October 2007 - Boost - lists.boost.org

boost.exception review and memory allocations
by Felipe Magno de Almeida 03 Oct '07

03 Oct '07

Hi, I would like to propose a way that could avoid allocations and would be customizable too. A allocator could be aggregated inside the boost.exception hierarchy. This class could be a template on the number of bytes it would have, and it would do the SBO for allocators. struct allocator_base { virtual /* all allocator interface */ }; template <std::size_t size> struct allocator : allocator_base { boost::array<char, size> buffer_; // all boilerpart to handle multiple sizes }; Then it is possible to dynamic_cast<> to allocator_base, where all allocations would take place. That way the allocator would not be allocated in the heap and the user could customize to a bigger size if it knows it will have to attach more info to that throwing, or less if it believes to be wasting space. It could have a partial specialization for 0 too to disable SBO completely. I guess objects should be default-constructed and swapped too, to avoid copying. Regards, -- Felipe Magno de Almeida

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reinterpret_cast : differents structs or classes with identical structural type
by Marco Servetto 03 Oct '07

03 Oct '07

Hello, Supposing having struct A{ int a1; double a2; int a3[3]; }; struct B{ int b1; double b2; int b3[3]; }; A and B are two structure with same strucutural type but different name, (and different name for fields...) The question is: C++ grants that A a; a.a2=8.8; assert(reinterpret_cast<B*>(&a)->b2==8.8); If the response is yes, it is even true for classes with inheritance? that is A1,B1,C1 have same struttural type of A2,B2,C2 and class A1 :public B1, class B1: public C1 and class A2 :public B2, class B2: public C2 can i safaly reinterpreter cast from A1 to A2 and acceding public fields (of A1,B1,C1) trought the field names of A2? Thanks in advance for your help.

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[review] Review of Boost.Exception
by Dean Michael Berris 03 Oct '07

03 Oct '07

Hi Everyone, Below is my review of the proposed Boost.Exception library. * What is your evaluation of the design? I personally like that the exception library is implemented in just a couple of header files, without having to drag in a lot of other libraries. It is succinctly implemented, though I have some reservations about some design decisions: get_error_info(T const &) returns a pointer instead of a const copy of the actual error_info it gets. This can be a potential headache to deal with especially in examining the error_info associated with the thrown exception. A proposed change would be to have get_error_info(T const &) return a const copy of the data, while get_error_info_ptr(T const &) would return a pointer as in the current version. Although it might also take quite some effort to remove the runtime polymorphic behavior which relies on shared_ptr<>'s and virtual tables with info_base, perhaps an expression templates-based approach to building exceptions that derive from a common base could mitigate these (theoretical) inefficiencies introduced with runtime polymorphic behavior? A linear inheritance encapsulating the different information using something like Boost.Fusion's or Boost.MPL's `fold` concept to create an exception type using expression templates might work. Then I haven't tried this myself, so it might be worth considering in the future. My concern with the use of shared_ptr<>'s is that in cases where an exception pertaining to "out of memory" conditions would arise, throwing a boost::exception and then having to allocate more memory just to encapsulate additional information would lead to undesirable termination. So keeping exception data statically allocated on the stack instead of being in the heap might be a good alternative. This is just my thinking though I don't have enough data to back this up. * What is your evaluation of the implementation? It's very cleanly implemented, and very much readable. * What is your evaluation of the documentation? The documentation is very well written although an alternative example aside from the case with file IO would be greatly appreciated. That said, the example given is sufficient enough to make a case for the utility of the library. * What is your evaluation of the potential usefulness of the library? In cases where minimal memory footprints are required, the current implementation introduces a lot of unnecessary allocations on the heap when additional information is being encapsulated into the exception. An expression templates-based approach as described above (though untested) should be able to mitigate this consequence. In all other cases where efficiency and performance are not critical, I can see this exception library to be very useful. * Did you try to use the library? With what compiler? Did you have any problems? Not yet, though I see myself using this in a project really soon. * How much effort did you put into your evaluation? A glance? A quick reading? In-depth study? I went into the implementation details, and the conceptual design based on the source code. I did a quick read of the documentation, which didn't seem to change since the first time this library has been introduced. * Are you knowledgeable about the problem domain? I have been writing ad-hoc exception hierarchies based on std::exception for quite a while already, and I can say that this implementation of exceptions is a welcome development. * Do you think the library should be accepted as a Boost library? Be sure to say this explicitly so that your other comments don't obscure your overall opinion. I think yes, it should be accepted as a Boost library. I can see it getting a lot of improvements in terms of implementation though, but being in the Boost C++ Library will allow more people to be able to propose changes and eventually extend this library in many different ways. -- Dean Michael C. Berris Software Engineer, Friendster, Inc. [http://cplusplus-soup.blogspot.com/] [mikhailberis(a)gmail.com] [+63 928 7291459] [+1 408 4049523]

