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• Why is quicksort better than other sorting algorithms in practice?

  - by Raphael

  This is a repost of a question on cs.SE by Janoma. Full credits and spoils to him or cs.SE. In a standard algorithms course we are taught that quicksort is O(n log n) on average and O(n²) in the worst case. At the same time, other sorting algorithms are studied which are O(n log n) in the worst case (like mergesort and heapsort), and even linear time in the best case (like bubblesort) but with some additional needs of memory. After a quick glance at some more running times it is natural to say that quicksort should not be as efficient as others. Also, consider that students learn in basic programming courses that recursion is not really good in general because it could use too much memory, etc. Therefore (and even though this is not a real argument), this gives the idea that quicksort might not be really good because it is a recursive algorithm. Why, then, does quicksort outperform other sorting algorithms in practice? Does it have to do with the structure of real-world data? Does it have to do with the way memory works in computers? I know that some memories are way faster than others, but I don't know if that's the real reason for this counter-intuitive performance (when compared to theoretical estimates).

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• Patterns: Local Singleton vs. Global Singleton?

  - by Mike Rosenblum

  There is a pattern that I use from time to time, but I'm not quite sure what it is called. I was hoping that the SO community could help me out. The pattern is pretty simple, and consists of two parts: A singleton factory, which creates objects based on the arguments passed to the factory method. Objects created by the factory. So far this is just a standard "singleton" pattern or "factory pattern". The issue that I'm asking about, however, is that the singleton factory in this case maintains a set of references to every object that it ever creates, held within a dictionary. These references can sometimes be strong references and sometimes weak references, but it can always reference any object that it has ever created. When receiving a request for a "new" object, the factory first searches the dictionary to see if an object with the required arguments already exits. If it does, it returns that object, if not, it returns a new object and also stores a reference to the new object within the dictionary. This pattern prevents having duplicative objects representing the same underlying "thing". This is useful where the created objects are relatively expensive. It can also be useful where these objects perform event handling or messaging - having one object per item being represented can prevent multiple messages/events for a single underlying source. There are probably other reasons to use this pattern, but this is where I've found this useful. My question is: what to call this? In a sense, each object is a singleton, at least with respect to the data it contains. Each is unique. But there are multiple instances of this class, however, so it's not at all a true singleton. In my own personal terminology, I tend to call the factory method a "global singleton". I then call the created objects "local singletons". I sometimes also say that the created objects have "reference equality", meaning that if two variables reference the same data (the same underlying item) then the reference they each hold must be to the same exact object, hence "reference equality". But these are my own invented terms, and I am not sure that they are good ones. Is there standard terminology for this concept? And if not, could some naming suggestions be made? Thanks in advance...

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• Persisting simple tree with (Fluent-)NHibernate leads to System.InvalidCastException

  - by fudge

  Hi there, there seems to be a problem with recursive data structures and (Fluent-)NHibernate or its just me, being a complete moron... here's the tree: public class SimpleNode { public SimpleNode () { this.Children = new List<SimpleNode> (); } public virtual SimpleNode Parent { get; private set; } public virtual List<SimpleNode> Children { get; private set; } public virtual void setParent (SimpleNode parent) { parent.AddChild (this); Parent = parent; } public virtual void AddChild (SimpleNode child) { this.Children.Add (child); } public virtual void AddChildren (IEnumerable<SimpleNode> children) { foreach (var child in children) { AddChild (child); } } } the mapping: public class SimpleNodeEntity : ClassMap<SimpleNode> { public SimpleNodeEntity () { Id (x => x.Id); References (x => x.Parent).Nullable (); HasMany (x => x.Children).Not.LazyLoad ().Inverse ().Cascade.All ().KeyNullable (); } } now, whenever I try to save a node, I get this: System.InvalidCastException: Cannot cast from source type to destination type. at (wrapper dynamic-method) SimpleNode. (object,object[],NHibernate.Bytecode.Lightweight.SetterCallback) at NHibernate.Bytecode.Lightweight.AccessOptimizer.SetPropertyValues (object,object[]) at NHibernate.Tuple.Entity.PocoEntityTuplizer.SetPropertyValuesWithOptimizer (object,object[]) My setup: Mono 2.8.1 (on OSX), NHibernate 2.1.2, FluentNHibernate 1.1.0

