- Boost - lists.boost.org

nested BOOST_FOREACH and -Wshadow
by Dustin Spicuzza 10 Feb '09

10 Feb '09

Hey, I really enjoy using BOOST_FOREACH on containers, saves a ton of typing. However, one semi-annoying problem that I've ran into is that nesting instances of BOOST_FOREACH causes the gcc argument -Wshadow to complain quite a bit. A useless example of this behavior is as follows: #include <vector> #include <iostream> using namespace std; #include <boost/foreach.hpp> int main() { vector < vector <int> > array; BOOST_FOREACH( vector<int>& i1, array) BOOST_FOREACH( int i2, i1) cout << i2; } And you get the following warnings: g++ -Wshadow shadowtest.cpp -o shadowtest shadowtest.cpp: In function ‘int main()’: shadowtest.cpp:13: warning: declaration of ‘_foreach_col’ shadows a previous local shadowtest.cpp:12: warning: shadowed declaration is here shadowtest.cpp:13: warning: declaration of ‘_foreach_cur’ shadows a previous local shadowtest.cpp:12: warning: shadowed declaration is here shadowtest.cpp:13: warning: declaration of ‘_foreach_end’ shadows a previous local shadowtest.cpp:12: warning: shadowed declaration is here shadowtest.cpp:13: warning: declaration of ‘_foreach_continue’ shadows a previous local shadowtest.cpp:12: warning: shadowed declaration is here Now, I know that some people don't believe in getting rid of warnings just for warnings sake, but the fix for this would be extremely trivial and useful (given the amount of noise that it generates) -- just add a new macro, BOOST_FOREACH_NESTED, which has a third parameter denoting the level of nesting. That parameter would be token-pasted onto the end of the variable name, and get rid of the warnings. One could define up to N more macros of the form BOOST_FOREACH_NESTED_N that get rid of the third parameter, by calling that macro appropriately. This wouldn't break any existing code, and would allow those that choose to get rid of the warnings. There are a number of arguments against such a change: (a) -Wshadow is gcc specific and isn't enabled by default, (b) If you don't like the warning, don't nest BOOST_FOREACH statements, or don't use -Wshadow. However, my feeling (and this is probably just a matter of personal preference) is that given the triviality of the fix, theres no reason to not fix it. Granted, I could easily just make my own and stick it in a header file, but it seems like it would make more sense to be in boost itself. I could create a patch and post it if so desired. Thanks! Dustin -- Innovation is just a problem away -- Innovation is just a problem away

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[MPI] Dynamic linking on Windows - mpi_datatype_cache() unresolved means BOOST_IS_MPI_DATATYPE unusable?
by Pete Bartlett 09 Feb '09

09 Feb '09

Dear all, It looks like boost::mpi::detail::mpi_data_type_cache() (in boost/mpi/detail/mpi_datatype_cache.hpp) is missing a declspec declaration, causing unresolved external symbols when trying to give a specialization of is_mpi_datatype. Is my use case (Windows, dynamic, "simple" types that qualify for is_mpi_datatype) unusual so that this hasn't been picked up before? Or am I missing something obvious? Thanks in advance for any help Pete

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[lexical_cast] A suggestion
by Vladimir Batov 09 Feb '09

09 Feb '09

I use boost::lexical_cast quite a bit. Possibly due to specifics of my task I constantly stumble upon two things: 1. It throws if/when conversion fails; 2. It requires the 'Target' class to be default-constructible. I'd like to suggest extending the existing template<class Target, class Source> Target lexical_cast(Source const& arg) interface with template<class Target, class Source> Target lexical_cast(Source const& arg, Target const& failure_value) The behavior of the latter would be to return the 'failure_value' if/when the conversion fails. That communicates the fact of the conversion failure differently (without an exception thrown), i.e. its usage would be as following: int value = boost::lexical_cast(some-string, -1); if (value == -1) conversion failed. In fact, my usage pattern is such that I often do not even need to check the success of the conversion -- if the conversion fails, the supplied failure/default value is returned/used and proceeded with. Secondly, the proposed lexical_cast requirements would be looser -- no need for the default-constructibility of the Target, only the copy-constructibility (as already required anyway). For that the current lexical_cast implementation if(interpreter << arg) { Target result; if (interpreter >> result) return result; } throw_exception(bad_lexical_cast(typeid(Source), typeid(Target))); return Target(); // normally never reached (throw_exception) would change (for the proposed interface) to something like if(interpreter << arg) { Target result(failure_value); if (interpreter >> result) return result; } return failure_value; I am not sure how much value such an interface extension might have for others but for my usage pattern the benefit would be quite visible as the majority of our classes are with no default constructors and handling conversion-failure exceptions is quite a hassle... not to mention that for whatever guided/misguided reasons exceptions are virtually banned for our mission-critical development. Best, V.

