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  • Derive abstract class from non-abstract class

    - by Jehof
    Is it OK to derive an abstract class from a non-abstract class or is there something wrong with this approach? Here´s a little example: public class Task { // Some Members } public abstract class PeriodicalTask : Task { // Represents a base class for task that has to be done periodicaly. // Some additional Members } public class DailyTask : PeriodicalTask { // Represents a Task that has to be done daily. // Some additional Members } public class WeeklyTask : PeriodicalTask { // Represents a Task that has to be done weekly. // Some additional Members } In the example above i do not want to make the class Task abstract, because i want to instantiate it directly. PeriodicalTask should inherit the functionality from Task and add some additional members but i do not want to instantiate it directly. Only derived class of PeriodicalTask should be instantiated.

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  • Access violation of member of pointer object

    - by Martin Lauridsen
    So I am coding this client/server program. This code is from the client side. The client has an instance of an object mpqs_sieve *instance_; The reason I make it as a pointer is, that mpqs_sieve only has a constructor that takes 3 arguments, and I want to instantiate it at a later point in time. The client first gets some data from the server, and uses this to instantiate instance_. After this, it will request some more data, and upon receiving this (these are three coeffecients for a quadratic polynomial), it should set these in the instance_ object. However upon calling a member function of instance_, I get an access violation on one of the members of instance_ within that function call. I posted my code here: on pastebin, and I get the error on line 100. The call comes from line 71, and before that line 21. Any ideas to solve this? Thanks!

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  • PHP Doctrine: Filter Table?

    - by ropstah
    I'm still not convinced after my previous question and some experience. Requirements: I don't want to use an SQL query everytime a filterBy() function is called and still be able to call -filterBy() on the returned table. Please find the comment @ ObjectsTable class: "How to instantiate another table and add records which match the filter criteria?" Usage: $globaltable = new ObjectsTable(); //globally accessible variable $globaltable->findAll(); //this call is made once at the beginning of the request $globaltable->filterBy('somefield', $someValue); //this function is used all over the place ObjectsTable class: class ObjectsTable extends Doctrine_Table { function filterBy($field, $value) { //How to instantiate another table and add records which match the criteria? } }

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  • Copy Structure To Another Program

    - by Steven
    Long story, long: I am adding a web interface (ASPX.NET: VB) to a data acquisition system developed with LabVIEW which outputs raw data files. These raw data files are the binary representation of a LabVIEW cluster (essentially a structure). LabVIEW provides functions to instantiate a class or structure or call a method defined in a .NET DLL file. I plan to create a DLL file containing a structure definition and a class with methods to transfer the structure. When the webpage requests data, it would call a LabVIEW executable with a filename parameter. The LabVIEW code would instantiate the structure, populate the structure from the data file, then call the method to transfer the data back to the website. Long story, short: How do you recommend I transfer (copy) an instance of a structure from one .NET program to a VB.NET program? Ideas considered: sockets, temp file, xml file, config file, web services, CSV, some type of serialization, shared memory

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  • Selecting a UserControl from XAML

    - by kmontgom
    I'm working on a problem right now where I need to embed a UserControl inside another UserControl. But, I need to determine at runtime which embedded UserControl to instantiate. This implies to me that some form of data binding and/or template selection mechanism has to be invoked, but I'm not sure how to proceed with the pure XAML approach. If I was to do this with code, I would define some sort of container control in the parent UserControl, and then in the code-behind, implement some logic which would instantiate the appropriate child UserControl and then insert it as Content into the specified container in the parent UserControl. Can this be done using only XAML, or is some sort of code-behind required?

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  • Bug in the official Android Fragments training sample?

