EGCS 1.1 C++ Features - GNU Project - Free Software Foundation (FSF)

EGCS 1.1 C++ Features

*** Changes since EGCS 1.0: * Namespaces are fully supported. The library has not yet been converted to use namespace std, however, and the old std-faking code is still on by default. To turn it off, you can use -fhonor-std. * Massive template improvements: + member template classes are supported. + template friends are supported. + template template parameters are supported. + local classes in templates are supported. + lots of bugs fixed. * operator new now throws bad_alloc where appropriate. * Exception handling is now thread safe, and supports nested exceptions and placement delete. Exception handling overhead on x86 is much lower with GNU as 2.9. * protected virtual inheritance is now supported. * Loops are optimized better; we now move the test to the end in most cases, like the C frontend does. * For class D derived from B which has a member 'int i', &D::i is now of type 'int B::*' instead of 'int D::*'. * An _experimental_ new ABI for g++ can be turned on with -fnew-abi. The current features of this are more efficient allocation of base classes (including the empty base optimization), and more compact mangling of C++ symbol names (which can be turned on separately with -fsquangle). This ABI is subject to change without notice, so don't use it for anything that you don't want to rebuild with every release of the compiler. As with all ABI-changing flags, this flag is for experts only, as all code (including the library code in libgcc and libstdc++) must be compiled with the same ABI. *** Changes in EGCS 1.0: * A public review copy of the December 1996 Draft of the ISO/ANSI C++ standard is now available. See http://www.cygnus.com/misc/wp/ for more information. * g++ now uses a new implementation of templates. The basic idea is that now templates are minimally parsed when seen and then expanded later. This allows conformant early name binding and instantiation controls, since instantiations no longer have to go through the parser. What you get: + Inlining of template functions works without any extra effort or modifications. + Instantiations of class templates and methods defined in the class body are deferred until they are actually needed (unless -fexternal-templates is specified). + Nested types in class templates work. + Static data member templates work. + Member function templates are now supported. + Partial specialization of class templates is now supported. + Explicit specification of template parameters to function templates is now supported. Things you may need to fix in your code: + Syntax errors in templates that are never instantiated will now be diagnosed. + Types and class templates used in templates must be declared first, or the compiler will assume they are not types, and fail. + Similarly, nested types of template type parameters must be tagged with the 'typename' keyword, except in base lists. In many cases, but not all, the compiler will tell you where you need to add 'typename'. For more information, see http://www.cygnus.com/misc/wp/dec96pub/template.html#temp.res + Guiding declarations are no longer supported. Function declarations, including friend declarations, do not refer to template instantiations. You can restore the old behavior with -fguiding-decls until you fix your code. Other features: + Default function arguments in templates will not be evaluated (or checked for semantic validity) unless they are needed. Default arguments in class bodies will not be parsed until the class definition is complete. + The -ftemplate-depth-NN flag can be used to increase the maximum recursive template instantiation depth, which defaults to 17. If you need to use this flag, the compiler will tell you. + Explicit instantiation of template constructors and destructors is now supported. For instance: template A<int>::A(const A&); Still not supported: + Member class templates. + Template friends. * Exception handling support has been significantly improved and is on by default. The compiler supports two mechanisms for walking back up the call stack; one relies on static information about how registers are saved, and causes no runtime overhead for code that does not throw exceptions. The other mechanism uses setjmp and longjmp equivalents, and can result in quite a bit of runtime overhead. You can determine which mechanism is the default for your target by compiling a testcase that uses exceptions and doing an 'nm' on the object file; if it uses __throw, it's using the first mechanism. If it uses __sjthrow, it's using the second. You can turn EH support off with -fno-exceptions. * RTTI support has been rewritten to work properly and is now on by default. This means code that uses virtual functions will have a modest space overhead. You can use the -fno-rtti flag to disable RTTI support. * On ELF systems, duplicate copies of symbols with 'initialized common' linkage (such as template instantiations, vtables, and extern inlines) will now be discarded by the GNU linker, so you don't need to use -frepo. This support requires GNU ld from binutils 2.8 or later. * The overload resolution code has been rewritten to conform to the latest C++ Working Paper. Built-in operators are now considered as candidates in operator overload resolution. Function template overloading chooses the more specialized template, and handles base classes in type deduction and guiding declarations properly. In this release the old code can still be selected with -fno-ansi-overloading, although this is not supported and will be removed in a future release. * Standard usage syntax for the std namespace is supported; std is treated as an alias for global scope. General namespaces are still not supported. * New flags: + New warning -Wno-pmf-conversion (don't warn about converting from a bound member function pointer to function pointer). + A flag -Weffc++ has been added for violations of some of the style guidelines in Scott Meyers' _Effective C++_ books. + -Woverloaded-virtual now warns if a virtual function in a base class is hidden in a derived class, rather than warning about virtual functions being overloaded (even if all of the inherited signatures are overridden) as it did before. + -Wall no longer implies -W. The new warning flag, -Wsign-compare, included in -Wall, warns about dangerous comparisons of signed and unsigned values. Only the flag is new; it was previously part of -W. + The new flag, -fno-weak, disables the use of weak symbols. * Synthesized methods are now emitted in any translation units that need an out-of-line copy. They are no longer affected by #pragma interface or #pragma implementation. * __FUNCTION__ and __PRETTY_FUNCTION__ are now treated as variables by the parser; previously they were treated as string constants. So code like `printf (__FUNCTION__ ": foo")' must be rewritten to `printf ("%s: foo", __FUNCTION__)'. This is necessary for templates. * local static variables in extern inline functions will be shared between translation units. * -fvtable-thunks is supported for all targets, and is the default for Linux with glibc 2.x (also called libc 6.x). * bool is now always the same size as another built-in type. Previously, a 64-bit RISC target using a 32-bit ABI would have 32-bit pointers and a 64-bit bool. This should only affect Irix 6, which was not supported in 2.7.2. * new (nothrow) is now supported. * Synthesized destructors are no longer made virtual just because the class already has virtual functions, only if they override a virtual destructor in a base class. The compiler will warn if this affects your code. * The g++ driver now only links against libstdc++, not libg++; it is functionally identical to the c++ driver. * (void *)0 is no longer considered a null pointer constant; NULL in <stddef.h> is now defined as __null, a magic constant of type (void *) normally, or (size_t) with -ansi. * The name of a class is now implicitly declared in its own scope; A::A refers to A. * Local classes are now supported. * __attribute__ can now be attached to types as well as declarations. * The compiler no longer emits a warning if an ellipsis is used as a function's argument list. * Definition of nested types outside of their containing class is now supported. For instance: struct A { struct B; B* bp; }; struct A::B { int member; }; * On the HPPA, some classes that do not define a copy constructor will be passed and returned in memory again so that functions returning those types can be inlined. *** The g++ team thanks everyone that contributed to this release, but especially: * Joe Buck <jbuck@synopsys.com>, the maintainer of the g++ FAQ. * Brendan Kehoe <brendan@cygnus.com>, who coordinates testing of g++. * Jason Merrill <jason@cygnus.com>, the g++ maintainer. * Mark Mitchell <mmitchell@usa.net>, who implemented member function templates and explicit qualification of function templates. * Mike Stump <mrs@wrs.com>, the previous g++ maintainer, who did most of the exception handling work.

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Last modified 2006-06-21