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• Trying to run multiple HTTP requests in parallel, but being limited by Windows (registry)

  - by Nailuj

  I'm developing an application (winforms C# .NET 4.0) where I access a lookup functionality from a 3rd party through a simple HTTP request. I call an url with a parameter, and in return I get a small string with the result of the lookup. Simple enough. The challenge is however, that I have to do lots of these lookups (a couple of thousands), and I would like to limit the time needed. Therefore I would like to run requests in parallel (say 10-20). I use a ThreadPool to do this, and the short version of my code looks like this: public void startAsyncLookup(Action<LookupResult> returnLookupResult) { this.returnLookupResult = returnLookupResult; foreach (string number in numbersToLookup) { ThreadPool.QueueUserWorkItem(lookupNumber, number); } } public void lookupNumber(Object threadContext) { string numberToLookup = (string)threadContext; string url = @"http://some.url.com/?number=" + numberToLookup; WebClient webClient = new WebClient(); Stream responseData = webClient.OpenRead(url); LookupResult lookupResult = parseLookupResult(responseData); returnLookupResult(lookupResult); } I fill up numbersToLookup (a List<String>) from another place, call startAsyncLookup and provide it with a call-back function returnLookupResult to return each result. This works, but I found that I'm not getting the throughput I want. Initially I thought it might be the 3rd party having a poor system on their end, but I excluded this by trying to run the same code from two different machines at the same time. Each of the two took as long as one did alone, so I could rule out that one. A colleague then tipped me that this might be a limitation in Windows. I googled a bit, and found amongst others this post saying that by default Windows limits the number of simultaneous request to the same web server to 4 for HTTP 1.0 and to 2 for HTTP 1.1 (for HTTP 1.1 this is actually according to the specification (RFC2068)). The same post referred to above also provided a way to increase these limits. By adding two registry values to [HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings] (MaxConnectionsPerServer and MaxConnectionsPer1_0Server), I could control this myself. So, I tried this (sat both to 20), restarted my computer, and tried to run my program again. Sadly though, it didn't seem to help any. I also kept an eye on the Resource Monitor (see screen shot) while running my batch lookup, and I noticed that my application (the one with the title blacked out) still only was using two TCP connections. So, the question is, why isn't this working? Is the post I linked to using the wrong registry values? Is this perhaps not possible to "hack" in Windows any longer (I'm on Windows 7)? Any ideas would be highly appreciated :) And just in case anyone should wonder, I have also tried with different settings for MaxThreads on ThreadPool (everyting from 10 to 100), and this didn't seem to affect my throughput at all, so the problem shouldn't be there either.

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• Transliteration API: Is it possible to make all input fields in the page transliteratable?

  - by SolidSnakeGTI

  Hello, I've used "Google AJAX Transliteration API" and it's going well with me. http://code.google.com/apis/ajaxlanguage/documentation/referenceTransliteration.html Currently I've a project that I need all input fields in every page (input & textarea tags) to be transliteratable, while these input fields differs from page to page (dynamic). As I know, I've to call makeTransliteratable(elementIds, opt_options) method in the API call to define which input fields to make transliteratable, and in my case here I can't predefine those fields manually. Is there a way to achieve this? Thanks in advance

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• Questions regarding PHPUnit mock feature

  - by Andree

  Can someone provide me a reference to a good PHPUnit mock guide? The one in the official documentation doesn't seem to be detailed enough. I need to know about the following: 1) How to expect multiple calls to a mock object's method, but each return a different sets of value? $tableMock->expects($this->exactly(2)) ->method('find') ->will($this->returnValue(2)); // I need the second call to return different value 2) How to expect a call to a mock object's method with multiple parameters?

