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• Several classes need to access the same data, where should the data be declared?

  - by Juicy

  I have a basic 2D tower defense game in C++. Each map is a separate class which inherits from GameState. The map delegates the logic and drawing code to each object in the game and sets data such as the map path. In pseudo-code the logic section might look something like this: update(): for each creep in creeps: creep.update() for each tower in towers: tower.update() for each missile in missiles: missile.update() The objects (creeps, towers and missiles) are stored in vector-of-pointers. The towers must have access to the vector-of-creeps and the vector-of-missiles to create new missiles and identify targets. The question is: where do I declare the vectors? Should they be members of the Map class, and passed as arguments to the tower.update() function? Or declared globally? Or are there other solutions I'm missing entirely?

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• Elastic Collision Formula in Java

  - by Shijima

  I'm trying to write a Java formula based on this tutorial: 2-D elastic collisions without Trigonometry. I am in the section "Elastic Collisions in 2 Dimensions". In step 1, it mentions "Next, find the unit vector of n, which we will call un. This is done by dividing by the magnitude of n". My below code represents the normal vector of 2 objects (I'm using a simple array to represent the normal vector), but I am not really sure what the tutorial means by dividing the magnitude of n to get the un. int[] normal = new int[2]; normal[0] = ball2.x - ball1.x; normal[1] = ball2.y - ball1.y; Can anyone please explain what un is, and how I can calculate it with my array in Java?

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• AWS elastic load balancer basic issues

  - by Jones

  I have an array of EC2 t1.micro instances behind a load balancer and each node can manage ~100 concurrent users before it starts to get wonky. i would THINK if i have 2 such instances it would allow my network to manage 200 concurrent users... apparently not. When i really slam the server (blitz.io) with a full 275 concurrents, it behaves the same as if there is just one node. it goes from 400ms response time to 1.6 seconds (which for a single t1.micro is expected, but not 6). So the question is, am i simply not doing something right or is ELB effectively worthless? Anyone have some wisdom on this? AB logs: Loadbalancer (3x m1.medium) Document Path: /ping/index.html Document Length: 185 bytes Concurrency Level: 100 Time taken for tests: 11.668 seconds Complete requests: 50000 Failed requests: 0 Write errors: 0 Non-2xx responses: 50001 Total transferred: 19850397 bytes HTML transferred: 9250185 bytes Requests per second: 4285.10 [#/sec] (mean) Time per request: 23.337 [ms] (mean) Time per request: 0.233 [ms] (mean, across all concurrent requests) Transfer rate: 1661.35 [Kbytes/sec] received Connection Times (ms) min mean[+/-sd] median max Connect: 1 2 4.3 2 63 Processing: 2 21 15.1 19 302 Waiting: 2 21 15.0 19 261 Total: 3 23 15.7 21 304 Single instance (1x m1.medium direct connection) Document Path: /ping/index.html Document Length: 185 bytes Concurrency Level: 100 Time taken for tests: 9.597 seconds Complete requests: 50000 Failed requests: 0 Write errors: 0 Non-2xx responses: 50001 Total transferred: 19850397 bytes HTML transferred: 9250185 bytes Requests per second: 5210.19 [#/sec] (mean) Time per request: 19.193 [ms] (mean) Time per request: 0.192 [ms] (mean, across all concurrent requests) Transfer rate: 2020.01 [Kbytes/sec] received Connection Times (ms) min mean[+/-sd] median max Connect: 1 9 128.9 3 3010 Processing: 1 10 8.7 9 141 Waiting: 1 9 8.7 8 140 Total: 2 19 129.0 12 3020

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• pdns-recursor allocates resources to non-existing queries

  - by azzid

  I've got a lab-server running pdns-recursor. I set it up to experiment with rate limiting, so it has been resolving requests openly from the whole internet for weeks. My idea was that sooner or later it would get abused, giving me a real user case to experiment with. To keep track of the usage I set up nagios to monitor the number of concurrent-queries to the server. Today I got notice from nagios that my specified limit had been reached. I logged in to start trimming away the malicious questions I was expecting, however, when I started looking at it I couldn't see the expected traffic. What I found is that even though I have over 20 concurrent-queries registered by the server I see no requests in the logs. The following command describes the situation well: $ sudo rec_control get concurrent-queries; sudo rec_control top-remotes 22 Over last 0 queries: How can there be 22 concurrent-queries when the server has 0 queries registered? EDIT: Figured it out! To get top-remotes working I needed to set ################################# # remotes-ringbuffer-entries maximum number of packets to store statistics for # remotes-ringbuffer-entries=100000 It defaults to 0 storing no information to base top-remotes statistics on.

