can use shared
libraries with collect.  The GNU linker does not (yet) support shared
libraries.

   Note: using existing shared libraries (X and libc, for example) works
very nicely; generating shared libraries from g++-compiled code is
another matter, generally requiring OS-dependent tricks if it is
possible at all.

   Ron Guilmette has written a set of patches for the g++ compiler that
will permit people using g++ on SVr4 systems to build ELF format shared
libraries.  Contact <[e-mail]> for further information.

   The GNU linker has not been ported to as many platforms as g++ has,
so you may be forced to use collect.

   If you use collect, you don't need to get something extra and figure
out how to install it; the standard gcc installation procedure will do
it for you.

   In conclusion, I don't see a clear win for either alternative at this
point.  Take your pick.


Should I use the GNU assembler, or my vendor's assembler?
=========================================================

   This depends on your platform and your decision about the GNU
linker.  For most platforms, you'll need to use gas if you use the GNU
linker.  For some platforms, you have no choice; check the gcc
installation notes to see whether you must use gas.  But you can
usually use the vendor's assembler if you don't use the GNU linker.

   The GNU assembler assembles faster than many native assemblers;
however, on many platforms it cannot support the local debugging format.

   If you want to build shared libraries from gcc/g++ output and you
are on a Sun, you must *not* use GNU as, as it cannot do
position-independent code correctly yet.


Should I use the GNU C library?
===============================

   At this point in time, no.  The GNU C library is still very young,
and libg++ still conflicts with it in some places.  Use your native C
library unless you know a lot about the gory details of libg++ and
gnu-libc.  This will probably change in the future.


Global constructors aren't being called
=======================================

   "I've installed gcc and it almost works, but constructors and
destructors for global objects and objects at file scope aren't being
called.  What did I do wrong?"

   It appears that you are running on a platform that requires you to
install either "collect2" or the GNU linker, and you have done neither.
For more information, see the section discussing the GNU linker (*Note
use GNU linker?::).


Strange assembler errors when linking C++ programs
==================================================

   "I've installed gcc and it seemed to go OK, but when I attempt to
link any C++ program, I'm getting strange errors from the assembler!
How can that be?"

   The messages in question might look something like

     as: "/usr/tmp/cca14605.s", line 8: error: statement syntax
     as: "/usr/tmp/cca14605.s", line 14: error: statement syntax

   (on a Sun, different on other platforms).  The important thing is
that the errors come out at the link step, *not* when a C++ file is
being compiled.

   Here's what's going on: the collect2 program uses the Unix "nm"
program to obtain a list of symbols for the global constructors and
destructors, and it builds a little assembly language module that will
permit them all to be called.  If you're seeing this symptom, you have
an old version of GNU nm somewhere on your path.  This old version
prints out symbol names in a format that the collect2 program does not
expect, so bad assembly code is generated.

   The solution is either to remove the old version of GNU nm from your
path (and that of everyone else who uses g++), or to install a newer
version (it is part of the GNU "binutils" package).  Recent versions of
GNU nm do not have this problem.


Problems building libg++ on 386/486
===================================

   Attempts to install libg++ on 386 or 486 systems running ports of
SVR4 have problems because of bugs in debugging support on that
platform.  Briefly, debugging does not currently work right yet for
C++.  You should be able to build the library successfully by deleting
the `-g' flag from the Makefiles (this should no longer be necessary
with gcc 2.4.x although debugging still doesn't work).

   See the section entitled "Debugging on SVR4 systems."


Other problems building libg++
==============================

   "I am having trouble building libg++.  Help!"

   On some platforms (for example, Ultrix), you may see errors
complaining about being unable to open dummy.o.  On other platforms
(for example, SunOS), you may see problems having to do with the type
of size_t.  The fix for these problems is to make libg++ by saying
"make CC=gcc".  According to Per Bothner, it should no longer be
necessary to specify "CC=gcc" for libg++-2.3.1 or later.

   "I built and installed libg++, but g++ can't find it.  Help!"

   The string given to `configure' that identifies your system must be
the same when you install libg++ as it was when you installed gcc.
Also, if you used the `--prefix' option to install gcc somewhere other
than `/usr/local', you must use the same value for `--prefix' when
installing libg++, or else g++ will not be able to find g++.


But I'm *still* having problems with `size_t'!
==============================================

   "I did all that, and I'm *still* having problems with disagreeing
definitions of size_t, SIZE_TYPE, and the type of functions like
`strlen'."