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• Problem in passing arrays from C# to C++

  - by Rakesh K

  Hi, I have an application in which I need to pass an array from C# to a C++ DLL. What is the best method to do it? I did some search on Internet and figured out that I need to pass the arrays from C# using ref. The code for the same: status = IterateCL(ref input, ref output); The input and output arrays are of length 20. and the corresponding code in C++ DLL is IterateCL(int *&inArray, int *&outArray) This works fine for once. But if I try to call the function from C# in a loop the second time, the input array in C# is showing up as an array of one element. Why is this happening and please help me how I can call this function iteratively from C#. Thanks, Rakesh.

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• C# Custom Control Properties load before Load and arrays are empties

  - by Wildhorn

  Hello, I made a custom control with custom properties. When I modify these properties in the "Form.cs [Design]", I need stuff to happens (it fill some arrays and modify the look of the control), so I call a function within the "set" of the property. All of this works good. My problem is that when I run the program with my custom control, it seems that properties "set" is called, which will call the function, but then, my arrays seems now to have lost all their values and the function use these arrays, but now because they are all empty, it crashes due to NullException blahblahblah. It also seems that properties "set" is called before the control Load (which I guess is called only when it is added to my Form, and not when the Form load). So question is, why does my arrays become empty once I try to run the Form and is there an event that is called before that when the Form load? Thanks

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• how to use @ in python.. and the @property and the @classmethods

  - by zjm1126

  this is my code: def a(): print 'sss' @a() def b(): print 'aaa' b() and the Traceback is: sss Traceback (most recent call last): File "D:\zjm_code\a.py", line 8, in <module> @a() TypeError: 'NoneType' object is not callable so how to use the '@' thanks updated class a: @property def b(x): print 'sss' aa=a() print aa.b it print : sss None how to use @property thanks updated2 and the classmethods: class a: @classmethods def b(x): print 'sss' aa=a() print aa.b the Traceback is : Traceback (most recent call last): File "D:\zjm_code\a.py", line 5, in <module> class a: File "D:\zjm_code\a.py", line 6, in a @classmethods NameError: name 'classmethods' is not defined

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• Perl: Asynchronous file monitoring

  - by Hussain

  I am wondering if it is possible, and if so how, one could create a perl script that constantly monitors a file/db, and then call a subroutine to perform text processing if the file is changed. I'm pretty sure this would be possible using sockets, but this needs to be used for a webchat application on a site running on a shared host, and I'm not so sure sockets would be allowed on it. The basic idea is: -create a listener for a chat file/database -when the file is updated with a new message, call a subroutine -the called subroutine will send the new message back to the browser to be displayed Thanks in advance.

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• cflock do not throw timeout for same url called in same browser

  - by Pritesh Patel

  I am trying lock block on page test.cfm and below is code written on page. <cfscript> writeOutput("Before lock at #now()#"); lock name="threadlock" timeout="3" type="exclusive" { writeOutput("<br/>started at #now()#"); thread action="sleep" duration="10000"; writeOutput("<br/>ended at #now()#"); } writeOutput("<br/>After lock at #now()#"); </cfscript> assuming my url for page is http://localhost.local/test.cfm and running it on browser in two different tabs. I was expecting one of the url will throw timeout error after 3 second since another url lock it atleast for 10 seconds due to thread sleep. Surprisingly I do not get any timeout error rather second page call run after 10 seconds as first call finish execution. But I am appending some url parameter (e.g. http://localhost.local/test.cfm?q=1) will throw error. Also I am calling same url in different browser then one of the call will throw timeout issue. Is lock based on session and url? Update Here is output for two different cases: Case 1: TAB1 Url: http://localhost.local/test/test.cfm Before lock at {ts '2013-10-18 09:21:35'} started at {ts '2013-10-18 09:21:35'} ended at {ts '2013-10-18 09:21:45'} After lock at {ts '2013-10-18 09:21:45'} TAB2 Url: http://localhost.local/test/test.cfm Before lock at {ts '2013-10-18 09:21:45'} started at {ts '2013-10-18 09:21:45'} ended at {ts '2013-10-18 09:21:55'} After lock at {ts '2013-10-18 09:21:55'} Case 2: TAB1 Url: http://localhost.local/test/test.cfm Before lock at {ts '2013-10-18 09:27:18'} started at {ts '2013-10-18 09:27:18'} ended at {ts '2013-10-18 09:27:28'} After lock at {ts '2013-10-18 09:27:28'} TAB2 Url: http://localhost.local/test/test.cfm? (Added ? at the end) Before lock at {ts '2013-10-18 09:27:20'} A timeout occurred while attempting to lock threadlock. The error occurred in C:/inetpub/wwwroot/test/test.cfm: line 13 11 : 12 : <cfoutput>Before lock at #now()#</cfoutput> 13 : <cflock name="threadlock" timeout="3" type="exclusive"> 14 : <cfoutput><br/>started at #now()#</cfoutput> 15 : <cfthread action="sleep" duration="10000"/> ... Result for case 2 as expected. For case 1, strange thing I just noticed is tab 2 output "Before lock at {ts '2013-10-18 09:21:45'} indicates that whole request start after 10 seconds (means after the complete execution of first tab) when I have fired it in second URL just after 2 seconds of first tabs.