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• How can I have multiple layers in my map array?

  - by Manl400

  How do I load Levels in my game, as in Layer 1 would be Objects, Layer 2 would be Characters and so on. I only need 3 layers, and they will all be put on top of each other. i.e having a flower with a transparent background to be put on grass or dirt on the layer below.I would like to Read From the same file too. How would i go about doing this? Any help would be appreciated. I load the map from a level file which are just numbers corresponding to a tile in the tilesheet. Here is the level file [Layer1] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 [Layer2] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 [Layer3] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 And here is the code that interprets it void LoadMap(const char *filename, std::vector< std::vector <int> > &map) { std::ifstream openfile(filename); if(openfile.is_open()) { std::string line, value; int space; while(!openfile.eof()) { std::getline(openfile, line); if(line.find("[TileSet]") != std::string::npos) { state = TileSet; continue; } else if (line.find("[Layer1]") != std::string::npos) { state = Map; continue; } switch(state) { case TileSet: if(line.length() > 0) tileSet = al_load_bitmap(line.c_str()); break; case Map: std::stringstream str(line); std::vector<int> tempVector; while(!str.eof()) { std::getline(str, value, ' '); if(value.length() > 0) tempVector.push_back(atoi(value.c_str())); } map.push_back(tempVector); break; } } } else { } } and this is how it draws the map. Also the tile sheet is 1280 by 1280 and the tilesizeX and tilesizeY is 64 void DrawMap(std::vector <std::vector <int> > map) { int mapRowCount = map.size(); for(int i, j = 0; i < mapRowCount; i ++) { int mapColCount = map[i].size(); for (int j = 0; j < mapColCount; ++j) { int tilesetIndex = map[i][j]; int tilesetRow = floor(tilesetIndex / TILESET_COLCOUNT); int tilesetCol = tilesetIndex % TILESET_COLCOUNT; al_draw_bitmap_region(tileSet, tilesetCol * TileSizeX, tilesetRow * TileSizeY, TileSizeX, TileSizeY, j * TileSizeX, i * TileSizeX, NULL); } } } EDIT: http://i.imgur.com/Ygu0zRE.jpg

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• Why lock-free data structures just aren't lock-free enough

