What's new in version 2.7.x of gcc/g++

The current version of gcc/g++ is 2.7.2; the current libg++ is 2.7.1.

By the time you read this, two minor bug-fix releases, gcc 2.7.3 and libg++ 2.7.2, may be available. These new releases are mainly intended to fix platform-specific bugs and do not affect the C++ "front end" of the compiler.

The 2.7.x releases represent a great deal of work on the part of the g++ maintainers to fix outstanding bugs and move the compiler closer to the current ANSI/ISO standards committee's working paper, including supporting many of the new features that have been added to the language. I recommend that everyone read the NEWS file contained in the distribution (and that system administrators make the file available to their users). I've borrowed liberally from this file here.

If any features seem unfamiliar, you will probably want to look at the recently-released public review copy of the C++ Working Paper:

• PostScript and PDF (Adobe Acrobat): see `ftp://research.att.com/dist/c++std/WP'.
• HTML and ASCII versions: see `ftp://ftp.cygnus.com/pub/g++'.
• World Wide Web: see `http://www.cygnus.com/misc/wp/'.

Here are the main points:

• As described above, the scope of variables declared in the initialization part of a for statement has been changed; such variables are now visible only in the loop body. Use -fno-for-scope to get the old behavior. You'll need this flag to build groff version 1.09, Ptolemy, and many other free software packages.
• Code that does not use #pragma interface/implementation will most likely shrink dramatically, as g++ now only emits the vtable for a class in the translation unit where its first non-inline, non-abstract virtual function is defined.
• Support for automatic template instantiation has not been enabled in the official distribution, due to a disagreement over design philosophies. But you can get a patch from Cygnus to turn it on; retrieve the patch from `ftp://ftp.cygnus.com/pub/g++/gcc-2.7.2-repo.gz' to patch gcc-2.7.2 (there are also patches for earlier gcc versions).
• See section Problems and limitations with exceptions
• Support for Run-Time Type Identification has been added with -frtti. This support is still in alpha; one major restriction is that any file compiled with -frtti must include <typeinfo> (not typeinfo.h as the NEWS file says). Also, all code you link with (including libg++) has to be built with -frtti, so it's still tricky to use.
• Synthesis of compiler-generated constructors, destructors and assignment operators is now deferred until the functions are used.
• The parsing of expressions such as a ? b : c = 1 has changed from (a ? b : c) = 1 to a ? b : (c = 1). This is a new C/C++ incompatibility brought to you by the ANSI/ISO standards committee.
• The operator keywords and, and_eq, bitand, bitor, compl, not, not_eq, or, or_eq, xor and xor_eq are now supported. Use -ansi or -foperator-names to enable them.
• The explicit keyword is now supported. explicit is used to mark constructors and type conversion operators that should not be used implicitly.
• Handling of user-defined type conversion has been improved.
• Explicit instantiation of template methods is now supported. Also, inline template class foo<int>; can be used to emit only the vtable for a template class.
• With -fcheck-new, g++ will check the return value of all calls to operator new, and not attempt to modify a returned null pointer.
• collect2 now demangles linker output, and c++filt has become part of the gcc distribution.
• Improvements to template instantiation: only members actually used are instantiated. (Actually this is not quite true: some inline templates that are not successfully inlined may be expanded even though they are not needed).