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• WCF REST Does Not Contain All of the Relative File Path

  - by Brandon

  I have a RESTful WCF 3.5 endpoint as such: System.Security.User.svc This is supposed to represent the namespace of the User class and is desired behavior by our client. I have another endpoint I created for testing called: Echo.svc I am writing an overridden IHttpModule and in my module, I follow what almost everyone does by doing: string path = HttpContext.Current.Request.AppRelativeCurrentExecutionFilePath; If I make a call to: http://localhost/services/Echo/test My path variable has a value of '~/echo/test' However, when I make a call to: http://localhost/services/System.Security.User/test My path variable has a value of '~/system.security.user' In my 2nd situation, it is stripping off the '/test' on the end of any endpoint that contains multiple periods. This is undesired behavior and the only solution I have found to fixing this is some ugly string manipulation using the property which does contain the complete URL path: string rawPath = HttpContext.Current.Request.RawUrl; This returns '/services/system.security.user/test'. Does anyone know why my first situation does not return the rest of the URL path for endpoints that contain multiple periods in the name?

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• Writing a line in a JTextArea from another class

  - by Christopher Nastovski

  I'm trying to use a Method in my class 'Visual' to write a line of code into a JTextArea and it prints when I call it from 'Visual' but when I call it from 'Login' It only prints Text into the Console, not to the TextArea. private static Visual Visual; Visual.WriteLine("I'm sorry, your username or password is incorrect. Please try again."); or private static Visual Visual = new Visual(); Visual.WriteLine("I'm sorry, your username or password is incorrect. Please try again."); Dont work form 'Login' But, WriteLine("Test"); works from 'Visual', the class that the method is in. Here's the method in 'Visual' public void WriteLine(String Text) { System.out.println(Text); SystemFeed.append(Text.toString() + "\n"); SystemFeed.setCaretPosition(SystemFeed.getDocument().getLength()); }

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• Passing variables from SQL proceedure to PHP

  - by Sam Corbet

  I am trying to create an sql proceedure that will return the results back to the php page. I want to be able to call the procedure as follows from the php call procedure_name($var1) which will run this script: -- --------------------------------------------------------------------------------- -- pUIGetCliStmtGenFlag -- -- This procedure returns the status of the Trading Period: -- -- --------------------------------------------------------------------------------- drop procedure if exists pUIGetCliStmtGenFlag; delimiter // create procedure pUIGetCliStmtGenFlag( IN pTradingPeriodMonth DATE ) MODIFIES SQL DATA COMMENT 'Checks if the TP has been closed' begin SELECT trading_period_month, dt_end, amt_traded_system_ccy FROM ca_trading_period WHERE trading_period_month=$var1 -- If amt_traded_system_ccy is NULL give the TP an open status otherwise mark as closed IF amt_traded_system_ccy is NULL $tpstatus='open' ELSE $tpstatus='closed' end; // delimiter ; I then want to be able to use $tpstatus in the rest of the php script. I know this is simple but this is completely new to me and I cant find the correct method

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• How to implement async pattern in windows forms application?