  - by Alex.Davies

  Today's post will explore why the current ways to communicate between threads don't scale, and show you a possible way to build scalable parallel programming on top of shared memory. The problem with shared memory Soon, we will have dozens, hundreds and then millions of cores in our computers. It's inevitable, because individual cores just can't get much faster. At some point, that's going to mean that we have to rethink our architecture entirely, as millions of cores can't all access a shared memory space efficiently. But millions of cores are still a long way off, and in the meantime we'll see machines with dozens of cores, struggling with shared memory. Alex's tip: The best way for an application to make use of that increasing parallel power is to use a concurrency model like actors, that deals with synchronisation issues for you. Then, the maintainer of the actors framework can find the most efficient way to coordinate access to shared memory to allow your actors to pass messages to each other efficiently. At the moment, NAct uses the .NET thread pool and a few locks to marshal messages. It works well on dual and quad core machines, but it won't scale to more cores. Every time we use a lock, our core performs an atomic memory operation (eg. CAS) on a cell of memory representing the lock, so it's sure that no other core can possibly have that lock. This is very fast when the lock isn't contended, but we need to notify all the other cores, in case they held the cell of memory in a cache. As the number of cores increases, the total cost of a lock increases linearly. A lot of work has been done on "lock-free" data structures, which avoid locks by using atomic memory operations directly. These give fairly dramatic performance improvements, particularly on systems with a few (2 to 4) cores. The .NET 4 concurrent collections in System.Collections.Concurrent are mostly lock-free. However, lock-free data structures still don't scale indefinitely, because any use of an atomic memory operation still involves every core in the system. A sync-free data structure Some concurrent data structures are possible to write in a completely synchronization-free way, without using any atomic memory operations. One useful example is a single producer, single consumer (SPSC) queue. It's easy to write a sync-free fixed size SPSC queue using a circular buffer*. Slightly trickier is a queue that grows as needed. You can use a linked list to represent the queue, but if you leave the nodes to be garbage collected once you're done with them, the GC will need to involve all the cores in collecting the finished nodes. Instead, I've implemented a proof of concept inspired by this intel article which reuses the nodes by putting them in a second queue to send back to the producer. * In all these cases, you need to use memory barriers correctly, but these are local to a core, so don't have the same scalability problems as atomic memory operations. Performance tests I tried benchmarking my SPSC queue against the .NET ConcurrentQueue, and against a standard Queue protected by locks. In some ways, this isn't a fair comparison, because both of these support multiple producers and multiple consumers, but I'll come to that later. I started on my dual-core laptop, running a simple test that had one thread producing 64 bit integers, and another consuming them, to measure the pure overhead of the queue. So, nothing very interesting here. Both concurrent collections perform better than the lock-based one as expected, but there's not a lot to choose between the ConcurrentQueue and my SPSC queue. I was a little disappointed, but then, the .NET Framework team spent a lot longer optimising it than I did. So I dug out a more powerful machine that Red Gate's DBA tools team had been using for testing. It is a 6 core Intel i7 machine with hyperthreading, adding up to 12 logical cores. Now the results get more interesting. As I increased the number of producer-consumer pairs to 6 (to saturate all 12 logical cores), the locking approach was slow, and got even slower, as you'd expect. What I didn't expect to be so clear was the drop-off in performance of the lock-free ConcurrentQueue. I could see the machine only using about 20% of available CPU cycles when it should have been saturated. My interpretation is that as all the cores used atomic memory operations to safely access the queue, they ended up spending most of the time notifying each other about cache lines that need invalidating. The sync-free approach scaled perfectly, despite still working via shared memory, which after all, should still be a bottleneck. I can't quite believe that the results are so clear, so if you can think of any other effects that might cause them, please comment! Obviously, this benchmark isn't realistic because we're only measuring the overhead of the queue. Any real workload, even on a machine with 12 cores, would dwarf the overhead, and there'd be no point worrying about this effect. But would that be true on a machine with 100 cores? Still to be solved. The trouble is, you can't build many concurrent algorithms using only an SPSC queue to communicate. In particular, I can't see a way to build something as general purpose as actors on top of just SPSC queues. Fundamentally, an actor needs to be able to receive messages from multiple other actors, which seems to need an MPSC queue. I've been thinking about ways to build a sync-free MPSC queue out of multiple SPSC queues and some kind of sign-up mechanism. Hopefully I'll have something to tell you about soon, but leave a comment if you have any ideas.

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• OO Design - polymorphism - how to design for handing streams of different file types

  - by Kache4

  I've little experience with advanced OO practices, and I want to design this properly as an exercise. I'm thinking of implementing the following, and I'm asking if I'm going about this the right way. I have a class PImage that holds the raw data and some information I need for an image file. Its header is currently something like this: #include <boost/filesytem.hpp> #include <vector> namespace fs = boost::filesystem; class PImage { public: PImage(const fs::path& path, const unsigned char* buffer, int bufferLen); const vector<char> data() const { return data_; } const char* rawData() const { return &data_[0]; } /*** other assorted accessors ***/ private: fs::path path_; int width_; int height_; int filesize_; vector<char> data_; } I want to fill the width_ and height_ by looking through the file's header. The trivial/inelegant solution would be to have a lot of messy control flow that identifies the type of image file (.gif, .jpg, .png, etc) and then parse the header accordingly. Instead of using vector<char> data_, I was thinking of having PImage use a class, RawImageStream data_ that inherits from vector<char>. Each type of file I plan to support would then inherit from RawImageStream, e.g. RawGifStream, RawPngStream. Each RawXYZStream would encapsulate the respective header-parsing functions, and PImage would only have to do something like height_ = data_.getHeight();. Am I thinking this through correctly? How would I create the proper RawImageStream subclass for data_ to be in the PImage ctor? Is this where I could use an object factory? Anything I'm forgetting?

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• rand() generating the same number – even with srand(time(NULL)) in my main!

  - by Nick Sweet

  So, I'm trying to create a random vector (think geometry, not an expandable array), and every time I call my random vector function I get the same x value, thought y and z are different. int main () { srand ( (unsigned)time(NULL)); Vector<double> a; a.randvec(); cout << a << endl; return 0; } using the function //random Vector template <class T> void Vector<T>::randvec() { const int min=-10, max=10; int randx, randy, randz; const int bucket_size = RAND_MAX/(max-min); do randx = (rand()/bucket_size)+min; while (randx <= min && randx >= max); x = randx; do randy = (rand()/bucket_size)+min; while (randy <= min && randy >= max); y = randy; do randz = (rand()/bucket_size)+min; while (randz <= min && randz >= max); z = randz; } For some reason, randx will consistently return 8, whereas the other numbers seem to be following the (pseudo) randomness perfectly. However, if I put the call to define, say, randy before randx, randy will always return 8. Why is my first random number always 8? Am I seeding incorrectly?