   The problem may be that you have an old version of `_G_config.h'
lying around.  As of libg++ version 2.4, `_G_config.h', since it is
platform-specific, is inserted into a different directory; most include
files are in `$prefix/lib/g++-include', but this file now lives in
`$prefix/$arch/include'.  If, after upgrading your libg++, you find that
there is an old copy of `_G_config.h' left around, remove it, otherwise
g++ will find the old one first.


Do I need to rebuild libg++ to go with my new g++?
==================================================

   "After I upgraded g++ to the latest version, I'm seeing undefined
symbols."

   or

   "If I upgrade to a new version of g++, do I need to reinstall
libg++?"

   This depends; as a rule, some upgrades will require rebuilding libg++
and others will not.  Both versions 2.3.3 and 2.4.0 introduced some
incompatibilities with previous versions.  For 2.3.3, the name mangling
of certain virtual table names changed, which introduced an
incompatiblity.  For 2.4.0, the type of "size_t" changed on Suns from
int (as declared by the include files provided by Sun) to unsigned long
(the ANSI C and draft ANSI C++ standards declare that size_t must be
unsigned, and the GCC maintainers are now correcting this "bug").

   With version 2.5 of g++ and libg++, major changes were made in the
way that prototypes are provided for system functions.  Before, libg++
provided prototypes for such functions in the g++-include directory.
As of 2.5, gcc creates fixed headers suitable for both C and C++ as part
of its installation process.  You definitely need to install a new
libg++ to go with a 2.5.x release of g++.

   As a rule, the first two digits of your g++ and libg++ should be the
same.


User Problems
*************


gcc 2.5.x broke my code!  Changes in function overloading
=========================================================

   "I have a program that worked just fine with older g++ versions, but
as of version 2.5.x it doesn't work anymore.  Help!"

   While it's always possible that a new bug has been introduced into
the compiler, it's also possible that you have been relying on bugs in
older versions of g++.  For example, version 2.5.0 was the first
version of g++ to correctly implement the "hiding rule."  That is, if
you have an overloaded function in a base class, and in a derived class
you redefine one of the names, the other names are effectively "hidden".
*All* the names from the baseclass need to be redefined in the derived
class.  See section 13.1 of the ARM: "A function member of a derived
class is *not* in the same scope as a function member of the same name
in a base class".

   Here's an example that is handled incorrectly by g++ versions before
2.5.0 and correctly by newer versions:

     class Base {
     public:
         void foo(int);
     };

     class Derived : public Base {
     public:
         void foo(double);  // *note that Base::foo(int) is hidden*
     };

     main() {
         Derived d;
         d.foo(2); // *Derived::foo(double), not Base::foo(int), is called*
     }


Where can I find a demangler?
=============================

   A g++-compatible demangler named `c++filt' can be found in the
`binutils' distribution.  This distribution (which also contains the
GNU linker) can be found at any GNU archive site.


Where can I find a version of etags for C++?
============================================

   The libg++ distribution contains a version of etags that works for
C++ code.  Look in `libg++/utils'.  It's not built by default when you
install libg++, but you can cd to that directory and type

     make etags

   after you've installed libg++.


Linker reports undefined symbols for static data members
========================================================

   "g++ reports undefined symbols for all my static data members when I
link, even though the program works correctly for compiler XYZ.  What's
going on?"

   The problem is almost certainly that you don't give definitions for
your static data members.  If you have

     class Foo {
         ...
         void method();
         static int bar;
     };

   you have only declared that there is an int named Foo::bar and a
member function named Foo::method that is defined somewhere.  You still
need to defined BOTH method() and bar in some source file.  According
to the draft ANSI standard, you must supply an initializer, such as

     int Foo::bar = 0;

in one (and only one) source file.


g++ won't accept the placement new syntax.
==========================================

   "I have a program that uses the "placement syntax" of operator new,
e.g.

        new (somewhere) T;

and g++ won't accept it."

   Up until version 2.3.1, g++ accepted an alternate form of the
placement syntax, for historical reasons; use

        new {somewhere} T;

if you are using g++-2.2.2 or older.

   As of 2.3.1, g++ finally fixed this, using the standard ARM syntax
for "placement new".  A few remaining glitches were fixed in 2.3.2.  The
only remaining problem is with declarators for pointers to functions;

     new (void (*)(int));  // confuses gcc 2.3.2
     new (a) (void (*)(int)); // ditto

   These can be worked around with a typedef:

     typedef void (*fun)(int);
     new fun;
     new (a) fun;


Overloaded increment (`++') and decrement (`--') operators
==========================================================

   "g++ doesn't seem to distinguish the prefix and postfix forms of
`operator++'.  What gives?"