  - by Alkersan

  I'm using an MVC pattern in winforms application. I need to call remote service asynchronously. So On some event in View I invoke corresponding Presenter method. In Presenter I call BeginInvoke method of service. But to View must be updated only in Main Thread. I could actualy point CallBack to some function in View, and update it`s controls state, but this conflicts with MVP pattern - View must not be responsible for data it carries. This callback function must be in Presenter. But how then invoke View in Main Thread?

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• Zend Framework: The form filter I am using is not filtering!

  - by Andrew

  So I have a form that is using the Zend_Filter_Null filter. When I call it directly, it works: $makeZeroNull = new Zend_Filter_Null(); $null = $makeZeroNull->filter('0'); //$null === null However, when I try to add it to an element in my form, it doesn't filter the value when I call getValue(). class My_Form extends Zend_Form { public function init() { $makeZeroNull = new Zend_Filter_Null(); $this->addElement('text', 'State_ID', array('filters' => array($makeZeroNull))); } } //in controller if ($form->isValid($_POST)) { $zero = $form->State_ID->getValue(); //getValue() should return null, but it is returning 0 } What is going on? What am I doing wrong?

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• NSNotifications vs delegate for multiple instances of same protocol

  - by Brent Traut

  I could use some architectural advice. I've run into the following problem a few times now and I've never found a truly elegant way to solve it. The issue, described at the highest level possible:I have a parent class that would like to act as the delegate for multiple children (all using the same protocol), but when the children call methods on the parent, the parent no longer knows which child is making the call. I would like to use loose coupling (delegates/protocols or notifications) rather than direct calls. I don't need multiple handlers, so notifications seem like they might be overkill. To illustrate the problem, let me try a super-simplified example: I start with a parent view controller (and corresponding view). I create three child views and insert each of them into the parent view. I would like the parent view controller to be notified whenever the user touches one of the children. There are a few options to notify the parent: Define a protocol. The parent implements the protocol and sets itself as the delegate to each of the children. When the user touches a child view, its view controller calls its delegate (the parent). In this case, the parent is notified that a view is touched, but it doesn't know which one. Not good enough. Same as #1, but define the methods in the protocol to also pass some sort of identifier. When the child tells its delegate that it was touched, it also passes a pointer to itself. This way, the parent know exactly which view was touched. It just seems really strange for an object to pass a reference to itself. Use NSNotifications. The parent defines a separate method for each of the three children and then subscribes to the "viewWasTouched" notification for each of the three children as the notification sender. The children don't need to attach themselves to the user dictionary, but they do need to send the notification with a pointer to themselves as the scope. Same as #4, but rather than using separate methods, the parent could just use one with a switch case or other branching along with the notification's sender to determine which path to take. Create multiple man-in-the-middle classes that act as the delegates to the child views and then call methods on the parent either with a pointer to the child or with some other differentiating factor. This approach doesn't seem scalable. Are any of these approaches considered best practice? I can't say for sure, but it feels like I'm missing something more obvious/elegant.

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• Best way to test if a generic type is a string? (c#)

  - by Rex M

  I have a generic class that should allow any type, primitive or otherwise. The only problem with this is using default(T). When you call default on a value type or a string, it initializes it to a reasonable value (such as empty string). When you call default(T) on an object, it returns null. For various reasons we need to ensure that if it is not a primitive type, then we will have a default instance of the type, not null. Here is attempt 1: T createDefault() { if(typeof(T).IsValueType) { return default(T); } else { return Activator.CreateInstance<T>(); } } Problem - string is not a value type, but it does not have a parameterless constructor. So, the current solution is: T createDefault() { if(typeof(T).IsValueType || typeof(T).FullName == "System.String") { return default(T); } else { return Activator.CreateInstance<T>(); } } But this feels like a kludge. Is there a nicer way to handle the string case?

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• The shortest way to convert infix expressions to postfix (RPN) in C

  - by kuszi

  Original formulation is given here (you can try also your program for correctness) . Additional rules: 1. The program should read from standard input and write do standard output. 2. The program should return zero to the calling system/program. 3. The program should compile and run with gcc -O2 -lm -s -fomit-frame-pointer. The challenge has some history: the call for short implementations has been announced at the Polish programming contest blog in September 2009. After the contest, the shortest code was 81 chars long. Later on the second call has been made for even shorter code and after the year matix2267 published his solution in 78 bytes: main(c){read(0,&c,1)?c-41&&main(c-40&&(c%96<27||main(c),putchar(c))):exit(0);} Anyone to make it even shorter or prove this is impossible?