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• Boost tuple + transform

  - by JH

  Is it possible to do the following. Say my boost tuple has <String, int> I would like to use std::transform + mem_fun to insert only the String element in a corresponding vector. Is it possible or are we required to use a loop and push_back(get<0) Ie the following doesn't like to compile... (unknown types...) result.resize(storage.size()) std::transform(storage.begin(), storage.end(), result.begin(), std::mem_fun(&boost::get<0>)); Here is an example (trying one of the comments): #include <boost/tuple/tuple.hpp> #include <vector> #include <string> #include <algorithm> int main(int argc, char**argv) { std::vector< boost::tuple<std::string, int> > storage; std::vector< std::string> result; result.resize(storage.size()); std::transform(storage.begin(), storage.end(), result.begin(), &boost::get<0, boost::tuple<std::string, int> >); return 0; } Output: g++ test.cpp /usr/include/boost/tuple/detail/tuple_basic.hpp: In instantiation of `boost::tuples::cons<boost::tuples::tuple<std::string, int, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type>, TT>': /usr/include/boost/tuple/detail/tuple_basic.hpp:151: instantiated from `boost::tuples::element<0, boost::tuples::cons<boost::tuples::tuple<std::string, int, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type>, TT> >' test.cpp:14: instantiated from here /usr/include/boost/tuple/detail/tuple_basic.hpp:329: error: `boost::tuples::cons<HT, TT>::tail' has incomplete type /usr/include/boost/tuple/detail/tuple_basic.hpp:329: error: invalid use of template type parameter test.cpp: In function `int main(int, char**)': test.cpp:14: error: no matching function for call to `transform(__gnu_cxx::__normal_iterator<boost::tuples::tuple<std::string, int, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type>*, std::vector<boost::tuples::tuple<std::string, int, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type>, std::allocator<boost::tuples::tuple<std::string, int, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type> > > >, __gnu_cxx::__normal_iterator<boost::tuples::tuple<std::string, int, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type>*, std::vector<boost::tuples::tuple<std::string, int, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type>, std::allocator<boost::tuples::tuple<std::string, int, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type> > > >, __gnu_cxx::__normal_iterator<std::string*, std::vector<std::string, std::allocator<std::string> > >, <unresolved overloaded function type>)'

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• C++ templates - matrix class

  - by lastOfMohicans

  Hi I'm learning templates in C++ so I decied to write matrix class which would be a template class. In Matrix.h file I wrote #pragma once #include "stdafx.h" #include <vector> using namespace std; template<class T> class Matrix { public: Matrix(); ~Matrix(); GetDataVector(); SetDataVector(vector<vector<T>> dataVector); bool operator == (Matrix* matrix); bool operator < (Matrix* matrix); bool operator <= (Matrix* matrix); bool operator > (Matrix* matrix); bool operator >= (Matrix* matrix); Matrix* operator + (Matrix* matrix); Matrix* operator - (Matrix* matrix); Matrix* operator * (Matrix* matrix); private: vector<vector<T>> datavector; int columns,rows; }; In Matrix cpp Visual Stuio automaticlly generated code for default constructors #include "StdAfx.h" #include "Matrix.h" Matrix::Matrix() { } Matrix::~Matrix() { } However if I want to compile this I get an error 'Matrix' : use of class template requires template argument list The error are in file Matrix.cpp in default constructors What may be the problem ??

