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  • How to use the vtable to determine class type

    - by Alex Silverman
    I was recently on an interview for a position where C/C++ is the primary language and during one question I was told that it's possible to use the vtable to determine which class in a hierarchy a base pointer actually stores. So if, for example you have class A { public: A() {} virtual ~A() {} virtual void method1() {} }; class B : public A { public: B() {} virtual ~B() {} virtual void method1() {} }; and you instantiate A * pFoo = new B(), is it indeed possible to use the vtable to determine whether pFoo contains a pointer to an instance of A or B?

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  • Catching KeyboardInterrupt when working with PyGame

    - by Sebastian P.
    I have written a small Python application where I use PyGame for displaying some simple graphics. I have a somewhat simple PyGame loop going in the base of my application, like so: stopEvent = Event() # Just imagine that this eventually sets the stopEvent # as soon as the program is finished with its task. disp = SortDisplay(algorithm, stopEvent) def update(): """ Update loop; updates the screen every few seconds. """ while True: stopEvent.wait(options.delay) disp.update() if stopEvent.isSet(): break disp.step() t = Thread(target=update) t.start() while not stopEvent.isSet(): for event in pygame.event.get(): if event.type == pygame.QUIT: stopEvent.set() It works all fine and dandy for the normal program termination; if the PyGame window gets closed, the application closes; if the application finishes its task, the application closes. The trouble I'm having is, if I Ctrl-C in the Python console, the application throws a KeyboardInterrupt, but keeps on running. The question would therefore be: What have I done wrong in my update loop, and how do I rectify it so a KeyboardInterrupt causes the application to terminate?

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  • Converting ObjC Blocks to C# Lambas

    - by Sam
    I need some help converting an Objective-C block to C#. Here is the source ObjC: NSDate* addYear = [_calendar dateByAddingComponents:((^{ NSDateComponents* components = [NSDateComponents new]; components.month = 12; return components; })()) toDate:now options:0]; Now I tried the following in C#: NSDate date = _calendar.DateByAddingComponents((() => { NSDateComponents components = new NSDateComponents(); components.Month = 12; return components; })(), now, NSCalendarOptions.None); To which I get the following compiler error: Expression denotes a 'anonymous method' where a 'method group' was expected. Removing the parentheses around the lambda yields Cannot convert 'lambda expression' to non-delegate type 'MonoTouch.Foundation.NSDateComponents'. What is the correct C# syntax? I need to retain the closures as there are a lot more in the code base that I am porting.

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  • Efficient alternatives to merge for larger data.frames R

    - by Etienne Low-Décarie
    I am looking for an efficient (both computer resource wise and learning/implementation wise) method to merge two larger (size1 million / 300 KB RData file) data frames. "merge" in base R and "join" in plyr appear to use up all my memory effectively crashing my system. Example load test data frame and try test.merged<-merge(test, test) or test.merged<-join(test, test, type="all") - The following post provides a list of merge and alternatives: How to join data frames in R (inner, outer, left, right)? The following allows object size inspection: https://heuristically.wordpress.com/2010/01/04/r-memory-usage-statistics-variable/ Data produced by anonym

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  • .NET 4.0 Generic Invariant, Covariant, Contravariant

    - by Sameer Shariff
    Here's the scenario i am faced with: public abstract class Record { } public abstract class TableRecord : Record { } public abstract class LookupTableRecord : TableRecord { } public sealed class UserRecord : LookupTableRecord { } public interface IDataAccessLayer<TRecord> where TRecord : Record { } public interface ITableDataAccessLayer<TTableRecord> : IDataAccessLayer<TTableRecord> where TTableRecord : TableRecord { } public interface ILookupTableDataAccessLayer<TLookupTableRecord> : ITableDataAccessLayer<TLookupTableRecord> where TLookupTableRecord : LookupTableRecord { } public abstract class DataAccessLayer<TRecord> : IDataAccessLayer<TRecord> where TRecord : Record, new() { } public abstract class TableDataAccessLayer<TTableRecord> : DataAccessLayer<TTableRecord>, ITableDataAccessLayer<TTableRecord> where TTableRecord : TableRecord, new() { } public abstract class LookupTableDataAccessLayer<TLookupTableRecord> : TableDataAccessLayer<TLookupTableRecord>, ILookupTableDataAccessLayer<TLookupTableRecord> where TLookupTableRecord : LookupTableRecord, new() { } public sealed class UserDataAccessLayer : LookupTableDataAccessLayer<UserRecord> { } Now when i try to cast UserDataAccessLayer to it's generic base type ITableDataAccessLayer<TableRecord>, the compiler complains that it cannot implicitly convert the type.