   This is a relatively new feature in the C++ language.  The solution
is to upgrade your compiler; distinguishing the prefix and postfix cases
of `operator++' and `operator--' was first implemented in g++ version
2.4.1.

   For backward compatibility, if a class declares a prefix version of
`operator++' (or `operator--') but no postfix version, and code
attempts to use `++' (or `--') as a postfix operator, g++ will use the
prefix version (unless the `-pedantic' flag is set).  This feature is
to avoid breaking old code.


I think I have found a bug in g++.
==================================

   "I think I have found a bug in g++, but I'm not sure.  How do I know,
and who should I tell?"

   First, see the excellent section on bugs and bug reports in the gcc
manual (which is included in the gcc distribution).  As a short summary
of that section: if the compiler gets a fatal signal, for any input,
it's a bug (newer versions of g++ will ask you to send in a bug report
when they detect an error in themselves).  Same thing for producing
invalid assembly code.

   When you report a bug, make sure to describe your platform (the type
of computer, and the version of the operating system it is running) and
the version of the compiler that you are running.  Also provide enough
code so that the g++ maintainers can duplicate your bug.

   I will add some extra notes that are C++-specific, since the notes
from the gcc documentation are generally C-specific.

   First, mail your bug report to "[e-mail]".  You may
also post to gnu.g++.bug, but it's better to use mail, particularly if
you have any doubt as to whether your news software generates correct
reply addresses.  Don't mail C++ bugs to [e-mail].

   If your bug involves libg++ rather than the compiler, mail to
[e-mail].  If you're not sure, choose one, and if you
guessed wrong, the maintainers will forward it to the other list.

   Second, if your program does one thing, and you think it should do
something else, it is best to consult a good reference if in doubt.  The
standard reference is "The Annotated C++ Reference Manual", by Ellis and
Stroustrup (copyright 1990, ISBN #0-201-51459-1).  This is what they're
talking about on the net when they refer to "the ARM".

   The reference manual, without annotations, also appears in
Stroustrup's "The C++ Programming Language, Second Edition" (copyright
1991, ISBN #0-201-53992-6).  Both books are published by Addison-Wesley.

   Note that the behavior of (any version of) AT&T's "cfront" compiler
is NOT the standard for the language.


Porting programs from other compilers to g++
============================================

   "I have a program that runs on <some other C++ compiler>, and I want
to get it running under g++.  Is there anything I should watch out for?"

   First, see the questions on placement new syntax and static data
members.

   Secondly, if the porting problem relates to the resolution of
overloaded operators or functions, you might try the
`-fansi-overloading' switch in g++ 2.5.0 or later.  This switch enables
new code that attempts to match the ARM specification of overloaded
argument resolution better.

   There are two other reasons why a program that worked under one
compiler might fail under another: your program may depend on the order
of evaluation of side effects in an expression, or it may depend on the
lifetime of a temporary (you may be assuming that a temporary object
"lives" longer than the standard guarantees).  As an example of the
first:

     void func(int,int);

     int i = 3;
     func(i++,i++);

   Novice programmers think that the increments will be evaluated in
strict left-to-right order.  Neither C nor C++ guarantees this; the
second increment might happen first, for example.  func might get 3,4,
or it might get 4,3.

   The second problem often happens with classes like the libg++ String
class.  Let's say I have

     String func1();
     void func2(const char*);

   and I say

     func2(func1());

   because I know that class String has an "operator const char*".  So
what really happens is

     func2(func1().convert());

   where I'm pretending I have a convert() method that is the same as
the cast.  This is unsafe, because the temporary String object may be
deleted after its last use (the call to the conversion function),
leaving the pointer pointing to garbage, so by the time func2 is
called, it gets an invalid argument.

   Both the cfront and the g++ behaviors are legal according to the ARM,
but the powers that be have decided that compiler writers were given
too much freedom here.  The ANSI C++ committee has now come to a
resolution of the lifetime of temporaries problem: they specify that
temporaries should be deleted at end-of-statement (and at a couple of
other points).  This means that g++ now deletes temporaries too early,
and cfront deletes temporaries too late.

   For now, the safe way to write such code is to give the temporary a
name, which forces it to live until the end of the scope of the name.
For example:

     String& tmp = func1();
     func2(tmp);

   Finally, like all compilers (but especially C++ compilers, it seems),
g++ has bugs, and you may have tweaked one.  If so, please file a bug
report (after checking the above issues).


Why does g++ mangle names differently from other C++ compilers?
===============================================================

   See the answer to the next question.