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• Avoiding RTTI In Java

  - by destructo_gold

  Hi, If I have a superclass, say Animal, and two subclasses: Zebra and Giraffe, If I decide to define a Vector of Animals: Vector <Animal> animals = new Vector(); and I want to say: You can add Giraffes, but you must own at least one Zebra first. What is the best way to do this without using RTTI? (instanceof)

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• terminate called after throwing an instance of 'std::length_error'

  - by mark

  hello all, this is my first post here. As i am newbie, the problem might be stupid. I was writing a piece of code while the following error message shown, terminate called after throwing an instance of 'std::length_error' what(): basic_string::_S_create /home/gcj/finals /home/gcj/quals where Aborted the following is the offending code especially Line 39 to Line 52. It is weired for me as this block of code is almost same as the Line64 to Line79. int main(){ std::vector<std::string> dirs, need; std::string tmp_str; std::ifstream fp_in("small.in"); std::ofstream fp_out("output"); std::string::iterator iter_substr_begin, iter_substr_end; std::string slash("/"); int T, N, M; fp_in>>T; for (int t = 0; t < T; t++){ std::cout<<" time "<< t << std::endl; fp_in >> N >> M; for (int n =0; n<N; n++){ fp_in>>tmp_str; dirs.push_back(tmp_str); tmp_str.clear(); } for (int m=0; m<M; m++){ fp_in>>tmp_str; need.push_back(tmp_str); tmp_str.clear(); } for (std::vector<std::string>::iterator iter = dirs.begin(); iter!=dirs.end(); iter++){ for (std::string::iterator iter_str = (*iter).begin()+1; iter_str<(*iter).end(); ++iter_str){ if ((*iter_str)=='/') { std::string tmp_str2((*iter).begin(), iter_str); if (find(dirs.begin(), dirs.end(), tmp_str2)==dirs.end()) { dirs.push_back(tmp_str2); } } } } for (std::vector<std::string>::iterator iter_tmp = dirs.begin(); iter_tmp!= dirs.end(); ++iter_tmp) std::cout<<*iter_tmp<<" "; dirs.clear(); std::cout<<std::endl; std::cout<<" need "<<std::endl; //processing the next for (std::vector<std::string>::iterator iter_tmp = need.begin(); iter_tmp!=need.end(); ++iter_tmp) std::cout<<*iter_tmp<<" "; std::cout<<" where "; for (std::vector<std::string>::iterator iter = need.begin(); iter!=need.end(); iter++){ for (std::string::iterator iter_str = (*iter).begin()+1; iter_str<(*iter).end(); ++iter_str){ if ((*iter_str)=='/') { std::string tmp_str2((*iter).begin(), iter_str); if (find(need.begin(), need.end(), tmp_str2)==need.end()) { need.push_back(tmp_str2); } } } } for (std::vector<std::string>::iterator iter_tmp = need.begin(); iter_tmp!= need.end(); ++iter_tmp) std::cout<<*iter_tmp<<" "; need.clear(); std::cout<<std::endl; //finish processing the next } for (std::vector<std::string>::iterator iter= dirs.begin(); iter!=dirs.end(); iter++) std::cout<<*iter<<" "; std::cout<<std::endl; for (std::vector<std::string>::iterator iter= need.begin(); iter!=need.end(); iter++) std::cout<<*iter<<" "; std::cout<<std::endl; fp_out.close(); } best regards, Mark

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• Use of for_each on map elements

  - by Antonio

  I have a map where I'd like to perform a call on every data type object member function. I yet know how to do this on any sequence but, is it possible to do it on an associative container? The closest answer I could find was this: Boost.Bind to access std::map elements in std::for_each. But I cannot use boost in my project so, is there an STL alternative that I'm missing to boost::bind? If not possible, I thought on creating a temporary sequence for pointers to the data objects and then, call for_each on it, something like this: class MyClass { public: void Method() const; } std::map<int, MyClass> Map; //... std::vector<MyClass*> Vector; std::transform(Map.begin(), Map.end(), std::back_inserter(Vector), std::mem_fun_ref(&std::map<int, MyClass>::value_type::second)); std::for_each(Vector.begin(), Vector.end(), std::mem_fun(&MyClass::Method)); It looks too obfuscated and I don't really like it. Any suggestions?

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• ajax call to servlet puzzler

  - by vector

  Greetings! I'm having a problem getting a text value of a captcha from a servlet through ajax call. When my captcha gets created, its text value is written to session, but after refreshing the image itself though ajax call, I only get one old value of the text. Refreshing the image itself works ok, but I'm stuck getting the correct values from the session on subsequent call. On page reload I get both the new image and its new text value, no joy with ajax though. This works great for the image refresh: $("#asos").attr("src", "/ImageServlet?="+((new Date()).getTime()) ) This call to another method to get text value gives me old stuff: $.ajax({ url:"checkCaptcha", type:"GET", cache: false, success: function( data) { alert(data); } }); Any feedback will be appreciated. ps: here's the meat of the method getting the call: PrintWriter out = response.getWriter(); response.setContentType("text/html"); response.setDateHeader("Expires", 0 ); // Set standard HTTP/1.1 no-cache headers. response.setHeader("Cache-Control", "no-store, no-cache, must-revalidate"); // Set IE extended HTTP/1.1 no-cache headers (use addHeader). response.addHeader("Cache-Control", "post-check=0, pre-check=0"); // Set standard HTTP/1.0 no-cache header. response.setHeader("Pragma", "no-cache"); out.print( request.getSession( ).getAttribute("randomPixValue") ); out.close();