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  • What is the precision of the priority field in sitemap.xml?

    - by Christoph
    Unfortunately the specification does not tell anything about precision. The xml scheme definition states that it is of the type xsd:decimal: <xsd:restriction base="xsd:decimal"> <xsd:minInclusive value="0.0"/> <xsd:maxInclusive value="1.0"/> </xsd:restriction> I have a sitemap generator that uses up to 10 positions after decimal point. Where often only the last few positions differ. These numbers are perfectly right according to the xsd, but yet i found some pages(3,4) that state that only 0.0, 0.1, 0.2, .., 1.0 are valid values. How will the search engines react to such a sitemap? Will some just round the value? I know that it is unlikely that someone can provide an answer to that question, unless he works for that search engine, but i think experiences will also do.

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  • Pre-packaged Rails applications

    - by Craig
    Seems like most Rails applications have similar 'base' functionality. As such, it seems that there would be value in having pre-build Rails applications at various functionality points such as: basic User model with authentication using Authlogic #1 + openid integration #2 + authorization using declarative_authorization #3 + Administration module #4 + a Profile model Themes (useful stylesheets and such) Friendship model Geocoding ... In addition to the basic MVC stuff, these applications would include: testing harnesses seed data git support One could choose start from any of these functionality points. Other than the sample application that are available with the various gems/plugins, are there projects such as these? If not, I would certainly be willing to contribute what I have.

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  • Should I create protected constructor for my singleton classes?

    - by Vijay Shanker
    By design, in Singleton pattern the constructor should be marked private and provide a creational method retuning the private static member of the same type instance. I have created my singleton classes like this only. public class SingletonPattern {// singleton class private static SingletonPattern pattern = new SingletonPattern(); private SingletonPattern() { } public static SingletonPattern getInstance() { return pattern; } } Now, I have got to extend a singleton class to add new behaviors. But the private constructor is not letting be define the child class. I was thinking to change the default constructor to protected constructor for the singleton base class. What can be problems, if I define my constructors to be protected? Looking for expert views....

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  • About Googe map

    - by vishal
    Hello everyone I am PHP Developer and developing website .in that i used Google map with the facility of select state and city. for select state and city i put two drop down.so now my problem is when i select state then i can not get value of city and both time whenever use select state and city i refresh the page and so after refreshing the page the value i got is incorrect and does not match the state with city... i clarify again Step 1:- select state (here page will refresh and then i will get value) Step 2:- select city (here also will refresh and then i will get value)) problem :- on the base getting state id and city id i fire the query but result will unknown b'cz city id that does not in selected state actually that city is there

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  • 'Static/Constant' business ojects

    - by UpTheCreek
    I don't quite know how to ask this question, so I'll phase it as an example: Imagine in an application you have a Country object. There are two properties of this object: Name, and a 'Bordering Countries' collection. More properties might be added later, but it will be the kind of information that would change very rarely (e.g. changes of country names/borders) Lets say this application needs to know about all of the countries in the world. Where would you store these object's state? How would you new them up? It seems silly to store all this state in the DB, since it won't change very often. One option might be to have an abstract 'country' base object, and have a class for each country inheriting from this with the details of each country. But this doesn't seem quite right to me. What is the proper way of dealing with these kinds of objects?

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  • Is it okay if my ViewModel 'creates' bindable user controls for my View?

    - by j0rd4n
    I have an entry-point View with a tab control. Each tab is going to have a user control embedded within it. Each embedded view inherits from the same base class and will need to be updated as a key field on the entry-point view is updated. I'm thinking the easiest way to design this page is to have the entry-point ViewModel create and expose a collection of the tabbed views so the entry-point View can just bind to the user control elements using a DataTemplate on the tab control. Is it okay for a ViewModel to instantiate and provide UI elements for its View?

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  • Is it possible to do live events in YUI, similar to jQuery's live events?

    - by eculver
    It doesn't currently seem as though there is a way to do live events in YUI similar to jQuery: http://api.jquery.com/live/ It sure would be nice if something like: function handleClick(e) { // click! } YUI().use('node-base', function(Y) { Y.on("click", handleClick, ".foo"); }); caused handleClick to be fired when a node with the "foo" class was clicked on after being dynamically added to the DOM and sometime after the Y.on was evaluated. I feel like live events are one of jQuery's killer features that YUI lacks.