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Boost.Exception - my 2 euro cents
by John Torjo 03 Oct '07

03 Oct '07

Hi Emil, 1. First, the idea totally rocks! 2. From the docs: struct tag_errno: boost::error_info_value<int> { }; //(1) class my_error: public boost::exception, public std::exception { }; //(2) void f() { .... throw my_error() << boost::error_info<tag_errno>(errno); //(3) } Not sure why this extra complication: Why not have this: throw my_error() << tag_errno(errno); boost::exception should know it's a tag (it derives from error_info_value) - it's just syntactic sugar. 3. I've looked at the code, and in what() function, you actually write the typeinfo's name. Well, this is fine for VC8 - lets say, but some compilers will give you some really crazy string - which, when logging will mean exactly nothing. You could provide some helper macros to actually enforce compiler to give a user-friendly name, like: #define BOOST_EXCEPTION_TAG(name,type) struct name : boost::error_info_value<type> \ { name() : boost::error_info_value<type>(#name) {} }; I believe this would help a lot debugging/logging 4. Debugging. When debugging, and you catch an exception, unless you wait for .what() to be logged somewhere, and look at that log - or something, when you "watch" that exception, you won't find anything useful. I recommend you have a directive, which if turned on, will contain .what() as string - only for debugging purposes. Thus, by watching a pointer to boost::exception, you can find out what tags it contains. Well, that's it from me! Best, John -- http://John.Torjo.com -- C++ expert ... call me only if you want things done right

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Boost.Exception helper
by John Torjo 03 Oct '07

03 Oct '07

Hi all, I've watched the throw some_error<e1,e2,>(...) discussion: > > Could you please explain? How does using Boost Exception require more > > boilerplate code than the traditional approach? > > I mean the throw site: > > throw an_error(a,b,c) > > vs. > > throw an_error() << boost::error_info<a_tag>(a) > << boost::error_info<b_tag>(b) << boost::error_info<c_tag>(c) Why not have a class like this: // t1, t2, t3 - tag classes template<class t1, class t2, class t3...> struct tags { // not sure if "underlying_type" is the actual underlying type but you catch my drift typedef typename t1::underlying_type p1; typedef typename t2::underlying_type p2; // hold the arguments tags(p1 a1, p2 a2, ...) : m_p1(p1), m_p2(p2) ... {} p1 m_a1; p2 m_a2; .... }; template<class t1, class t2,...> boost::exception & operator<<(boost::exception & e, const tags<...> & t) { e << t1(t.m_a1) << t2(t.m_a2) << ...; return e; } This way, you can have your cake and eat it too: typedef tags<tag_file_name,tag_open_mode,tag_errno> file_err; throw my_error() << file_err(name,mode,errno); Best, John -- http://John.Torjo.com -- C++ expert ... call me only if you want things done right

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boost::exception questions
by Tom Brinkman 03 Oct '07