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• Iterator for boost::variant

  - by Ivan

  Hy there, I'm trying to adapt an existing code to boost::variant. The idea is to use boost::variant for a heterogeneous vector. The problem is that the rest of the code use iterators to access the elements of the vector. Is there a way to use the boost::variant with iterators? I've tried typedef boost::variant<Foo, Bar> Variant; std::vector<Variant> bag; std::vector<Variant>::iterator it; for(it= bag.begin(); it != bag.end(); ++it){ cout<<(*it)<<endl; } But it didn't work.

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• What C++ pitfalls should I avoid ?

  - by Craig H

  I remember first learning about vectors in the STL and after some time, I wanted to use a vector of bools for one of my projects. After seeing some strange behavior and doing some research, I learned that a vector of bools is not really a vector of bools. Anyone out there have any other common pitfalls to avoid in C++?

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• About variadic templates

  - by chedi

  Hi, I'm currently experiencing with the new c++0x variadic templates, and it's quit fun, Although I have a question about the process of member instanciation. in this example, I'm trying to emulate the strongly typed enum with the possibility of choose a random valid strong enum (this is used for unit testing). #include<vector> #include<iostream> using namespace std; template<unsigned... values> struct sp_enum; /* this is the solution I found, declaring a globar var vector<unsigned> _data; and it work just fine */ template<> struct sp_enum<>{ static const unsigned _count = 0; static vector<unsigned> _data; }; vector<unsigned> sp_enum<>::_data; template<unsigned T, unsigned... values> struct sp_enum<T, values...> : private sp_enum<values...>{ static const unsigned _count = sp_enum<values...>::_count+1; static vector<unsigned> _data; sp_enum( ) : sp_enum<values...>(values...) {_data.push_back(T);} sp_enum(unsigned v ) {_data.push_back(v);} sp_enum(unsigned v, unsigned...) : sp_enum<values...>(values...) {_data.push_back(v);} }; template<unsigned T, unsigned... values> vector<unsigned> sp_enum<T, values...>::_data; int main(){ enum class t:unsigned{Default = 5, t1, t2}; sp_enum<t::Default, t::t1, t::t2> test; cout <<test._count << endl << test._data.size() << endl; for(auto i= test._data.rbegin();i != test._data.rend();++i){cout<< *i<< ":";} } the result I'm getting with this code is : 3 1 5: can someone point me what I'm messing here ??? Ps: using gcc 4.4.3

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• cakePHP - ACL tutorial & SQL Error: 1104

  - by vector

  Greetings! I run into a problem/bug in production environment with SQL Error: 1104 when deploying a project secured with the ACL tutorial. The full error: SQL Error: 1104: The SELECT would examine more than MAX_JOIN_SIZE rows; check your WHERE and use SET SQL_BIG_SELECTS=1 or SET SQL_MAX_JOIN_SIZE=# if the SELECT is okay [CORE/cake/libs/model/datasources/dbo_source.php, line 666] I've build a small site, worked through the ACL tutorial from the official cake book and locally everything is spiffy. dev. setup consists of: XAMPP: myslq 5.1.33, php 5.3 cakePHP 1.2.7 prod. setup: mysql 5.0.33 php 5.2.2 One of the differences I noticed with mysql setup between local and production servers, is that SQL_BIG_SELECTS is disabled on the production server ( and I don't have privileges to change that, posted a support ticked, didn't hear anything yet) Is there anything I can do about this from my end? Thanks in advance.

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• Basic doubt about sensor usage

  - by Al

  Suppose I have a cellphone with accelerometer and magnetometer, and want to determine its absolute (wrt North/East/South/West) 3d position. Imagine the phone is laid vertically, with the screen facing me, the "up" vector pointing to the ceil. Whenever I tilt, the accelerometer allows me to get the "up" vector info change. The problem is that if I tilt the device and put it horizontally (screen now facing ceil, and "up" vector pointing to the opposite of where I am), then the up vector doesn't get updated any more if I rotate the phone horizontally on the table. This is something that clearly is detected by the magnetometer now. So, the question is, when to know where to use acc or mag for each case? Is there a generic way to achieve this?

