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  • Why "object reference not set to an instance of an object" doesn't tell us which object?

    - by Saeed Neamati
    We're launching a system, and we sometimes get the famous exception NullReferenceException with the message Object reference not set to an instance of an object. However, in a method where we have almost 20 objects, having a log which says an object is null, is really of no use at all. It's like telling you, when you are the security agent of a seminar, that a man among 100 attendees is a terrorist. That's really of no use to you at all. You should get more information, if you want to detect which man is the threatening man. Likewise, if we want to remove the bug, we do need to know which object is null. Now, something has obsessed my mind for several months, and that is: Why .NET doesn't give us the name, or at least the type of the object reference, which is null?. Can't it understand the type from reflection or any other source? Also, what are the best practices to understand which object is null? Should we always test nullability of objects in these contexts manually and log the result? Is there a better way?

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  • Associating an object with another object for GC clearup

    - by thecoop
    Is there any way of associating an object instance (object A) with a second object (object B) in a generalised way, so that when B gets collected A becomes eligable for collection? The same behaviour that would happen if B had an instance variable pointing to A, but without explicitly changing the class definition of B, and being able to do this in a dynamic way? The same sort of effect could be done by using the Component.Disposed event in a funky way, but I don't want to make B disposable EDIT I'm basically creating a cache of objects that are associated with a single 'root' object, and I don't want the cache to be static, as there can be lots of root objects using different caches, so lots of memory will be used up when a root object is no longer used but the cached objects are still around. So, I want a collection of cached objects to be associated with each 'root' object, without changing the root object definition. Sort of like metadata of an extra object reference attached to each root object instance. That way, each collection will get collected when the root object is collected, and not hang around like they would if a static cache was used.

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  • Are instance initializers good or bad?

    - by berry120
    I personally quite like instance initializers - I use them to assign default values to things such as collections so when writing constructors I don't have to remember to assign them the same default values each time. It seems quite elegant to me - avoids annoying NPE's popping up and avoids duplicate code. A private method doesn't seem as nice because a) it can't assign values to final fields, b) it could be run elsewhere in code and c) the method still needs to be explicitly called at the start of each constructor. However, the flip side with others I have spoken to is that they're confusing, some people reading the code might not understand what they do or when they're called and thus they could cause more problems than they solve. Are proper use of these initializers something to be encouraged or avoided? Or is it an "each to their own" case?

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  • Use a custom value object or a Guid as an entity identifier in a distributed system?

    - by Kazark
    tl;dr I've been told that in domain-driven design, an identifier for an entity could be a custom value object, i.e. something other than Guid, string, int, etc. Can this really be advisable in a distributed system? Long version I will invent an situation analogous to the one I am currently facing. Say I have a distributed system in which a central concept is an egg. The system allows you to order eggs and see spending reports and inventory-centric data such as quantity on hand, usage, valuation and what have you. There area variety of services backing these behaviors. And say there is also another app which allows you to compose recipes that link to a particular egg type. Now egg type is broken down by the species—ostrich, goose, duck, chicken, quail. This is fine and dandy because it means that users don't end up with ostrich eggs when they wanted quail eggs and whatnot. However, we've been getting complaints because jumbo chicken eggs are not even close to equivalent to small ones. The price is different, and they really aren't substitutable in recipes. And here we thought we were doing users a favor by not overwhelming them with too many options. Currently each of the services (say, OrderSubmitter, EggTypeDefiner, SpendingReportsGenerator, InventoryTracker, RecipeCreator, RecipeTracker, or whatever) are identifying egg types with an industry-standard integer representation the species (let's call it speciesCode). We realize we've goofed up because this change could effect every service. There are two basic proposed solutions: Use a predefined identifier type like Guid as the eggTypeID throughout all the services, but make EggTypeDefiner the only service that knows that this maps to a speciesCode and eggSizeCode (and potentially to an isOrganic flag in the future, or whatever). Use an EggTypeID value object which is a combination of speciesCode and eggSizeCode in every service. I've proposed the first solution because I'm hoping it better encapsulates the definition of what an egg type is in the EggTypeDefiner and will be more resilient to changes, say if some people now want to differentiate eggs by whether or not they are "organic". The second solution is being suggested by some people who understand DDD better than I do in the hopes that less enrichment and lookup will be necessary that way, with the justification that in DDD using a value object as an ID is fine. Also, they are saying that EggTypeDefiner is not a domain and EggType is not an entity and as such should not have a Guid for an ID. However, I'm not sure the second solution is viable. This "value object" is going to have to be serialized into JSON and URLs for GET requests and used with a variety of technologies (C#, JavaScript...) which breaks encapsulation and thus removes any behavior of the identifier value object (is either of the fields optional? etc.) Is this a case where we want to avoid something that would normally be fine in DDD because we are trying to do DDD in a distributed fashion? Summary Can it be a good idea to use a custom value object as an identifier in a distributed system (solution #2)?