    - by Jeff Axelrod
    It seems to me that there must be a bug in the Android Fragments demo. As background, Fragments are apparently sometimes instantiated by the Android OS and thus need a public no-arg constructor: All subclasses of Fragment must include a public empty constructor. The framework will often re-instantiate a fragment class when needed, in particular during state restore, and needs to be able to find this constructor to instantiate it. If the empty constructor is not available, a runtime exception will occur in some cases during state restore. But the NewsReader demo from the official Android training on Fragments constructs the HeadlinesFragment class and configures it with setOnHeadlineSelectedListener(this) from NewsReaderActivity.onCreate(). If the Android OS re-instantiates this fragment, the mHeadlineSelectedListener field will be null because HeadlinesFragment doesn't save or restore its state. Is this a bug or am I missing something?

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  • Static lib that links another static lib and qmake? Odd linking error

    - by Dan O
    I have two qt .pro files, both using the lib TEMPLATE and staticlib CONFIG. The first library (lets call it 'core') is a dependency for the second lib (I'll call it 'foo'). In fact, there's a class in foo that extends a class in core, I will call this class Bar. When I instantiate the class (which is defined and implemented in foo, but extends a class (Bar) from core) in another project (not a lib) I get the following linking error: /usr/bin/ld: Undefined symbols: Bar::Bar() Basically, the linker cannot find the class in the core lib that has been derived in the foo lib, but ONLY when I instantiate the class in a third project that is using both libs. Is this behaviour expected? Regards, Dan O Update: I fixed it by directly invoking the Bars constructor in the third project before using derived class... does anyone know why I need to do this?

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  • Design pattern question: encapsulation or inheritance

    - by Matt
    Hey all, I have a question I have been toiling over for quite a while. I am building a templating engine with two main classes Template.php and Tag.php, with a bunch of extension classes like Img.php and String.php. The program works like this: A Template object creates a Tag objects. Each tag object determines which extension class (img, string, etc.) to implement. The point of the Tag class is to provide helper functions for each extension class such as wrap('div'), addClass('slideshow'), etc. Each Img or String class is used to render code specific to what is required, so $Img->render() would give something like <img src='blah.jpg' /> My Question is: Should I encapsulate all extension functionality within the Tag object like so: Tag.php function __construct($namespace, $args) { // Sort out namespace to determine which extension to call $this->extension = new $namespace($this); // Pass in Tag object so it can be used within extension return $this; // Tag object } function render() { return $this->extension->render(); } Img.php function __construct(Tag $T) { $args = $T->getArgs(); $T->addClass('img'); } function render() { return '<img src="blah.jpg" />'; } Usage: $T = new Tag("img", array(...); $T->render(); .... or should I create more of an inheritance structure because "Img is a Tag" Tag.php public static create($namespace, $args) { // Sort out namespace to determine which extension to call return new $namespace($args); } Img.php class Img extends Tag { function __construct($args) { // Determine namespace then call create tag $T = parent::__construct($namespace, $args); } function render() { return '<img src="blah.jpg" />'; } } Usage: $Img = Tag::create('img', array(...)); $Img->render(); One thing I do need is a common interface for creating custom tags, ie I can instantiate Img(...) then instantiate String(...), I do need to instantiate each extension using Tag. I know this is somewhat vague of a question, I'm hoping some of you have dealt with this in the past and can foresee certain issues with choosing each design pattern. If you have any other suggestions I would love to hear them. Thanks! Matt Mueller

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  • Using StructureMap, how do you explicitly trigger the reinstantiation of a object with InstanceScope

    - by Mark Rogers
    I have an integration test harness where I want to teardown and then re-instantiate some of the singleton-scoped objects I've registered with StructureMap, after and before each test. This way I can simulate the actual run time environment, but not have the singleton's state being passed from one test to another. Maybe this isn't a great way to do an integration test, but I'm running out of alternative solutions (read open to any advice). So can an object with InstanceScope.Singleton, be re-instantiated? What's the best way to do re-instantiate a singleton-scoped object with StructureMap?

