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• C++ Thread Safe Integer

  - by Paul Ridgway

  Hello everyone, I have currently created a C++ class for a thread safe integer which simply stores an integer privately and has public get a set functions which use a boost::mutex to ensure that only one change at a time can be applied to the integer. Is this the most efficient way to do it, I have been informed that mutexes are quite resource intensive? The class is used a lot, very rapidly so it could well be a bottleneck... Googleing C++ Thread Safe Integer returns unclear views and oppinions on the thread safety of integer operations on different architectures. Some say that a 32bit int on a 32bit arch is safe, but 64 on 32 isn't due to 'alignment' Others say it is compiler/OS specific (which I don't doubt). I am using Ubuntu 9.10 on 32 bit machines, some have dual cores and so threads may be executed simultaneously on different cores in some cases and I am using GCC 4.4's g++ compiler. Thanks in advance...

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• What type of websites does memcached speed up

  - by Saif Bechan

  I have read this article about 400% boost of your website. This is done by a combination of nginx and memcached. The how-to part of this website is quite good, but i mis the part where it says to what types of websites this applies. I know nginx is a http engine, I need no explanation for that. I thought memcached had something to do with caching database result. However i don't understand what this has to do with the http request, can someone please explain that to me. Another question I have is for what types of websites is this used. I have a website where the important part of the website consist of data that changes often. Often being minutes. Will this method still apply to me, or should I just stick with the basic boring setup of apache and nothing else.

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• Conditional type definitions

  - by pythonic metaphor

  I'm sure that boost has some functions for doing this, but I don't know the relevant libraries well enough. I have a template class, which is pretty basic, except for one twist where I need to define a conditional type. Here is the psuedo code for what I want struct PlaceHolder {}; template <typename T> class C{ typedef (T == PlaceHolder ? void : T) usefulType; }; How do I write that type conditional?

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• Optimizing Dijkstra for dense graph?

  - by Jason

  Is there another way to calculate the shortest path for a near complete graph other than Dijkstra? I have about 8,000 nodes and about 18 million edges. I've gone through the thread "a to b on map" and decided to use Dijkstra. I wrote my script in Perl using the Boost::Graph library. But the result isn't what I expected. It took about 10+ minutes to calculate one shortest path using the call $graph-dijkstra_shortest_path($start_node,$end_node); I understand there are a lot of edges and it may be the reason behind the slow running time. Am I dead in the water? Is there any other way to speed this up?

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• Database structure for storing Bank-like accounts and transactions

  - by user1241320

  We're in the process of adding a bank-like sub-system to our own shop. We already have customers, so each will be given a sort of account and transactions of some kind will be possible (adding to the account or subtracting from it). So we at least need the account entity, the transaction one and operations will then have to recalculate overall balances. How would you structure your database to handle this? Is there any standard bank system have to use that I could mock? By the way, we're on mysql but will also look at some nosql solution for performance boost.

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• How return a std::string from C's "getcwd" function

  - by rubenvb

  Sorry to keep hammering on this, but I'm trying to learn :). Is this any good? And yes, I care about memory leaks. I can't find a decent way of preallocating the char*, because there simply seems to be no cross-platform way. const string getcwd() { char* a_cwd = getcwd(NULL,0); string s_cwd(a_cwd); free(a_cwd); return s_cwd; } UPDATE2: without Boost or Qt, the most common stuff can get long-winded (see accepted answer)

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• Potential problems porting to different architectures

  - by Brendan Long

  I'm writing a Linux program that currently compiles and works fine on x86 and x86_64, and now I'm wondering if there's anything special I'll need to do to make it work on other architectures. What I've heard is that for cross platform code I should: Don't assume anything about the size of a pointer, int or size_t Don't make assumptions about byte order (I don't do any bit shifting -- I assume gcc will optimize my power of two multiplication/division for me) Don't use assembly blocks (obvious) Make sure your libraries work (I'm using SQLite, libcurl and Boost, which all seem pretty cross-platform) Is there anything else I need to worry about? I'm not currently targeting any other architectures, but I expect to support ARM at some point, and I figure I might as well make it work on any architecture if I can. Also, regarding my second point about byte order, do I need to do anything special with text input? I read files with getline(), so it seems like that should be done automatically as well.