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  • JavaScriptSerializer deserialize object "collection" as property in object failing

    - by bill
    Hi All, I have a js object structured like: object.property1 = "some string"; object.property2 = "some string"; object.property3.property1 = "some string"; object.property3.property2 = "some string"; object.property3.property2 = "some string"; i'm using JSON.stringify(object) to pass this with ajax request. When i try to deserialize this using JavaScriptSerializer.Deserialize as a Dictionary i get the following error: No parameterless constructor defined for type of 'System.String'. This exact same process is working for regular object with non "collection" properties.. thanks for any help!

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  • Object mapping in objective-c (iphone) from JSON

    - by freshfunk
    For my iPhone app, I'm consuming a RESTful service and getting JSON. I've found libraries to deserialize this into an NSDictionary. However, I'm wondering if there are any libraries to deserialize the JSON/NSDictionary/Property List into my object (an arbitrary one on my side). The java equivalent would be the object-relational mappers although the sort of object mapping I'm looking for is relatively straightforward (simple data types, no complex relationships, etc.). I noticed that Objective-C does have introspection so it seems theoretically possible but I haven't found a library to do it. Or is there a simple way to load an object from an NSDictionary/Property List object that doesn't require modification every time the object changes? For example: { "id" : "user1", "name" : "mister foobar" "age" : 20 } gets loaded into object @interface User : NSObject { NSString *id; NSString *name; int *age; }

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  • using an existing object in ajax-called php files?

    - by noname
    i have in my index.php created an object and set some property values. then i use jquery ajax to call some php files and i want to use the object created. i tried this one but it didn´t work: ---- in index.php ---- // Create a new object session_start(); $object = new stdClass(); $object->value = 'something'; $object->other_value = 'something else'; // Save the object in the user's session $_SESSION['object'] = $object; ---- Then in the next page that loads from AJAX ---- // Start the session saved from last time session_start(); // Get the object out $object = $_SESSION['object']; // Prints "something" print $object->value; how do i accomplish this. cause i dont want to recreate the object in every ajaxcalled php script. thanks in advance!

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  • What's the benefit of object-oriented programming over procedural programming?

    - by niko
    I'm trying to understand the difference between procedural languages like C and object-oriented languages like C++. I've never used C++, but I've been discussing with my friends on how to differentiate the two. I've been told C++ has object-oriented concepts as well as public and private modes for definition of variables: things C does not have. I've never had to use these for while developing programs in Visual Basic.NET: what are the benefits of these? I've also been told that if a variable is public, it can be accessed anywhere, but it's not clear how that's different from a global variable in a language like C. It's also not clear how a private variable differs from a local variable. Another thing I've heard is that, for security reasons, if a function needs to be accessed it should be inherited first. The use-case is that an administrator should only have as much rights as they need and not everything, but it seems a conditional would work as well: if ( login == "admin") { // invoke the function } Why is this not ideal? Given that there seems to be a procedural way to do everything object-oriented, why should I care about object-oriented programming?

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  • Questioning the motivation for dependency injection: Why is creating an object graph hard?

    - by oberlies
    Dependency injection frameworks like Google Guice give the following motivation for their usage (source): To construct an object, you first build its dependencies. But to build each dependency, you need its dependencies, and so on. So when you build an object, you really need to build an object graph. Building object graphs by hand is labour intensive (...) and makes testing difficult. But I don't buy this argument: Even without dependency injection, I can write classes which are both easy to instantiate and convenient to test. E.g. the example from the Guice motivation page could be rewritten in the following way: class BillingService { private final CreditCardProcessor processor; private final TransactionLog transactionLog; // constructor for tests, taking all collaborators as parameters BillingService(CreditCardProcessor processor, TransactionLog transactionLog) { this.processor = processor; this.transactionLog = transactionLog; } // constructor for production, calling the (productive) constructors of the collaborators public BillingService() { this(new PaypalCreditCardProcessor(), new DatabaseTransactionLog()); } public Receipt chargeOrder(PizzaOrder order, CreditCard creditCard) { ... } } So dependency injection may really be an advantage in advanced use cases, but I don't need it for easy construction and testability, do I?

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  • Is the Entity Component System architecture object oriented by definition?

    - by tieTYT
    Is the Entity Component System architecture object oriented, by definition? It seems more procedural or functional to me. My opinion is that it doesn't prevent you from implementing it in an OO language, but it would not be idiomatic to do so in a staunchly OO way. It seems like ECS separates data (E & C) from behavior (S). As evidence: The idea is to have no game methods embedded in the entity. And: The component consists of a minimal set of data needed for a specific purpose Systems are single purpose functions that take a set of entities which have a specific component I think this is not object oriented because a big part of being object oriented is combining your data and behavior together. As evidence: In contrast, the object-oriented approach encourages the programmer to place data where it is not directly accessible by the rest of the program. Instead, the data is accessed by calling specially written functions, commonly called methods, which are bundled in with the data. ECS, on the other hand, seems to be all about separating your data from your behavior.

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  • Why can't Java servlet sent out an object ?

