March 2006 - Boost-users - lists.boost.org

06 Mar '06

Hi, all, I'm a fresh hand to boost. I encounter some problems with ptr_map, in boost/ptr_container/detail/associative_ptr_container.hpp, line 107 & 111. size_type erase( const key_type& x ) // nothrow { BOOST_ASSERT( !this->empty() ); iterator i = find( x ); // nothrow // line 107 if( i == this->end() ) // nothrow return 0; // nothrow this->remove( i ); // nothrow return this->c_private().erase( i.base() ); // nothrow // line 111 } In fact I doubt the correctness of these two lines. First, find() is defined in ptr_map_adapter class, which is a descendant of associative_ptr_container. No other find() functions could match the case here. As for the erase() in line 111, it's actually std::map::erase(). But this overload should return nothing as specified in C++ standard (in the case of MS implementation, it returns an iterator). In no way can it be converted to size_type. Simple modifications on the code could solve this problem. Another thing for ptr_map_adapter is one of its Insert() overloads std::pair<iterator,bool> insert( key_type& key, value_type x ) I wonder why we don't use const key_type&? Without const modifier, the compiler would not allow passing constants to it. At last, never forget to mention my environment: VC 8.0 No test has been done in other environment. -- Jason

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Thread creation problem
by Olaf van der Spek 05 Mar '06

05 Mar '06

Hi, I'm converting code using pthreads to boost::thread. I've now got this code: Cdns_worker::Cdns_worker(): m_run(true), m_thread(boost::bind(&Cdns_worker::run, this)) { // pthread_cond_init(&m_condition, NULL); // pthread_mutex_init(&m_mutex, NULL); // pthread_create(&m_thread, NULL, Cdns_worker::run, this); } The data the thread is using is in Cdns_worker, but it seems impossible to delay the thread creation until the end of the constructor, like I could with pthreads. What should I do in this case? I also noticed boost::thread has a default constructor but it's not copyable. What is the purpose of this default constructor? And are there any boost::thread examples? I couldn't find them in the documentation.

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Support for reading/writing binary 'streams'
by Olaf van der Spek 05 Mar '06

05 Mar '06

Hi, When reading/writing binary streams, you often need to read/write an (unaligned) integer in big, little or native endian. Is there support for this in Boost? I think iostreams isn't usable as 'zero-copy' is required.

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serializatoin + auto_ptr
by Denis Kuznetsov 05 Mar '06

05 Mar '06

If I build the boost demo_auto_ptr example from http://www.boost.org/libs/serialization/example/demo_auto_ptr.cpp it works fine. But if I change my code (see attach) it doesnt't compile. The main difference in my code from boost's sample is that I don't use save() and load() functions. If I add and call them it works fine. The reason my code doesn't compile is that I get BOOST_STATIC_ASSERT. Compiler (VC 7.1 and VC 8.0) points to the code: template<class Archive, class T> inline void save(Archive & ar, T &t){ // if your program traps here, it indicates taht your doing one of the following: // a) serializing an object of a type marked "track_never" through a pointer. // b) saving an non-const object of a type not markd "track_never) // Either of these conditions may be an indicator of an error usage of the // serialization library and should be double checked. See documentation on // object tracking. BOOST_STATIC_ASSERT(check_tracking<T>::value); // <---------- compile error save(ar, const_cast<const T &>(t)); } Why my code doesn't compile and what should I do to build it?

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Boost.Python and Civ4
by Shin 05 Mar '06

05 Mar '06

So, I don't know if folks in the Boost.Python community are aware, but the game Civilization 4 makes use of Python as its primary in-game scripting/modding language. The Python scripts interact with the Civ4 engine via Boost.Python (1.32 I believe). Up until a month ago, I hadn't really used Python, and I'd never heard of Boost.Python :/. As part of my learning experience, I've been learning more about the tools available in this space. My current point in this process is trying to get PyDev and Eclipse configured to allow me to edit Civ4 mods. In pursuit of that goal, I've been talking with Fabio Zadrozny over on the PyDev site about configuring PyDev. He's been very helpful, but I've hit a snag for which he doesn't recall the issue and has sent me here in hopes that someone here might be able to help. I have Python (2.2, 2.3 and 2.4 ;), installed on my machine. I've managed to build Boost.Python 1.32 static and dynamic libs, and have that dir in my path. I know that Civ4's Python API objects are stored in a file called CvGameCoreDLL.dll. When I try to import it from the command line I get the following error: ImportError: dynamic module does not define init function (initCvGameCoreDLL) This being a step forward from when it was saying: This application has failed to start because boost_python-vc71-mt-1_32.dll was not found. Re-installing the application may fix this problem. What I am trying to accomplish, btw, is to be able to get the signatures of the API functions and classes (via intellisense help) from within PyDev. Fabio assures me that if I can get the import call to work from the command line, PyDev will be able to get the signatures. Anyone have any idea what's missing? (This is all taking place on a windows box, btw)