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• How might one implement FileTimeToSystemTime?

  - by Billy ONeal

  Hello :) I'm writing a simple wrapper around the Win32 FILETIME structure. boost::datetime has most of what I want, except I need whatever date type I end up using to interpolate with Windows APIs without issues. To that end, I've decided to write my own things for doing this -- most of the operations aren't all that complicated. I'm implementing the TimeSpan - like type at this point, but I'm unsure how I'd implement FileTimeToSystemTime. I could just use the system's built-in FileTimeToSystemTime function, except FileTimeToSystemTime cannot handle negative dates -- I need to be able to represent something like "-12 seconds". How should something like this be implemented? Billy3

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• what this `^` mean here in solr

  - by Rahul Mehta

  I am confuse her but i want to clear my doubt. I think it is stupid question but i want to know. Use a TokenFilter that outputs two tokens (one original and one lowercased) for each input token. For queries, the client would need to expand any search terms containing upper case characters to two terms, one lowercased and one original. The original search term may be given a boost, although it may not be necessary given that a match on both terms will produce a higher score. text:NeXT ==> (text:NeXT^10 OR text:next) what this ^ mean here . http://wiki.apache.org/solr/SolrRelevancyCookbook#Relevancy_and_Case_Matching

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• C++ domain specific embedded language operators

  - by aaa

  hi. In numerical oriented languages (Matlab, Fortran) range operator and semantics is very handy when working with multidimensional data. For example: A(i:j,k,:n) // represents two-dimensional slice B(i:j,0:n) of A at index k unfortunately C++ does not have range operator (:). of course it can be emulated using range/slice functor, but semantics is less clean than Matlab. I am prototyping matrix/tensor domain language in C++ and am wondering if there any options to reproduce range operator. I still would like to rely on C++/prprocessor framework exclusively. So far I have looked through boost wave which might be an suitable option. is there any other means to introduce new operators to C++ DSL?

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• When to trash hashmap contents to avoid performance degradation?

  - by Jack

  Hello, I'm woking on Java with a large (millions) hashmap that is actually built with a capacity of 10.000.000 and a load factor of .75 and it's used to cache some values since cached values become useless with time (not accessed anymore) but I can't remove useless ones while on the way I would like to entirely empty the cache when its performance starts to degrade. How can I decide when it's good to do it? For example, with 10 millions capacity and .75 should I empty it when it reaches 7.5 millions of elements? Because I tried various threshold values but I would like to have an analytic one. I've already tested the fact that emping it when it's quite full is a boost for perfomance (first 2-3 algorithm iterations after the wipe just fill it back, then it starts running faster than before the wipe) Thanks

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• Getting meaningful error messages from fstream's in C++

  - by Hassan Syed

  What is the best way to get meaningful file access error messages, in a portable way from std::fstreams ? The primitiveness of badbits and failbits is getting to be bit annoying. I have written my own exception hierarchies against win32 and POSIX before, and that was far more flexible than the way the STL does it. I am getting "basic::ios_clear" as an error message from the what method of a downcasted catch (std::exception) of a fstream which has exceptions enabled. This doesn't mean much to me, although I do know what the problem is I'd like my program to be a tad more informative so that when I start deployment a few months later my life will be easier. Is there anything in Boost to extract meaningful messages out of the ofstream's implementation cross platform and cross STL implementation ?

