G++ FAQ

Frequently asked questions about the GNU C++ compiler

February 23, 1994

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. One thing that’s missing is a section on templates and template problems with g++; I’m looking for contributions on this score. You can mail comments, suggestions, flames, etc. to jbuck@synopsys.com. 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).

 — The Detailed Node Listing —

Obtaining Source Code

Installation Issues and Problems

User Problems

1 Obtaining Source Code

1.1 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 gnu@prep.ai.mit.edu, phone (617) 876-3296 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 uucp@cis.ohio-state.edu 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!

1.2 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.u4. 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.u5. The site also has GDB (4.12.u1), GAS (2.2.u4), and libg++ (2.5.2.u3).

I recommend that HP users use the Utah versions of the tools (see above). HP GNU users can also find useful stuff on the site geod.emr.ca in the ‘/pub/GNU-HP’ directory.

1.3 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.

1.4 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.

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 <wild@nessie.cs.id.ethz.ch>, 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 cjensen@netcom.com.

Eberhard Mattes did the EMX port. His address is mattes@azu.informatik.uni-stuttgart.de.

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.

1.5 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.”

1.6 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.

2 Installation Issues and Problems

2.1 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.

2.2 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++.

2.3 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 <rfg@netcom.com> 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.

2.4 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.

2.5 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.

2.6 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.

2.7 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.”

2.8 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++.

2.9 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.

2.10 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.

3 User Problems

3.1 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
}

3.2 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.

3.3 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++.

3.4 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.

3.5 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;

3.6 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.

3.7 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 "bug-g++@prep.ai.mit.edu". 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 bug-gcc@prep.ai.mit.edu.

If your bug involves libg++ rather than the compiler, mail to bug-lib-g++@prep.ai.mit.edu. 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.

3.8 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).

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

See the answer to the next question.

3.10 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.

3.11 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.

3.12 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.

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].

3.13 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 <steinarb@idt.unit.no>]

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 <garp@isy.liu.se> 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.

3.14 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 <rfg@netcom.com>.

4 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.

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").

Appendix A Concept Index

Table of Contents

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