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• Memory leak dyld dlopen

  - by imthi

  I am getting leak and I cannot detect from where this is happening. The stack trace does not give full info after dyld open. For few leaks I am not getting any stack trace info. All I get is only object memory address. Is anyone else facing the same issue. I am using XCode 3.2 on show leopard. 18 0x103038 17 0x1033c7 16 0x1034a1 15 0x90145f48 14 dyld dlopen 13 dyld dyld::link(ImageLoader*, bool, ImageLoader::RPathChain const&) 12 dyld ImageLoader::link(ImageLoader::LinkContext const&, bool, bool, ImageLoader::RPathChain const&) 11 dyld ImageLoader::recursiveLoadLibraries(ImageLoader::LinkContext const&, bool, ImageLoader::RPathChain const&) 10 dyld dyld::libraryLocator(char const*, bool, char const*, ImageLoader::RPathChain const*) 9 dyld dyld::load(char const*, dyld::LoadContext const&) 8 dyld dyld::loadPhase0(char const*, dyld::LoadContext const&, std::vector<char const*, std::allocator<char const*> >*) 7 dyld dyld::loadPhase1(char const*, dyld::LoadContext const&, std::vector<char const*, std::allocator<char const*> >*) 6 dyld dyld::loadPhase3(char const*, dyld::LoadContext const&, std::vector<char const*, std::allocator<char const*> >*) 5 dyld dyld::loadPhase4(char const*, dyld::LoadContext const&, std::vector<char const*, std::allocator<char const*> >*) 4 dyld dyld::loadPhase5(char const*, dyld::LoadContext const&, std::vector<char const*, std::allocator<char const*> >*) 3 dyld dyld::mkstringf(char const*, ...) 2 dyld strdup 1 dyld mallocenter

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• Architecture for Qt SIGNAL with subclass-specific, templated argument type

  - by Barry Wark

  I am developing a scientific data acquisition application using Qt. Since I'm not a deep expert in Qt, I'd like some architecture advise from the community on the following problem: The application supports several hardware acquisition interfaces but I would like to provide an common API on top of those interfaces. Each interface has a sample data type and a units for its data. So I'm representing a vector of samples from each device as a std::vector of Boost.Units quantities (i.e. std::vector<boost::units::quantity<unit,sample_type> >). I'd like to use a multi-cast style architecture, where each data source broadcasts newly received data to 1 or more interested parties. Qt's Signal/Slot mechanism is an obvious fit for this style. So, I'd like each data source to emit a signal like typedef std::vector<boost::units::quantity<unit,sample_type> > SampleVector signals: void samplesAcquired(SampleVector sampleVector); for the unit and sample_type appropriate for that device. Since tempalted QObject subclasses aren't supported by the meta-object compiler, there doesn't seem to be a way to have a (tempalted) base class for all data sources which defines the samplesAcquired Signal. In other words, the following won't work: template<T,U> //sample type and units class DataSource : public QObject { Q_OBJECT ... public: typedef std::vector<boost::units::quantity<U,T> > SampleVector signals: void samplesAcquired(SampleVector sampleVector); }; The best option I've been able to come up with is a two-layered approach: template<T,U> //sample type and units class IAcquiredSamples { public: typedef std::vector<boost::units::quantity<U,T> > SampleVector virtual shared_ptr<SampleVector> acquiredData(TimeStamp ts, unsigned long nsamples); }; class DataSource : public QObject { ... signals: void samplesAcquired(TimeStamp ts, unsigned long nsamples); }; The samplesAcquired signal now gives a timestamp and number of samples for the acquisition and clients must use the IAcquiredSamples API to retrieve those samples. Obviously data sources must subclass both DataSource and IAcquiredSamples. The disadvantage of this approach appears to be a loss of simplicity in the API... it would be much nicer if clients could get the acquired samples in the Slot connected. Being able to use Qt's queued connections would also make threading issues easier instead of having to manage them in the acquiredData method within each subclass. One other possibility, is to use a QVariant argument. This necessarily puts the onus on subclass to register their particular sample vector type with Q_REGISTER_METATYPE/qRegisterMetaType. Not really a big deal. Clients of the base class however, will have no way of knowing what type the QVariant value type is, unless a tag struct is also passed with the signal. I consider this solution at least as convoluted as the one above, as it forces clients of the abstract base class API to deal with some of the gnarlier aspects of type system. So, is there a way to achieve the templated signal parameter? Is there a better architecture than the one I've proposed?