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• C++ build systems

  - by flo

  I will start a new C++ project (it may have some C components as well) soon and I am looking for a modern, industrial-strength (i.e. non-beta) build system. The software will be created by several developers in 3-5 years and will run on Linux (Mac OS X and Windows might be supported later). I am looking for something that has better comprehensibility, ease-of-use and maintainability than e.g. make but is still powerful enough to handle a complex project. Open source software is preferred. I started looking into Boost.Build, CMake, Maven and SCons so far and liked features and concepts of all of those, but I'm lacking the experience to make a decision for a large project.

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• What to do with missing fields in sunspot-rails?

  - by chrismealy

  I'm using sunspot/rails version 2. It's working great, but I can't figure out how to handle missing fields. If I don't have latitude and longitude this code will map it to 0,0 (near Africa): searchable do text :resume, :stored => true text :city, :boost => 5 latlon(:geo) { Sunspot::Util::Coordinates.new(latitude, longitude) } end I tried using two search blocks, each with a different conditional, but sunspot just uses the first searchable block. What I want to happen is for things missing locations to still be searchable, just not by location.

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• How do you debug c/c++ source code in linux using emacs?

  - by vmihai

  Hello all, I am using emacs and autotools, to write and compile c/c++ sources on linux. I am using gdb via GUD in emacs. I have defined for convenience: F7:compile, F10:gud-next, F11:gud-step, F5:gud-cont, F9:gud-tbreak, F8:gud-until, F4:gud-print. I am mainly interested in debugging c/c++ source code on linux from emacs and I would like to get the most gdb can give. Unfortunately I am using only F4 which prints the variable under cursor. So my question is how do you guys debug the source code ? What programs do you use ? What key bindings (functionality) do you use mostly ? What do you need the debugger to do for you ? If you do weird stuff it doesn't matter. I would like to know everything to boost my speed a bit here. Thanks in advance. Mihai

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• LALR(1) or GLR on Windows - Alternatives to Bison++ / Flex++ that are current?

  - by mrjoltcola

  I have been using the same version of bison++ (1.21-8) and flex++ (2.3.8-7) since 2002. I'm not looking for an alternative to LALR(1) or GLR at this time, just looking for the most current options. Is anyone aware of any later ports of these than the original that aren't Cygwin dependent? What are other folks using in Windows environments for C++ compiler development (besides ANTLR or Boost.spirit)? Commercial options are ok, if you have firsthand experience. I do need to compile on Linux as well.

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• FastCGI, PHP, Sendmail, and Codeigniter

  - by Kyle J. Dye

  Hi Everyone. I am experiencing an odd issue. I just switched to FastCGI (Apache) because of the big performance boost. Everything is working great, except when I attempt to use sendmail (Codeigniter Class or just raw PHP). I have tested with and without CI and still get a 500 internal server error when trying to send. Could this be getting caused by a discrepency in how FastCGI utilizes sendmail? Has anyone else experienced this issue? Also, the email will send, it just errors afterwards. Please let me know a solution if you have one! Thanks! :)

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• Is there an easy way in Python to wait until certain condition is true?

  - by Checkers

  I need to wait in a script until a certain number of conditions become true? I know I can roll my own eventing using condition variables and friends, but I don't want to go through all the trouble of implementing it, since some object property changes come from external thread in a wrapped C++ library (Boost.Python), so I can't just hijack __setattr__ in a class and put a condition variable there, which leaves me with either trying to create and signal a Python condition variable from C++, or wrap a native one and wait on it in Python, both of which sound fiddly, needlessly complicated and boring. Is there an easier way to do it, barring continuous polling of the condition? Ideally it would be along the lines of res = wait_until(lambda: some_predicate, timeout) if (not res): print 'timed out'

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• Matrix multiplication using Matrix Template library (MTL 4)

  - by Lxc

  The program is as following: #include <iostream> #include <boost/numeric/mtl/mtl.hpp> using namespace mtl; int main(int argc, char* argv[]) { dense_vector<double> a(5,1.0); dense_vector<double> b(5,2.0); a * trans(b); } I want to calculate a * trans(b), but there is a compling error :C2893. Will someone help me? Thanks a lot!

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• Compiler reordering around mutex boundaries?