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  • has_many through and partials

    - by user307428
    I have a User model, a Post model, and an Interest model. Using User has_many posts through interests Using User has_many interests Using Post has_many users through interests Using Post has_many interests Using Interest belongs to Post Using Interest belongs to User Application_Controller is as follows: class ApplicationController < ActionController::Base before_filter :login_from_cookie before_filter :find_user_interests helper :all # include all helpers, all the time session :session_key = '_blah_session' include AuthenticatedSystem def find_user_interests @user_interests = current_user ? current_user.interests : [] true end Application.html.erb has as follows: <%= render :partial = "users/interests", :object = @user_interests % _interests.html.erb partial is as follows: ul <% unless current_user.nil? then -% <% @user_interests.each do |interest| -% li<%= interest.post.title %/li <% end % <% end -% /ul Given all this when I at localhost:3000/posts/1 my partial shows up fine, but when in localhost:3000/posts I get an error "undefined method `title' for nil:NilClass" thus an error in the line li<%= interest.post.title %/li shown above in the _interests.html.erb partial. What the heck would be the issue? TIA end

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  • [0-9a-zA-Z]* string expressed with primes or prime-factorization-style way to break it into parts?

    - by HH
    Suppose a string consists of numbers and alphabets. You want to break it into parts, an analogy is primes' factorization, but how can you do similar thing with strings [0-9a-zA-Z]* or even with arbitrary strings? I could express it in alphabets and such things with octal values and then prime-factorize it but then I need to keep track of places where I had the non-numbers things. Is there some simple way to do it? I am looking for simple succinct solutions and don't want too much side-effects. [Update] mvds has the correct idea, to change the base, how would you implement it?

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  • Is there a variable in Rails that equates to the template that is being rendered?

    - by Sean Ahrens
    I can do request.path_parameters['controller'] and request.path_parameters['action'], but is there anything like request.path_parameters['template'] so I can discern which template file (such as index.html.erb) is being rendered? I'm writing a method that automatically sets the body id to the template being rendered, for easy css manipulation: class ApplicationController < ActionController::Base ... after_filter :define_body_selector ... def define_body_selector # sets @body_id to the name of the template that will be rendered # ie. if users/index.html.erb was just rendered, @body_id gets set to "index" @body_id = ??? end ...

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  • LinkButton child controls render

    - by Alex
    Hi. I want to have a LinkButton that adds 'span' tag around the text. protected override void Render(HtmlTextWriter writer) { Text = String.Concat("<span>", Text, "</span>"); base.Render(writer); } It's works perfectly, but only if I add text like this: <cc:TestLinkButton ID="TestLinkButton" runat="server" Text="SomeText"> </cc:TestLinkButton> If I want to add some image I will write something like this: <cc:TestLinkButton ID="LinkButton1" runat="server"> <asp:Image ID="Image1" runat="server" ImageUrl="~/Images/icon_holiday.png" BorderWidth="0" /> SomeText </cc:TestLinkButton> In this case Text property will be empty, because actualy "SomeText" is child control property. So question is is how to add tag around child controls.

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  • Rewriting usort function because of fatal error (PHP bug)

    - by Lionel
    The two following usort functions throw fatal error Base lambda function for closure not found in our productive environment (PHP 5.4). This seems to be a known PHP bug that should be fixed by now (https://bugs.php.net/bug.php?id=52144), but it still occurs for us. Anyway, we unfortunately don't have time to figure out what's wrong with our PHP configurations etc. We would like to rewrite these two functions without the use of anonymous functions, so that the error doesn't occur anymore. Ordering of a multidimensional array ($array) by value of key "position": usort($array, function($a, $b) { return $a['position'] - $b['position']; }); Ordering of a multidimensional array ($array) according to the order of a second array ($position_order): usort($array, function($a, $b) use($position_order) { return (isset($position_order[$a['ftid']]) ? ($position_order[$a['ftid']] - $position_order[$b['ftid']]) : 1); }); Especially the latter causes some headache, as we don't know how to pass the "outside" array $position_order.

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  • Google apps API problem with read-only feed?

    - by brandonprry
    Hi all, I am beginning to use the google data api (specifically for the finance app). I can read my portfolio's just fine, so I am authenticating correctly (or so I think). However, when i try and create a portfolio, I get a 'feed is read-only' error. The constructor for the service: public class FinanceService : Service, IService { public FinanceService(string applicationName) : base ("finance", applicationName) { this.RequestFactory = new GDataGAuthRequestFactory("finance", applicationName) { ProtocolMajor = 3 }; } } and saving it is private const string _schema = "http://schemas.google.com/finance/2007"; private const string _feed = "http://finance.google.com/finance/feeds/default/portfolios"; AtomFeed atomFeed = new AtomFeed(new Uri(_feed), this.FinanceService); return this.FinanceService.Insert(atomFeed, this as AtomEntry) as PortfolioEntry; Any idea why the atomFeed would come back ReadOnly? The credentials are legit, and I can get my current portfolios without a problem.