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• How to pass parameters to manage_shared_memory.construct() in Boost.Interprocess

  - by recipriversexclusion

  I've stared at the Boost.Interprocess documentation for hours but still haven't been able to figure this out. In the doc, they have an example of creating a vector in shared memory like so: //Define an STL compatible allocator of ints that allocates from the managed_shared_memory. //This allocator will allow placing containers in the segment typedef allocator<int, managed_shared_memory::segment_manager> ShmemAllocator; //Alias a vector that uses the previous STL-like allocator so that allocates //its values from the segment typedef vector<int, ShmemAllocator> MyVector; int main(int argc, char *argv[]) { //Create a new segment with given name and size managed_shared_memory segment(create_only, "MySharedMemory", 65536); //Initialize shared memory STL-compatible allocator const ShmemAllocator alloc_inst (segment.get_segment_manager()); //Construct a vector named "MyVector" in shared memory with argument alloc_inst MyVector *myvector = segment.construct<MyVector>("MyVector")(alloc_inst); Now, I understand this. What I'm stuck is how to pass a second parameter to segment.construct() to specify the number of elements. The interprocess document gives the prototype for construct() as MyType *ptr = managed_memory_segment.construct<MyType>("Name") (par1, par2...); but when I try MyVector *myvector = segment.construct<MyVector>("MyVector")(100, alloc_inst); I get compilation errors. My questions are: Who actually gets passed the parameters par1, par2 from segment.construct, the constructor of the object, e.g. vector? My understanding is that the template allocator parameter is being passed. Is that correct? How can I add another parameter, in addition to alloc_inst that is required by the constructor of the object being created in shared memory? There's very little information other than the terse Boost docs on this.

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• How to create a function with vectors and vertical asymptote using MATLAB

  - by Pedro J Trinidad

  Plot the function f(x) = 1.5x / x-4 for -10 equal or less than X equal or less than 10. Notice that the function have a vertical asymptote at x = 4. Plot the function by creating two vectors for the domain of x. The first vector (call it x1) with elements from -10 to 3.7, and the second vector (calle it x2) with elements from 4.3 to 10. For each of the X vector create a Y vector (call them y1 and y2) with the corresponding values of Y according to the function. To plot the function make two curves in the same plot (y1 vs. x1 and y2 vs. x2).

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• Iterating backward

  - by MBennett

  Suppose I have a vector<int> myvec and I want to loop through all of the elements in reverse. I can think of a few ways of doing this: for (vector<int>::iterator it = myvec.end() - 1; it >= myvec.begin(); --it) { // do stuff here } for (vector<int>::reverse_iterator rit = myvec.rbegin(); rit != myvec.rend(); ++rit) { // do stuff here } for (int i = myvec.size() - 1; i >= 0; --i) { // do stuff here } So my question is when should I use each? Is there a difference? I know that the first one is dangerous because if I pass in an empty vector, then myvec.end() - 1 is undefined, but are there any other hazards or inefficiencies with this?

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• Good way to maintain Muliple level selection menu list in J2ME

  - by geoaxis

  Hello, I need a good way to maintain multiple form level data for menue selection. So for example If I have A and B, each might Have 1 2 3 so A A1 A2 A3 B B1 B2 B3 And this can continue for long, so that I could have A - A1 - A1.1 - A1.1.1 -.... I have the following class in place, works ok But I suspect we could have better. I just need to perform selection ni a selection tree like Widget, but each level of selection comes in another form (in J2ME) import java.util.Vector; public class Tag { private String tag; private Vector childTags; private Tag parent; Tag(String tag, Vector childtag) { this.tag = tag; this.childTags= childTags; } public void setChildTags(Vector childTags) { this.childTags = childTags; } public Vector getChildTags() { return this.childTags; } public String getTag() { return this.tag; } public String toString(int depth) { String a =""; if(depth==0) { a = a + this.getTag(); } if(this.getChildTags()!= null) { for(int k=0;k <this.getChildTags().capacity(); k++) { for (int i=0; i<depth; i++ ) { a = a + ("-"); } a = a+ ( ((Tag)this.getChildTags().elementAt(k)).toString(depth++)); } } return a; } }

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