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• I'm confused with block in ruby, compared to smalltalk.

  - by weakish

  What does block in ruby mean? It looks similar with smalltalk, but you can't send messages to it. For example, in smalltalk: [:x | x + 3] value: 3 returns 6. But in ruby: {|x| x + 3}.call 3 will cause SyntaxError. Well, you can pass messages to lambda in ruby, though: irb(main):025:0> ->(x){x+3}.call 3 => 6 So in ruby, block is not a block, but lambda is a block? Is this true? I mean, are there any differences between ruby lambda and smalltalk block? If this is true, then what is ruby block?

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• ASP.NET MCV 2 controller-url problems

  - by cc0

  I am still very new to the MVC framework, but I managed to create a controller that reads from a database and writes JSON to an url; host.com/Controllername?minValue=something&maxValue=something However when I move the site to a subfolder; host.com/mvc/ it doesn't seem to be able to call the controller from there when I do it like this; host.com/mvc/Procedure?minValue=something&maxValue=something Did I forget to do something somewhere to make this url call valid from that subfolder? Any help here would be greatly appreciated.

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• file_get_contents() returns "failed to open stream" when hitting HTTPS

  - by TMC

  file_get_contents() is returning "failed to open stream" when I call it on an HTTPS URL. Warning: file_get_contents(https://google.com) [function.file-get-contents]: failed to open stream: No error in E:\\htdocs\callback3.php on line 5 Same call will work with a non-SSL URL. At first, I thought it was a security issue with my webhoster, but I have verified with phpinfo() that allow url open is indeed allowed. I have also tried this code and verified it works: Anyone have any ideas why file_get_contents() is failing with an HTTPS URL? update: People correctly pointed out this was an HTTPS issue. My webhoster claims this should work, and has no idea how to resolve this. Anyone have specific directions I can give them for their IIS7 setup?

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• drawImage in Java applet flickers in Safari

  - by lcvinny

  I'm having a flicker problem in a Java applet in Safari (Mac). However, it's not the usual double buffering problem. I have isolated it down to one single drawImage call (no redundant repaint, no clear is called), which gives a white flicker before painting the image but not on every repaint. In fact, I measured the duration of the drawImage call, which is normally about 1ms, but up to 30ms about every 5th time paint is called, which is when it flickers. Repaints are triggered when I drag a component or when the window is scrolled. Is this a bug in the java implementation on OSX, and is there a fix for it?

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• Do I need to invoke MessageBox calls?

  - by mafutrct

  To pop-up a message box, I'm using MessageBox.Show(...). I usually wrap this call in an Invoke: BeginInvoke (new Action (() => { MessageBox.Show ()); })); I've got 2 questions: Do I always need to wrap the MessageBox call in an Invoke if I'm calling from a non-GUI thread? If so, should I use BeginInvoke or Invoke? I found not much difference in my tests, BeginInvoke is, as expected (and unlike Invoke), displayed with a slight delay.

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• C Class Instance from Void Pointer using Ctypes

  - by g.d.d.c

  I've got a C DLL that exposes a handful of methods that return void pointers to a Class like so: void *GetLicense() { static AppLicenseImpl ipds_; return (void *) &ipds_; } In C++, after loading the DLL, I'd do this to work with it: typedef void *(* FPGetLicense)(); GetLicense_ = (FPGetLicense)GetAddress("GetLicense"); license_ = (AppLicense *) GetLicense_(); license_->GetApplicationStatus(); // Load data so that other calls don't fail I can't figure out how to parallel that in Python. This gets me the pointer: d = ctypes.cdll.LoadLibrary('license.dll') d.GetLicense.restype = ctypes.c_void_p p = d.GetLicense() # returns ptr loc, something like 8791433660848L But I obviously can't call p.GetApplicationStatus() in Python. Does anyone have a suggestion on how I'd instantiate that Class the rest of the way in Python so that I can call GetApplicationStatus()?

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