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  • Ruby on Rails: one-to-one mapping. Just semantics or really a different structure.

    - by Sam
    So I'm creating a plugin for Ruby on Rails to make implemented addresses including country, state, city, and zip_code for countries that can follow that paradigm a lot easier but that's beside the point expect for how the address model is associated. So starting with my address model. class Address < ActiveRecord::Base has_one :country has_one :state has_one :city has_one :zip_code end What's the difference between saying belongs_to and has_one Seems to be the same thing because both only require one model to declare ownership and foreign_key And it also seems that both are logical to say. an address belongs to an account and an account has one address Is this only semantics or is there are real difference

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  • how to get SOURCE

    - by laknath27
    i do some development with jena ontology API.my ontology file in my local machine..when i'm going to read the model.. there is an error.. and i made ontology with protege and tried to read that file. String SOURCE = "http://www.owl-ontologies.com/Ontology1275995702";(it's XML:base value) //String NS = SOURCE + "#"; //InputStream in = FileManager.get().open("tourism.owl"); OntModel model = ModelFactory.createOntologyModel(OntModelSpec.OWL_MEM); model.read(SOURCE,"RDF/XML"); OntClass paper = model.getOntClass( SOURCE + "srilanka" ); how can i fix this?

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  • Best Tools for Software Maintenance Engineering

    - by Pev
    Yes, the dreaded 'M' word. You've got a workstation, source control and half a million lines of source code that you didn't write. The documentation was out of date the moment that it was approved and published. The original developers are LTAO, at the next project/startup/loony bin and not answering email. What are you going to do? {favourite editor} and Grep will get you started on your spelunking through the gnarling guts of the code base but what other tools should be in the maintenance engineers toolbox? To start the ball-rolling; I don't think I could live without source-insight for C/C++ spelunking. (DISCLAIMER: I don't work for 'em).

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  • ruby on rails has_many through relationship