Why can't g++ code link with code from other C++ compilers?
===========================================================

   "Why can't I link g++-compiled programs against libraries compiled by
some other C++ compiler?"

   Some people think that, if only the FSF and Cygnus Support folks
would stop being stubborn and mangle names the same way that, say,
cfront does, then any g++-compiled program would link successfully
against any cfront-compiled library and vice versa.  Name mangling is
the least of the problems.  Compilers differ as to how objects are laid
out, how multiple inheritance is implemented, how virtual function
calls are handled, and so on, so if the name mangling were made the
same, your programs would link against libraries provided from other
compilers but then crash when run.  For this reason, the ARM
*encourages* compiler writers to make their name mangling different
from that of other compilers for the same platform.  Incompatible
libraries are then detected at link time, rather than at run time.


What documentation exists for g++ 2.x?
======================================

   Relatively little.  While the gcc manual that comes with the
distribution has some coverage of the C++ part of the compiler, it
focuses mainly on the C compiler (though the information on the "back
end" pertains to C++ as well).  Still, there is useful information on
the command line options and the #pragma interface and #pragma
implementation directives in the manual.  There is a Unix-style manual
entry, "g++.1", in the gcc-2.x distribution; the information here is a
subset of what is in the manual.

   You can buy a nicely printed and bound copy of this manual from the
FSF; see above for ordering information.

   A draft of a document describing the g++ internals appears in the gcc
distribution (called g++int.texi); it is still incomplete.


What are the differences between g++ and the ARM specification of C++?
======================================================================

   The chief thing missing from g++ that is in the ARM is exceptions.
There are bits and pieces of exception code present, but it is not
presently usable.

   The template implementation is still new.  The implementation in
2.5.x represents a considerable improvement over that of previous
releases, but it has a long way to go.  This continues to improve from
release to release.

   g++ does not implement a separate pass to instantiate template
functions and classes at this point; for this reason, it will not work,
for the most part, to declare your template functions in one file and
define them in another.  The compiler will need to see the entire
definition of the function, and will generate a static copy of the
function in each file in which it is used.  For 2.5.0, however, a new
switch `-fexternal-templates' was added; this makes it possible to have
only one globally visible copy of a given template expansion in your
executable.  See the gcc manual for details.

   As of version 2.5.8, there are still a few limitations in the
template implementation besides the above (thanks to Jason Merrill for
this info):

  1. Static data member templates are not supported.

          template <class T> struct A {
            static T t;
          };

          template <class T> T A<T>::t = 0; // gets bogus error
          int A<int>::t = 0;                // OK

  2. Template member names are not available when defining member
     function templates.

          template <class T> struct A {
            typedef T foo;
            void f (foo);
            void g (foo arg) { ... }; // this works
          };

          template <class T> void A<T>::f (foo) { } // gets bogus error

  3. Templates are instantiated using the parser.  This results in two
     problems:

     a) Class templates are instantiated in some situations where such
     instantiation should not occur.

          template <class T> class A { };
          A<int> *aip = 0; // should not instantiate A<int> (but does)

     b) Function templates cannot be inlined at the site of their
     instantiation.

          template <class T> inline T min (T a, T b) { return a < b ? a : b; }

          void f () {
            int i = min (1, 0);           // not inlined
            int j = min (1, 0);           // inlined
          }

  4. Member function templates are always instantiated when their
     containing class is.  This is wrong.

   Some features that the ANSI/ISO standardization committee has voted
in that don't appear in the ARM are supported, notably the `mutable'
keyword, in version 2.5.x.

   As with any beta-test compiler, there are bugs.  You can help improve
the compiler by submitting detailed bug reports.

   One of the weakest areas of g++ other than templates is the
resolution of overloaded functions and operators in complex cases.  The
usual symptom is that in a case where the ARM says that it is ambiguous
which function should be chosen, g++ chooses one (often the first one
declared).  This is usually not a problem when porting C++ code from
other compilers to g++, but shows up as errors when code developed under
g++ is ported to other compilers.

   As of 2.5.0, the overloading code has been rewritten.  For now, you
must specify the option `-fansi-overloading' to get the new code, since
there were some important users actually depending on g++'s incorrect
resolution of ambiguities.  This switch should disappear in the future.
If a program that compiled under previous g++ versions now reports that
a use of an overloaded function is ambiguous, it is likely that the old
g++ was letting you write buggy code and the new one is detecting the
problem.  If in doubt, consult the ARM.

   [A full bug list would be very long indeed, so I won't put one here.
I may add a list of frequently-reported bugs and "non-bugs" like the
static class members issue mentioned above].