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  • Flex SWF assets loaded into Flash SWF at runtime within same ApplicationDomain

    - by Xyre
    I'm trying to load a swf compiled by the Flex SDK into a swf exported by the Flash IDE and instantiate the assets by way of getDefinition(). Normally this works fine with assets exported from the Flash IDE then loaded into another swf also from Flash IDE. This is how I could normally do this using only the Flash IDE: Loader - Using same ApplicationDomain - getDefinition(class) Now, using the 'Test.as' compiled from Flex SDK using the [Embed] metadata tag: Loader - Using same ApplicationDomain - getDefinition("Test_" + class) The problem is I'd rather not have to keep track of the asset libraries loaded to prefix the class name I'd like to get (('Test_" + class) vs (class)). Is there any way of doing this without referencing the library the class is being pulled from or without accessing the original loader? This way I don't need to know which swf the asset is coming from, just the class name that I could instantiate from the current ApplicaitonDomain. Thanks

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  • What to Return? Error String, Bool with Error String Out, or Void with Exception

    - by Ranger Pretzel
    I spend most of my time in C# and am trying to figure out which is the best practice for handling an exception and cleanly return an error message from a called method back to the calling method. For example, here is some ActiveDirectory authentication code. Please imagine this Method as part of a Class (and not just a standalone function.) bool IsUserAuthenticated(string domain, string user, string pass, out errStr) { bool authentic = false; try { // Instantiate Directory Entry object DirectoryEntry entry = new DirectoryEntry("LDAP://" + domain, user, pass); // Force connection over network to authenticate object nativeObject = entry.NativeObject; // No exception thrown? We must be good, then. authentic = true; } catch (Exception e) { errStr = e.Message().ToString(); } return authentic; } The advantages of doing it this way are a clear YES or NO that you can embed right in your If-Then-Else statement. The downside is that it also requires the person using the method to supply a string to get the Error back (if any.) I guess I could overload this method with the same parameters minus the "out errStr", but ignoring the error seems like a bad idea since there can be many reasons for such a failure... Alternatively, I could write a method that returns an Error String (instead of using "out errStr") in which a returned empty string means that the user authenticated fine. string AuthenticateUser(string domain, string user, string pass) { string errStr = ""; try { // Instantiate Directory Entry object DirectoryEntry entry = new DirectoryEntry("LDAP://" + domain, user, pass); // Force connection over network to authenticate object nativeObject = entry.NativeObject; } catch (Exception e) { errStr = e.Message().ToString(); } return errStr; } But this seems like a "weak" way of doing things. Or should I just make my method "void" and just not handle the exception so that it gets passed back to the calling function? void AuthenticateUser(string domain, string user, string pass) { // Instantiate Directory Entry object DirectoryEntry entry = new DirectoryEntry("LDAP://" + domain, user, pass); // Force connection over network to authenticate object nativeObject = entry.NativeObject; } This seems the most sane to me (for some reason). Yet at the same time, the only real advantage of wrapping those 2 lines over just typing those 2 lines everywhere I need to authenticate is that I don't need to include the "LDAP://" string. The downside with this way of doing it is that the user has to put this method in a try-catch block. Thoughts? Is there another way of doing this that I'm not thinking of?

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  • Detecting/Repairing NSConnection failure

    - by anthony
    I would like to use NSConnection/NSDistributedObject for interprocess communication. I would like the client to be able to handle the case where the server is only occasionally reachable. How can I determine if sending a message to the NSConnection will fail or has failed? Currently if my server (the process that has vended the remote object) dies, the client will crash if it sends a selector to the remote object. Ideally I'd like to have a wrapper for the remote object that can lazily instantiate (or reinstantiate) the connection, and return a default value in the case where the connection could not be instantiated, or the connection has failed. I don't really know the correct way to do this using objective c. Here's some pseudocode representing this logic: if myConnection is null: instantiate myConnection if MyConnection is null: return defaultValue try return [myConnection someMethod] catch myConnection = null return defaultValue

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  • Problem with lazy loading implementation