03 Oct '07

1) Why cant I just throw an boost::exception object? throw boost::exception() << boost::error_info<tag_errno>(100) << boost::error_info<tag_file_name>("hello"); 2) It might be nice to support "std::cerr << e", instead of requiring the use of e.what(). std::cerr << e; 3) Support for linking multiple error tags in a single expression might be useful and help in standarizing errors throughout an application. throw boost:::exception() << boost::error_info<tag_errorno,tag_filename,tag_size,tag_width>( (1,"output.txt",100,200);

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boost.python error with DirectX
by Elaine Wong 03 Oct '07

03 Oct '07

I am using boost.python to wrap C++ function which includes directmusic libraries to simply play the midi, but lots of linkage errors "error LNK2001: unresolved external symbol". I wonder if it is possible to work with DirectX - directmusic libs. Are there any ways to wrap it? *********************compilation errors ************************* msvc.link.dll bin\msvc-7.1\debug\threading-multi\playmusic.pyd bin \msvc-7.1\debu g\threading-multi\playmusic.lib Creating library bin\msvc-7.1\debug\threading-multi\playmusic.lib and object bin\msvc-7.1\debug\threading-multi\playmusic.exp playmusic.obj : error LNK2001: unresolved external symbol _GUID_PerfMasterTempo playmusic.obj : error LNK2019: unresolved external symbol __imp__CoUninitialize@ 0 referenced in function "void __cdecl `anonymous namespace'::StartPlay(void)" ( ?StartPlay@?A0xc386fc44@@YAXXZ) playmusic.obj : error LNK2001: unresolved external symbol _CLSID_DirectMusicSegm ent playmusic.obj : error LNK2001: unresolved external symbol _IID_IDirectMusicSegme nt8 playmusic.obj : error LNK2001: unresolved external symbol _GUID_DirectMusicAllTy pes playmusic.obj : error LNK2001: unresolved external symbol _CLSID_DirectMusicPerf ormance playmusic.obj : error LNK2001: unresolved external symbol _IID_IDirectMusicPerfo rmance8 playmusic.obj : error LNK2019: unresolved external symbol __imp__CoCreateInstanc e@20 referenced in function "void __cdecl `anonymous namespace'::StartPlay(void) " (?StartPlay@?A0xc386fc44@@YAXXZ) playmusic.obj : error LNK2001: unresolved external symbol _CLSID_DirectMusicLoad er playmusic.obj : error LNK2001: unresolved external symbol _IID_IDirectMusicLoade r8 playmusic.obj : error LNK2019: unresolved external symbol __imp__CoInitialize@4 referenced in function "void __cdecl `anonymous namespace'::StartPlay(void)" (?S tartPlay@?A0xc386fc44@@YAXXZ) bin\msvc-7.1\debug\threading-multi\playmusic.pyd : fatal error LNK1120: 11 unres olved externals call "C:\Program Files\Microsoft Visual Studio .NET 2003\Vc7\bin\vcvars3 2.bat" >nul link /NOLOGO /INCREMENTAL:NO /DLL /DEBUG /subsystem:console /out:"bin \msvc-7.1\d ebug\threading-multi\playmusic.pyd" /IMPLIB:"bin\msvc-7.1\debug \threading-multi\ playmusic.lib" /LIBPATH:"C:\Panda3D-1.4.0\python\libs" @"bin \msvc-7.1\debug\th reading-multi\playmusic.pyd.rsp" if %ERRORLEVEL% NEQ 0 EXIT %ERRORLEVEL% if exist "bin\msvc-7.1\debug\threading-multi \playmusic.pyd.manifest" ( mt -nologo -manifest "bin\msvc-7.1\debug\threading-multi \playmusic.p yd.manifest" "-outputresource:bin\msvc-7.1\debug\threading-multi \playmusic.pyd;2 " ) ...failed msvc.link.dll bin\msvc-7.1\debug\threading-multi \playmusic.pyd bin\msv c-7.1\debug\threading-multi\playmusic.lib... ...removing bin\msvc-7.1\debug\threading-multi\playmusic.lib ...failed updating 2 targets... ******************************************compiliation errors************** In C++, Inside "playmusic.cpp", it needs to include following DirectMusic libs. The link errors mainly related to directmusic stuff. #include <dmusicc.h> #include <dmusici.h> #include <dmusicf.h> ************* playmusic.cpp************************* #include <boost/python/class.hpp> #include <boost/python/module.hpp> #include <boost/python/def.hpp> #include "boost/python/detail/wrap_python.hpp" //using namespace std; #include <windows.h> #include <dmusicc.h> #include <dmusici.h> #include <dmusicf.h> namespace { //----------------------------------------------------------------------------- // Defines, constants, and global variables //----------------------------------------------------------------------------- void StartPlay(); IDirectMusicLoader8* g_pLoader = NULL; IDirectMusicPerformance8* g_pPerformance = NULL; IDirectMusicSegment8* g_pSegment = NULL; FLOAT fTempo; void StartPlay() { // Initialize COM CoInitialize(NULL); // Create loader object CoCreateInstance( CLSID_DirectMusicLoader, NULL, CLSCTX_INPROC, IID_IDirectMusicLoader8, (void**)&g_pLoader ); ............. ............. namespace python = boost::python; BOOST_PYTHON_MODULE(playmusic) { python::def("StartPlay", StartPlay); } *********** playmusic.cpp*************************** The code in void StartPlay() inside "playmusic.cpp" can be compiled and executed successfully before in C++, so I extract that part. Any ideas? Thanks so much for your help --------------------------------- Yahoo! 全新升級網上相簿，讓你由相片中分享生活點滴! 立即全面試用