Will g++ compile InterViews?  The NIH class library?
====================================================

   The NIH class library uses a non-portable, compiler-dependent hack
to initialize itself, which makes life difficult for g++ users.  It
will not work without modification, and I don't know what modifications
are required or whether anyone has done them successfully.

   In short, it's not going to happen any time soon (previous FAQs
referred to patches that a new NIHCL release would hopefully contain,
but this hasn't happened).

   [ From Steinar Bang <[e-mail]>]

   InterViews 3.1 compiles and runs with gcc-2.3.3 and libg++-2.3,
except that the "doc" application immediately dumps core when you try
to run it.  There is also a small glitch with idraw.

   There is a patch for InterViews 3.1 from Johan Garpendahl
<[e-mail]> available for FTP from site "ugle.unit.no".  It is in
the file

   `/pub/X11/contrib/InterViews/g++/3.1-beta3-patch'.

   This fixes two things: the Doc coredump, and the pattern menu of
idraw.  Read the instructions at the start of the file.

   I think that as of version 2.5.6, the standard g++ will compile the
standard 3.1 InterViews completely successfully.  I'd appreciate a
confirmation.


Debugging on SVR4 systems
=========================

   "When I use the `-g' flag on C++ code on a System V Release 4 system,
I get lots of undefined symbols at link time.  Why?  Help!"

   Most systems based on System V Release 4 (except Solaris) encode
symbolic debugging information in a format known as `DWARF'.

   Although the GNU C compiler already knows how to write out symbolic
debugging information in the DWARF format, the GNU C++ compiler does
not yet have this feature, nor is it likely to in the immediate future.

   Ron Guilmette has done a great deal of work to try to get the GNU
C++ com- piler to produce DWARF format symbolic debugging information
(for C++ code) but he gave up on the project because of a lack of
funding and/or interest from the g++ user community.  If you have a
strong desire to see this project completed, contact Ron at
<[e-mail]>.


What are the rules for shipping code built with g++ and libg++?
***************************************************************

   "Is it is possible to distribute programs for profit that are created
with g++ and use the g++ libraries?"

   I am not a lawyer, and this is not legal advice.  In any case, I have
little interest in telling people how to violate the spirit of the GNU
licenses without violating the letter.  This section tells you how to
comply with the intention of the GNU licenses as best I understand them.

   The FSF has no objection to your making money.  Its only interest is
that source code to their programs, and libraries, and to modified
versions of their programs and libraries, is always available.

   The short answer is that you do not need to release the source to
your program, but you can't just ship a stripped executable either,
unless you use only the subset of libg++ that includes the iostreams
classes (see discussion below).

   Compiling your code with a GNU compiler does not affect its
copyright; it is still yours.  However, in order to ship code that
links in a GNU library such as libg++ there are certain rules you must
follow.  The rules are described in the file COPYING.LIB that
accompanies gcc distributions; it is also included in the libg++
distribution.  See that file for the exact rules.  The agreement is
called the Library GNU Public License or LGPL.  It is much "looser"
than the GNU Public License, or GPL, that covers must GNU programs.

   Here's the deal: let's say that you use some version of libg++,
completely unchanged, in your software, and you want to ship only a
binary form of your code.  You can do this, but there are several
special requirements.  If you want to use libg++ but ship only object
code for your code, you have to ship source for libg++ (or ensure
somehow that your customer already has the source for the exact version
you are using), and ship your application in linkable form.  You cannot
forbid your customer from reverse-engineering or extending your program
by exploiting its linkable form.

   Furthermore, if you modify libg++ itself, you must provide source
for your modifications (making a derived class does not count as
modifying the library - that is "a work that uses the library").

   For certain portions of libg++ that implement required parts of the
C++ language (such as iostreams), the FSF has loosened the copyright
requirement still more by adding the "special exception" clause, which
reads as follows:

     As a special exception, if you link this library with files
     compiled with GCC to produce an executable, this does not cause
     the resulting executable to be covered by the GNU General Public
     License.  This exception does not however invalidate any other
     reasons why the executable file might be covered by the GNU
     General Public License.

   If your only use of libg++ uses code with this exception, you may
ship stripped executables or license your executables under different
conditions without fear of violating an FSF copyright.  It is the intent
of FSF and Cygnus that, as the other classes required by the ANSI/ISO
draft standard are developed, these will also be placed under this
"special exception" license.

   To avoid coming under the influence of the LGPL, you can link with
`-liostream' rather than `-lg++'.


Concept Index
*************

--
-- Joe Buck     <[e-mail]>
Posting from but not speaking for Synopsys, Inc.
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