    - by Mehran
    Hi, I have implemented lazy loading in my program. it's done through a proxy class like: class Order { public virtual IList<Item> Items {get; set;} } class OrderProxy { public override IList<Item> Items { get { if (base.Items == null) Items = GetItems(base.OrderID); return base.Items; } set { base.Items = value; } } } the problem is that whenever i instantiate proxy class,without even touching the Items property, it tries to load Items! as you may know,i want to instantiate proxy class and return the instance to BLL instead of domain object itself. what's the problem? Does .NET CLR access(read) properties in a class, when it's instatiating the class? any other methods? Thanks

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  • polymorphism pass instantiated base to deriver

    - by Eric
    I was wondering how to do this, consider the following classes public class Fruit { public string Name { get; set; } public Color Color { get; set; } } public class Apple : Fruit { public Apple() { } } How can I instantiate a new fruit but upcast to Apple, is there a way to instantiate a bunch of Fruit and make them apples with the name & color set. Do I need to manually deep copy? Of course this fails Fruit a = new Fruit(); a.Name = "FirstApple"; a.Color = Color.Red; Apple wa = a as Apple; System.Diagnostics.Debug.Print("Apple name: " + wa.Name); Do I need to pass in a Fruit to the AppleCTor and manually set the name and color( or 1-n properties) Is there an better design to do this?

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  • Instantiating a python class in C#

    - by Jekke
    I've written a class in python that I want to wrap into a .net assembly via IronPython and instantiate in a C# application. I've migrated the class to IronPython, created a library assembly and referenced it. Now, how do I actually get an instance of that class? The class looks (partially) like this: class PokerCard: "A card for playing poker, immutable and unique." def __init__(self, cardName): The test stub I wrote in C# is: using System; namespace pokerapp { class Program { static void Main(string[] args) { var card = new PokerCard(); // I also tried new PokerCard("Ah") Console.WriteLine(card.ToString()); Console.ReadLine(); } } } What do I have to do in order to instantiate this class in C#?

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  • Sending string from class to Form1

    - by Farstucker
    Although there are some similar questions I’m having difficulties finding an answer on how to receive data in my form from a class. I have been trying to read about instantiation and its actually one of the few things that does make sense to me :) but if I were to instantiate my form, would I not have two form objects? To simplify things, lets say I have a some data in Class1 and I would like to pass a string into a label on Form1. Is it legal to instantiate another form1? When trying to do so it looks like I can then access label1.Text but the label isn’t updating. The only thing I can think of is that the form needs to be redrawn or there is some threading issue that I’m unaware of. Any insight you could provide would be greatly appreciated.

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  • Creating New Objects in JavaScript

    - by Ken Ray
    I'm a relatively newbie to object oriented programming in JavaScript, and I'm unsure of the "best" way to define and use objects in JavaScript. I've seen the "canonical" way to define objects and instantiate a new instance, as shown below. function myObjectType(property1, propterty2) { this.property1 = property1, this.property2 = property2 } // now create a new instance var myNewvariable = new myObjectType('value for property1', 'value for property2'); But I've seen other ways to create new instances of objects in this manner: var anotherVariable = new someObjectType({ property1: "Some value for this named property", property2: "This is the value for property 2" }); I like how that second way appears - the code is self documenting. But my questions are: Which way is "better"? Can I use that second way to instantiate a variable of an object type that has been defined using the "classical"way of defining the object type with that implicit constructor? If I want to create an array of these objects, are there any other considerations? Thanks in advance.

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  • Create a Stream without having a physical file to create from.

    - by jhorton
    I'm needing to create a zip file containing documents that exist on the server. I am using the .Net Package class to do so, and to create a new Package (which is the zip file) I have to have either a path to a physical file or a stream. I am trying to not create an actual file that would be the zip file, instead just create a stream that would exist in memory or something. My question is how do you instantiate a new Stream (i.e. FileStream, MemoryStream, etc) without having a physical file to instantiate from.