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Re: [boost] [Boost-users] [Iostreams] Issue with mapped_file
by Marshall Clow 02 Oct '07

02 Oct '07

At 7:42 AM +0200 10/2/07, JD wrote: >Subsidiary question: Who is maintaining this library? Jonathan Turkanis is the maintainer of the iostreams library. -- -- Marshall Marshall Clow Idio Software <mailto:marshall@idio.com> It is by caffeine alone I set my mind in motion. It is by the beans of Java that thoughts acquire speed, the hands acquire shaking, the shaking becomes a warning. It is by caffeine alone I set my mind in motion.

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Re: [boost] [Iostreams] Issue with mapped_file
by eg 02 Oct '07

02 Oct '07

JD wrote: > > Nobody? > I have played around with the example, and when I force the filename > used to open the mapped_file to something like "toto", the example > fails! Like if the example would only work on the particular filename > used in it. But the filename of the example does not work in my code. I > really don't understand what's going on here, can someone help? Ok... I debugged into the library and found that you have stumbled upon a bug in the code. A prior call is setting the last error which some subsequent logic depends on. At line 198 in mapped_file.cpp. This triggered a recollection... so I searched in the bug database at http://svn.boost.org/trac It turns out this was previously reported as Ticket # 699 where it suggests a solution and some related issues. See: http://svn.boost.org/trac/boost/ticket/699 > > Subsidiary question: Who is maintaining this library? > No idea.

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Regression testing: Progress report
by Beman Dawes 02 Oct '07

02 Oct '07

For those of you not following the boost-testing list, here are some of the good things that have been happening recently: * Rene Rivera has tracked down and fixed several sources of past failures, as part of our general effort to make regression testing more robust and reliable. * Noel Belcourt has gotten timeouts working on POSIX-based systems, so that a hanging test can no longer hang the whole testing system. * With help from Sebastian Redl, Phil Endecott, and Rene, I've added the Subversion revision number to the report headings. That makes it easier to tell what changes were included in a test run, and has already saved me from some wheel spinning in looking at test failures. It will a couple of more days for the revision number code to propagate to all runners. See http://beta.boost.org/development/tests/trunk/developer/summary.html --Beman

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