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An extension to boost::variant
by Chris Hamilton 09 Feb '09

09 Feb '09

While boost::variant is very capable at what it does, there is one thing it doesn't do (and maybe with very good reason, which I'm sure somebody on this list will be capable of pointing out to me if this is the case ;) Anyhow, boost::variant requires each of the enclosed types to be assignable, which is clearly not met of a reference type. My question is this: why can't boost::variant be clever about reference data types (detected using type_traits is_reference), and internally store them as pointers (type derived by using type_traits remove_reference). The outside API (assignment, output streaming, visitation, comparison, etc) could still treat the object as a reference. The reason I want this, is to have a nice String variant, that encapsulates the various forms a string can take in a current project: std::strings, char*'s, and std::string* and boost::shared_ptr<std::string>. I want functions to be able to handle all of these types transparently by using variants as the argument types. Allowing "const std::string&" as a variant type allows this to be done while not requiring a wasteful string copy. Are there any caveats to this that I should be aware of? (I have a small custom variant class implementing this, and it appears to work well. It's not as 'full fledged' as the boost::variant class, but the approach I've taken could surely be grafted on to boost::variant.) Cheers, Chris Hamilton

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1.38.0 release done ?
by Stefan Seefeld 09 Feb '09

09 Feb '09

While I'm casually reading this ML, I notice some ongoing discussion concerning the 1.38.0 release. Looking at the boost.org website, I trip over the 1.38.0 release announcement, and I wonder whether I just missed the 1.38.0 announcement here on the list, or whether someone mistakenly announced a release-candidate for a final release. Could anybody clarify please ? Thanks, Stefan -- ...ich hab' noch einen Koffer in Berlin...

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[graph]
by Andrew Sutton 09 Feb '09

09 Feb '09

Update on the BGL. I've just ported my SOC work en masse from 2007 to the BGL trunk. New features include: * [un]directed_graph classes - wrappers around adjacency lists. These serve as introductory classes to help with prototyping. They also try to hide all of the problems with vertex indices and property maps - as in every algorithm requires vertex maps, but only VerteSet == vecS provides them. * degree_centraility - algorithms for computing the degree centrality of vertices in a graph. Includes measures of influence and prestige. * closeness_centraltiy - algoritghms for computing the closeness centraility (based on SSPs) for vertices in a graph. * geodesic_distance - Computes the small-world and mean geodesic distances of graphs. * eccentricity - Algorithms for computing eccentricity, and the radius and diameter of the graph. * clustering_coefficient - Returns the clustering coefficient of a vertex or the average CC for the entire graph. * core_numbers - Partitioning of a graph into cores based on vertex degree (from David Gleich) * all_cycles - Find and visit all cycles in a graph (Tiernan) * all_cliques - Find and visit all cliques in a graph (Bron & Kerbosch) * exterior_properties - A new framework for helping define exterior properties (and property maps) for graphs. New graph concepts: * VertexIndexGraph - any graph for which get(vertex_index, g) is valid * EdgeIndexGraph - any graph for which get(edge_index, g) is valid Since nearly every algorithm requires a vertex index map in one way or another, these may as well be concepts. Also, its a convenient documentary artifact that lets you write, "If your graph is not a *IndexMap, then you must provide an index map as an exterior property." Documentation for these features is forthcoming. Its part of my project to (finally) migrate the BGL docs over to quickbook. There are tests for most of these features - some are in progress - others are woefully inadequate and under development. Since I'm generally pretty busy, these will probably materialize slowly. I'm aiming at 1.40. Questions? Comments? Andrew Sutton andrew.n.sutton(a)gmail.com

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[thread|interprocess] Boost.Synchro: Towards a generic view of synchronization mechanisms
by vicente.botet 09 Feb '09