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  • Python 2.6, 3 abstract base class misunderstanding

    - by Aaron
    I'm not seeing what I expect when I use ABCMeta and abstractmethod. This works fine in python3: from abc import ABCMeta, abstractmethod class Super(metaclass=ABCMeta): @abstractmethod def method(self): pass a = Super() TypeError: Can't instantiate abstract class Super ... And in 2.6: class Super(): __metaclass__ = ABCMeta @abstractmethod def method(self): pass a = Super() TypeError: Can't instantiate abstract class Super ... They both also work fine (I get the expected exception) if I derive Super from object, in addition to ABCMeta. They both "fail" (no exception raised) if I derive Super from list. I want an abstract base class to be a list but abstract, and concrete in sub classes. Am I doing it wrong, or should I not want this in python?

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  • Why's a simple change to rt.jar causing the Java Runtime Environment to crash silently?

    - by Tom
    This is what I'm doing: extract contents of my JRE's rt.jar extract src.zip of my JDK (same version) Now, if I copy Runtime.java from the extracted src folder and compile it using javac.exe without any modifications and then put it in the extracted rt folder to finally put everything back in a jar file using jar.exe, everything works as expected. The JRE runs fine. However, if I make the slightest change to Runtime.java and compile it and put it in rt.jar, the JRE crashes whenever I attempt to start it. This is an example of a slight change that causes the silent crash: /** Don't let anyone else instantiate this class */ private Runtime() { System.out.println("This is a test."); } Instead of: /** Don't let anyone else instantiate this class */ private Runtime() {} Could anyone tell me why this is causing my JRE to crash? Thanks in advance.

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  • How to implement an abstract class in ruby?

    - by Chirantan
    I know there is no concept of abstract class in ruby. But if at all it needs to be implemented, how to go about it? I tried something like... class A def self.new raise 'Doh! You are trying to instantiate an abstract class!' end end class B < A ... ... end But when I try to instantiate B, it is internally going to call A.new which is going to raise the exception. Also, modules cannot be instantiated but they cannot be inherited too. making the new method private will also not work. Any pointers?

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  • Grails design for domain class initialization from static data

    - by Allison Eer
    I have some data, stateNames, to instantiate an instance of the object Country. Right now, I will only have one Country but stateNames for each country should be different. What is the best way to instantiate the instance of Country with my data? I am new to grails and would appreciate any "best practices" or common designs. One solution I can think of is to use BootStrap to save the unitedStates instance of Country to the database. What are the cons of this approach? Another solution would be to save the data in a file (in xml?) under web-app folder. If I did this approach, should the unitedStates instance of Country be instantiated by a controller?

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  • How to find the entity with the greatest primary key?

    - by simpatico
    I've an entity LearningUnit that has an int primary key. Actually, it has nothing more. Entity Concept has the following relationship with it: @ManyToOne @Size(min=1,max=7) private LearningUnit learningUnit; In a constructor of Concept I need to retrieve the LearningUnit with the greatest primary key. If no LearningUnit exists yet I instantiate one. I then set this.learningUnit to the retrieved/instantied. Finally, I call the empty constructor of Concept in a try-catch block, to have the entitymanager do the cardinality check. If an exception is thrown (I expect one in the case that already another 7 Concepts are referring to the same LearningUnit. In that case, I case instantiate a new LearningUnit with a new greater primary key. Please, also point out, if any, clear pitfalls in my outlined algorithm above.

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  • What's the best way to return different cells depending on position in a UITableView

    - by Robert Conn
    I have a grouped UITableView that has 3 sections and a number of cells in each section that each need to be a different custom cell, with different display requirements. I currently have a large if statement in cellForRowAtIndexPath, with each branch instantiating and returning the appropriate custom cell based on interrogating the indexPath's section and row properties. e.g. if (indexPath.section == 0 && indexPath.row == 0) { // instantiate and return cell A } else if (indexPath.section == 1 && indexPath.row == 2) { // instantiate and return cell B } //etc etc What is best practice for this scenario? A large if statement does the job, but is it the best implementation?

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