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Declaring an iterators for std::list contain weak ptrs
by James Hughes 05 Mar '06

05 Mar '06

Hello all, I am pretty sure I am not getting something conceptually here, but can anyone shed some light please. I am writing a template class which contains a std::list of weak_ptr<t1> template<typename T1> class fred { private : typedef boost::weak_ptr<T1> MyPtr; typdef std::list<MyPtr> MyPtrList; MyPtrList MyList; public : DoSomething() { MyPtrList::iterator pos; for (pos = MyList.begin(); pos < MyList.end(); ++pos) { ...do stuff.. } } } Now that looks fine to me, but the line where I declare the iterator fails with a error: Expect ';' before 'pos' and I don't know why - it seems fine to me. Using boost 1.33.1 recent cross compiler to ARM. What have I missed? I have tried direct declaration (not using the typedefs') and get the same result. If I put list<int>::iterator that compiles fine. James

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[boost website] Inconsistent search?
by Delfin Rojas 05 Mar '06

05 Mar '06

I just wanted to report something I just found with regards to searching on the website. When I go to www.boost.org there is a search box in the top right corner. Next to the search box there is a dropdown to select between searching "Boost", "Documentation" or "Mailing Lists". Upon starting the search, the user is redirected to a customized Google page with results. That Google page also let's you choose between "Search WWW", "Search www.boost.org" and "Search lists.boost.org". This is what I found: case a) 1. In the boost.org main page selected "Boost" under search dropdown. 2. Searched for "thread priority". 3. Got Google results page. Results did not include matches in boost mailing lists (as expected). 4. Selected "Search lists.boost.org" at the top of Google results page. 5. Clicked search again (same string). 6. No results were found. case b) 1. In the boost.org main page selected "Mailing Lists" under search dropdown. 2. Searched for "thread priority" 3) Got results. In summary: if you start a search from the boost main page and then try to change the search "scope" in the Google results page the scope does not change correctly. However, if you go back to the boost main page and select the new "scope" there then the search shows correct results. This may not be at all a boost problem (may be a Google bug) but I think it is confusing (particularly for newbies). If changing the search scope does not work as expected from the results page then I think it should not give you the option. -delfin

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boost::shared_ptr and custom handling of delete
by Arun Dharankar 05 Mar '06

05 Mar '06

Hello, The type of shared pointer I have in mind is boost::shared_ptr. Is there some way I can extend/modify (short of re-writing) cleanly its functionality such that when its reference counter becomes zero, a method in the actual pointer's class is called instead of that class's destructor being called? Consider the following snippet to allow me to explain: class A { public: A() { } ~A() { } void deleteA() { } }; boost::shared_ptr<A> a(new A); a.reset(); In this example, when a.reset() is called A's I want the deleteA() method to be called instead of the destructor for A. It would then be up to deleteA() to decide if/when to cause the delete of the real object. Best regards, -Arun.

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Boost thread questions
by Calderon Hector 05 Mar '06

05 Mar '06

HI all, Thread support in boost is good. However, 1.- I don't see how to lock a file (fstream) for reading/writing. Do I have to use OS functions? 2.- How do I set a thread to be low priority? Do I have to go to the OS again? I think these are somewhat basic operations. If they are already implemented, can you tell me how to use them. If not, can you tell me if there are plans/steps taken to implement something like 'boost/filesystem/convenience.hpp' (a file with convenience operations). ++Hector C. ---------------------------------- --------------------------------- Yahoo! Mail Bring photos to life! New PhotoMail makes sharing a breeze.

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Lambda library if_then question
by Meryl Silverburgh 05 Mar '06

05 Mar '06

I have a function which check the x of a RectList if it matches certain value, it put the y in an int vector, like this. class Rect{ int x; int y; }; typedef list<Rect*> RectList; // this compiles: void addKey(RectList& rectList, vector<int>& keys, int value) { for (RectList::iterator itr = rectList.begin(); itr != rectList.end(); itr++) { Rect* r = (*itr); if (r->x == value) { keys.push_back(r->y); } } } But when I try to replace the code using boost lambda, I can't get it to compile. can you please tell me what am I missing? void addKey(RectList& rectList, vector<int>& keys, int value) { for_each(rectList.begin(), rectList.end(), if_then( bind (&Rect::x, _1) == value, deviantKey.push_back( bind (&Rect::y, _1) ))); } error: oid addDeviantKey(RectList&, std::vector<int, std::allocator<int> >&, int)': ../DeviantPatternMatcher.cpp:146: error: no matching function for call to 'std::vector<int, std::allocator<int> >::push_back(const boost::lambda::lambda_functor<boost::lambda::lambda_functor_base<boost::lambda::ac

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