  - by shojtsy

  Suppose I have my own non-inline functions LockMutex and UnlockMutex, which are using some proper mutex - such as boost - inside. How will the compiler know not to reorder other operations with regard to calls to the LockMutex and UnlockMutex? It can not possibly know how will I implement these functions in some other compilation unit. void SomeClass::store(int i) { LockMutex(_m); _field = i; // could the compiler move this around? UnlockMutex(_m); } ps: One is supposed to use instances of classes for holding locks to guarantee unlocking. I have left this out to simplify the example.

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• Function template accepting nothing less than a bidirectional iterator or a pointer

  - by san

  I need a function template that accepts two iterators that could be pointers. If the two arguments are random_access iterators I want the return type to be an object of std::iterator<random_access_iterator_tag, ...> type else a std::iterator<bidirectional_iterator_tag, ...> type. I also want the code to refuse compilation if the arguments are neither a bidirectional iterator, nor a pointer. I cannot have dependency on third party libraries e.g. Boost Could you help me with the signature of this function so that it accepts bidirectional iterators as well as pointers, but not say input_iterator, output_iterator, forward_iterators. One partial solution I can think of is the following template<class T> T foo( T iter1, T iter2) { const T tmp1 = reverse_iterator<T>(iter1); const T tmp2 = reverse_iterator<T>(iter2); // do something } The idea is that if it is not bidirectional the compiler will not let me construct a reverse_iterator from it.

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• Qt and finding partial matches in a QList

  - by ExpatEgghead

  I have a struct viz: struct NameKey { std::string fullName; std::string probeName; std::string format; std::string source; } which are held in a QList: QList<NameKey> keyList; what I need to do is find an occurence in keyList of a partial match where the search is for a NameKey that only has two members filled. All the keyList entries are full NameKey's. My current implementation is , well, boring in the extreme with too many if's and conditions. So, If I have a DataKey with a fullName and a format I need to find all the occurences in keyList which match. Any useful Qt/boost things available?

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• Allocation Target of std::aligned_storage (stack or heap?)

  - by Zenikoder

  I've been trying to get my head around the TR1 addition known as aligned_storage. Whilst reading the following documents N2165, N3190 and N2140 I can't for the life of me see a statement where it clearly describes stack or heap nature of the memory being used. I've had a look at the implementation provided by msvc2010, boost and gcc they all provide a stack based solution centered around the use of a union. In short: Is the memory type (stack or heap) used by aligned_storage implementation defined or is it always meant to be stack based? and, What the is the specific document that defines/determines that?

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• C++ - checking if a class has a certain method at compile time

  - by jetwolf

  Here's a question for the C++ gurus out there. Is there a way to check at compile time where a type has a certain method, and do one thing if it does, and another thing if it doesn't? Basically, I have a template function template <typename T> void function(T t); and I it to behave a certain way if T has a method g(), and another way if it doesn't. Perhaps there is something that can be used together with boost's enable_if? Something like this: template <typename T> enable_if<has_method<T, g, void ()>, void>::type function(T t) { // Superior implementation calling t.g() } template <typename T> disable_if<has_method<T, g, void ()>, void>::type function(T t) { // Inferior implementation in the case where T doesn't have a method g() } "has_method" would be something that preferably checks both that T has a method named 'g', and that the method has the correct signature (in this case, void ()). Any ideas?

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• iPhone audio Filter Question

  - by Joe

  Okay, I am going to try to make this totally not a "plz send teh codez kthxbai" I am considering an app which takes sound (eventually an audio track) and applies an audio filter to it. So I can play sounds with AudioServicesPlaySystemSound via AudioToolbox framework just fine. What I need is a very simple example of how I might take a sound and apply (for instance) midrange boost etc. Actually the kind of alteration is irrelevant -- if I can get my head around how the alteration is done I can figure out the rest. I am just finding both docs and examples of altering audio in code to be very scarce. Thanks for any help!

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