09 Feb '09

Hi, Currently there are two libraries defining mutex, condition variables and locks in Boost (Boost.Thread and Boost.Interprocess), which have similar interfaces but with some differences. This has as consequence that we can not interchange them. One of the reasons is that we are not working with concepts, and algorithms. Both boost::mutex and boost::interprocess_mutex are models of the same Lockable concept, but not because they share a common syntax but because they share a common semantics. Can we see a binary_semaphore as a Lockable? Well, usually a binary_semaphore will define operations such a post and wait, so binary_semaphore do no share the common syntax as boost::mutex and boost::interprocess_mutex. On the semantic side wait can be assimilated to lock and post to unlock. By the way, there are also other syntactical differences between the Boost.Thread and Boost.Interprocess classes. So how can we write gerenic code that can be used with every model of the concepts. The idea consist in switching from the OO paradigm to the functional paradigm. We found the same approach Boost.MPL and Boost.Fusion between others. So instead of requiring that a model of the concept Lockable defines the member functions lock and unlock, we require that the functional expressions lock(m) and unlock(m) are well founded. A generic Synchro library can define a default implementation that forwards to the member function. namespace synchro { template <typename Lockable> void lock(Lockable& lockable) { return lockable.lock(); } } Now we can overload the lock function on a binary_semaphore as namespace synchro { void lock(binary_semaphore& sem) { return sem.wait(); } } Just another example for a class template anotherLockableModel that models the Lockable concept but that with acquire/release functions. We need to overload again the lock function. namespace synchro { template< typename T> void lock( anotherLockableModel<T>& lockable ) { return lockable.acquire(); } } However, there are cases where overloads do not suffice (as described in N1295 - Partial specialization of function templates), and we need to use partial specialization of function templates. On compiler that do not support partial specialization of function templates we can use a workaround, that is, use partial specialization of class templates. The default implementation will now forward to a static function of a class which has forward to the member function namespace synchro { namespace partial_specialization_workaround { template< typename Lockable > struct lock { static void apply( Lockable& lockable ) { return lockable.lock(); } }; } template <typename Lockable> void lock(Lockable& lockable) { return partial_specialization_workaround::lock<Lockable>::apply(lockable); } } So now the user can partially specialize the class template partial_specialization_workaround::lock and define the apply function as needed: namespace synchro { namespace partial_specialization_workaround { template< typename T> struct lock <anotherLockableModel<T> >{ static void apply( anotherLockableModel<T>& lockable ) { return lockable.acquire(); } }; } } Once we have all the Lockable operations defined in this way, and do the same for ConditionVariable, we can define in a generic way unique_lock, shared_lock and upgrade_lock that works with boost::shared_mutex as boost::interprocess::interprocess_upgradable_mutex. This could leverage these libraries to define its own locks classes. Ion, Anthony, Boosters what do you think of applying this approach to get a generic Synchro library? Should the partial_specialization_workaround be applied systematicaly or it complicates things? Thanks, Vicente

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[factory] Including documentation
by Rutger ter Borg 09 Feb '09

09 Feb '09

Dear all, After about 30 minutes searching and digging I found out that apparently a Boost.Factory library is accepted into Boost, and committed to trunk under Boost.Functional. Would it be an idea to include its documentation under the main documentation as well -- or is this already done in 1.38.0? Thanks, Kind regards, Rutger ter Borg

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[1.38.0] Release candidate
by Beman Dawes 09 Feb '09

09 Feb '09

The release candidates are up at http://boost.cowic.de/rc/ Daniel, I'd appreciate it if you could check the docs to make sure there were no glitches. I'll hold off releasing the files on SourceForge until I've heard back from you. Thanks, --Beman

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[constrained-value] Some observations and questions.
by Jeroen N. Witmond [Bahco] 09 Feb '09