    - by BennyB
    Hi i'm having a little trouble with a has_many through relationship for my app and was hoping to find some help. So i've got Users & Lectures. Lectures are created by one user but then other users can then "join" the Lectures that have been created. Users have their own profile feed of the Lectures they have created & also have a feed of Lectures friends have created. This question however is not about creating a lecture but rather "Joining" a lecture that has been created already. I've created a "lecturerelationships" model & controller to handle this relationship between Lectures & the Users who have Joined (which i call "actives"). Users also then MUST "Exit" the Lecture (either by clicking "Exit" or navigating to one of the header navigation links). I'm grateful if anyone can work through some of this with me... I've got: Users.rb model Lectures.rb model Users_controller Lectures_controller then the following model lecturerelationship.rb class lecturerelationship < ActiveRecord::Base attr_accessible :active_id, :joinedlecture_id belongs_to :active, :class_name => "User" belongs_to :joinedlecture, :class_name => "Lecture" validates :active_id, :presence => true validates :joinedlecture_id, :presence => true end lecturerelationships_controller.rb class LecturerelationshipsController < ApplicationController before_filter :signed_in_user def create @lecture = Lecture.find(params[:lecturerelationship][:joinedlecture_id]) current_user.join!(@lecture) redirect_to @lecture end def destroy @lecture = Lecturerelationship.find(params[:id]).joinedlecture current_user.exit!(@user) redirect_to @user end end Lectures that have been created (by friends) show up on a users feed in the following file _activity_item.html.erb <li id="<%= activity_item.id %>"> <%= link_to gravatar_for(activity_item.user, :size => 200), activity_item.user %><br clear="all"> <%= render :partial => 'shared/join', :locals => {:activity_item => activity_item} %> <span class="title"><%= link_to activity_item.title, lecture_url(activity_item) %></span><br clear="all"> <span class="user"> Joined by <%= link_to activity_item.user.name, activity_item.user %> </span><br clear="all"> <span class="timestamp"> <%= time_ago_in_words(activity_item.created_at) %> ago. </span> <% if current_user?(activity_item.user) %> <%= link_to "delete", activity_item, :method => :delete, :confirm => "Are you sure?", :title => activity_item.content %> <% end %> </li> Then you see I link to the the 'shared/join' partial above which can be seen in the file below _join.html.erb <%= form_for(current_user.lecturerelationships.build(:joinedlecture_id => activity_item.id)) do |f| %> <div> <%= f.hidden_field :joinedlecture_id %> </div> <%= f.submit "Join", :class => "btn btn-large btn-info" %> <% end %> Some more files that might be needed: config/routes.rb SampleApp::Application.routes.draw do resources :users do member do get :following, :followers, :joined_lectures end end resources :sessions, :only => [:new, :create, :destroy] resources :lectures, :only => [:create, :destroy, :show] resources :relationships, :only => [:create, :destroy] #for users following each other resources :lecturerelationships, :only => [:create, :destroy] #users joining existing lectures So what happens is the lecture comes in my activity_feed with a Join button option at the bottom...which should create a lecturerelationship of an "active" & "joinedlecture" (which obviously are supposed to be coming from the user & lecture classes. But the error i get when i click the join button is as follows: ActiveRecord::StatementInvalid in LecturerelationshipsController#create SQLite3::ConstraintException: constraint failed: INSERT INTO "lecturerelationships" ("active_id", "created_at", "joinedlecture_id", "updated_at") VALUES (?, ?, ?, ?) Also i've included my user model (seems the error is referring to it) user.rb class User < ActiveRecord::Base attr_accessible :email, :name, :password, :password_confirmation has_secure_password has_many :lectures, :dependent => :destroy has_many :lecturerelationships, :foreign_key => "active_id", :dependent => :destroy has_many :joined_lectures, :through => :lecturerelationships, :source => :joinedlecture before_save { |user| user.email = email.downcase } before_save :create_remember_token validates :name, :presence => true, :length => { :maximum => 50 } VALID_EMAIL_REGEX = /\A[\w+\-.]+@[a-z\d\-.]+\.[a-z]+\z/i validates :email, :presence => true, :format => { :with => VALID_EMAIL_REGEX }, :uniqueness => { :case_sensitive => false } validates :password, :presence => true, :length => { :minimum => 6 } validates :password_confirmation, :presence => true def activity # This feed is for "My Activity" - basically lectures i've started Lecture.where("user_id = ?", id) end def friendactivity Lecture.from_users_followed_by(self) end # lECTURE TO USER (JOINING) RELATIONSHIPS def joined?(selected_lecture) lecturerelationships.find_by_joinedlecture_id(selected_lecture.id) end def join!(selected_lecture) lecturerelationships.create!(:joinedlecture_id => selected_lecture.id) end def exit!(selected_lecture) lecturerelationships.find_by_joinedlecture_id(selected_lecture.id).destroy end end Thanks for any and all help - i'll be on here for a while so as mentioned i'd GREATLY appreciate someone who may have the time to work through my issues with me...

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  • How often do you implement the big three?

    - by Neil Butterworth
    I was just musing about the number of questions here that either are about the "big three" (copy constructor, assignment operator and destructor) or about problems caused by them not being implemented correctly, when it occurred to me that I could not remember the last time I had implemented them myself. A swift grep on my two most active projects indicate that I implement all three in only one class out of about 150. That's not to say I don't implement/declare one or more of them - obviously base classes need a virtual destructor, and a large number of my classes forbid copying using the private copy ctor & assignment op idiom. But fully implemented, there is this single lonely class, which does some reference counting. So I was wondering am I unusual in this? How often do you implement all three of these functions? Is there any pattern to the classes where you do implement them?

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  • MySQL select two tables at the same time...

    - by Jerry
    Hi all I have two tables and want to make a query. I tried to get team AA and team BB's image base on table A. I used: SELECT tableA.team1, tableA.team2, tableB.team, tableB.image, FROM tableA LEFT JOIN tableB ON tableA.team1=tableB.team The result only display imageA on the column. Are there any ways to select imageA and image B without using the second query? I appreciate any helps! Thanks a lot! My table structure are: table A team1 team2 ------------ AA BB table B team image ------------- AA imagaA BB imageB

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  • Can't send flash message from Model method

    - by Andy
    Hello, I'm trying to prevent a record that has a relationship to another record from being deleted. I can stop the deletion but not send a flash message as I had hoped! class Purchaseitem < ActiveRecord::Base before_destroy :check_if_ingredient ... def check_if_ingredient i = Ingredient.find(:all, :conditions => "purchaseitem_id = #{self.id}") if i.length > 0 self.errors.add(:name) flash.now[:notice] = "#{self.name} is in use as an ingredient and cannot be deleted" return false end end This will prevent a the delete wihthout the flash line, and when I add it I get: undefined local variable or method `flash' for # Any help would be much appreciated!

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