09 Feb '09

Greetings, Please forgive me for intruding on the Boost Development mailing list. I'm rather new to Boost, but I'm in love with it, because IMHO it shows C++ as it should be used. :) While browsing this mailing list, I came across the review announcement[1] for the Boost Constrained Value library[2]. I like the concept, so I downloaded[3] and installed the library. I also installed file boost/utility/swap.hpp from the sandbox. I had to hack the example program[4] to get around "[boost] [utility/swap] Ambiguous overload"[5]. To get familiar with Boost and the Boost Constrained Value library, I first rewrote the example program into a Boost.Test testcase[6]. (I have tried not to disturb the documentation generated from this file, but I did not test this.) As testcase, it runs succesfully, with the hack mentioned above. (At least for me, Emacs has problems with font-lock-mode[7] on this file. The original example program does not demonstrate this problem.) In the section on Custom error policies in the example program, perhaps it should be explicitely stated that: - on entry into the error handler, constraint(new_value) will always be false. An error handler that neither changes the constraint, nor supplies a valid value can therefore throw unconditionally. - on entry into the error handler, constraint(value) will only be false during the construction of an object with an invalid value. If the error handler does not throw in this case, the library will raise signal SIGABRT (application abort requested). - the only case where the error handler can be empty is when the constraint always returns true, as in 'template <typename ValueType> class unconstrained'[8]. This code[9] illustrates these points. Next I was itching to create and more real-life use case for the Boost Constrained Value library, with special attention on improving error reporting. So I wrote four template classes[10], implementing non-positive, non-negative, positive and negative constraints over any integral type. (This code still can be improved in many ways.) These constrained integers can be declared like: positiveInteger<int> i; // Construction with value 0 would fail, BOOST_CHECK_EQUAL( i, 1 ); // so it is set to 1. They can used like teir base type. In case of constraint violations (e.g. 'i = 1;' for 'i' above), an exception is thrown, which derives from both std::logic_error and boost::exception. The std::logic_error::what() returns: Failed assert(1 <= 0); And from the boost::exception, the following information can be retrieved (slightly edited to fit the newsgroup): constrained-integer.hpp:168: In '[very long function name]': Failed assert(-1 > 0); constrained-integer.cpp:82: Constraint was defined here. See the first testcase in the Boost.Test testsuite[11] for the code that produced these messages from boost::exception. The remainder of the testsuite runs the four template classes both against fundamental types, and against classes that implement infinite precision integer arithmetic. This showed that I needed to implement 'bool operator == (const Integer & v) const' for these classes, but not for the fundamental types. As implementing this operator==() for fundamental types results in compiler errors, the implementation is present only when preprocessor symbol 'CLASS_TYPE' is defined. (My attempt to hack this in the Boost Constrained Value library source failed.) Note that no hackery is needed for the relational operators used in the constraints. Otherwise, the testsuite compiles correctly and runs almost as expected. It would appear that in the process I may have discovered a bug[12] in the mp_math library in the sandbox. One question I have is related to the requirement of the Constrained Value library that the constraint and error handler objects must be completely constructed before the constrained object is constructed. In these four template classes, I handle this by deriving my template type (e.g. nonPositiveInteger) from the constraint class and indirectly from the error handler class. The purpose is to allow these template types to be used as drop-in replacements for the types they constrain. In this context it appears to work, but I'm having second thoughts about its validity and advisability. Although it is not a Boost question, is there anybody who wants to comment? One thing that has gotten me completely stumped is deriving my own template class from constrained_value::unconstrained. Whatever I try, I get the weirdest of compiler error messages. See the bottom of [10] and [11] for my latest attempt in the section guarded by '#ifdef DO_UNCONSTRAINED'. I must be missing something obvious. Also all other comments will be appreciated. Regards, Jeroen. PS: All files referenced in this email and authored by me are released under the Boost Software License, Version 1.0[99]. Note however that this code is not of release quality. [ 1] http://lists.boost.org/Archives/boost/2008/12/145581.php [ 2] http://student.agh.edu.pl/~kawulak/constrained_value/index.html [ 3] http://rk.go.pl/f/constrained_value.zip [ 4] file libs/constrained_value/doc/src/examples.cpp in [3]. [ 5] http://lists.boost.org/Archives/boost/2007/10/128494.php [ 6] http://www.xs4all.nl/~jnw/boost/constrained-value/test-constrained-value.cpp [ 7] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=510475 [ 8] file boost/constrained_value/unconstrained.hpp in [3]. [ 9] http://www.xs4all.nl/~jnw/boost/constrained-value/understanding-ignore.cpp [10] http://www.xs4all.nl/~jnw/boost/constrained-value/constrained-integer.hpp [11] http://www.xs4all.nl/~jnw/boost/constrained-value/constrained-integer.cpp [12] http://lists.boost.org/boost-users/2009/01/43824.php [99] http://www.boost.org/LICENSE_1_0.txt

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