    - by Frank
    I use the following method to send out an object from a servlet : public void doGet(HttpServletRequest request,HttpServletResponse response) throws IOException { String Full_URL=request.getRequestURL().append("?"+request.getQueryString()).toString(); String Contact_Id=request.getParameter("Contact_Id"); String Time_Stamp=Get_Date_Format(6),query="select from "+Contact_Info_Entry.class.getName()+" where Contact_Id == '"+Contact_Id+"' order by Contact_Id desc"; PersistenceManager pm=null; try { pm=PMF.get().getPersistenceManager(); // note that this returns a list, there could be multiple, DataStore does not ensure uniqueness for non-primary key fields List<Contact_Info_Entry> results=(List<Contact_Info_Entry>)pm.newQuery(query).execute(); Write_Serialized_XML(response.getOutputStream(),results.get(0)); } catch (Exception e) { Send_Email(Email_From,Email_To,"Check_License_Servlet Error [ "+Time_Stamp+" ]",new Text(e.toString()+"\n"+Get_Stack_Trace(e)),null); } finally { pm.close(); } } /** Writes the object and CLOSES the stream. Uses the persistance delegate registered in this class. * @param os The stream to write to. * @param o The object to be serialized. */ public static void writeXMLObject(OutputStream os,Object o) { // Classloader reference must be set since netBeans uses another class loader to loead the bean wich will fail in some circumstances. ClassLoader oldClassLoader=Thread.currentThread().getContextClassLoader(); Thread.currentThread().setContextClassLoader(Check_License_Servlet.class.getClassLoader()); XMLEncoder encoder=new XMLEncoder(os); encoder.setExceptionListener(new ExceptionListener() { public void exceptionThrown(Exception e) { e.printStackTrace(); }}); encoder.writeObject(o); encoder.flush(); encoder.close(); Thread.currentThread().setContextClassLoader(oldClassLoader); } private static ByteArrayOutputStream writeOutputStream=new ByteArrayOutputStream(16384); /** Writes an object to XML. * @param out The boject out to write to. [ Will not be closed. ] * @param o The object to write. */ public static synchronized void writeAsXML(ObjectOutput out,Object o) throws IOException { writeOutputStream.reset(); writeXMLObject(writeOutputStream,o); byte[] Bt_1=writeOutputStream.toByteArray(); byte[] Bt_2=new Des_Encrypter().encrypt(Bt_1,Key); out.writeInt(Bt_2.length); out.write(Bt_2); out.flush(); out.close(); } public static synchronized void Write_Serialized_XML(OutputStream Output_Stream,Object o) throws IOException { writeAsXML(new ObjectOutputStream(Output_Stream),o); } At the receiving end the code look like this : File_Url="http://"+Site_Url+App_Dir+File_Name; try { Contact_Info_Entry Online_Contact_Entry=(Contact_Info_Entry)Read_Serialized_XML(new URL(File_Url)); } catch (Exception e) { e.printStackTrace(); } private static byte[] readBuf=new byte[16384]; public static synchronized Object readAsXML(ObjectInput in) throws IOException { // Classloader reference must be set since netBeans uses another class loader to load the bean which will fail under some circumstances. ClassLoader oldClassLoader=Thread.currentThread().getContextClassLoader(); Thread.currentThread().setContextClassLoader(Tool_Lib_Simple.class.getClassLoader()); int length=in.readInt(); readBuf=new byte[length]; in.readFully(readBuf,0,length); byte Bt[]=new Des_Encrypter().decrypt(readBuf,Key); XMLDecoder dec=new XMLDecoder(new ByteArrayInputStream(Bt,0,Bt.length)); Object o=dec.readObject(); Thread.currentThread().setContextClassLoader(oldClassLoader); in.close(); return o; } public static synchronized Object Read_Serialized_XML(URL File_Url) throws IOException { return readAsXML(new ObjectInputStream(File_Url.openStream())); } But I can't get the object from the Java app that's on the receiving end, why ? The error messages look like this : java.lang.ClassNotFoundException: PayPal_Monitor.Contact_Info_Entry Continuing ... java.lang.NullPointerException: target should not be null Continuing ... java.lang.NullPointerException: target should not be null Continuing ... java.lang.NullPointerException: target should not be null Continuing ...

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  • Game Object Factory: Fixing Memory Leaks

    - by Bunkai.Satori
    Dear all, this is going to be tough: I have created a game object factory that generates objects of my wish. However, I get memory leaks which I can not fix. Memory leaks are generated by return new Object(); in the bottom part of the code sample. static BaseObject * CreateObjectFunc() { return new Object(); } How and where to delete the pointers? I wrote bool ReleaseClassType(). Despite the factory works well, ReleaseClassType() does not fix memory leaks. bool ReleaseClassTypes() { unsigned int nRecordCount = vFactories.size(); for (unsigned int nLoop = 0; nLoop < nRecordCount; nLoop++ ) { // if the object exists in the container and is valid, then render it if( vFactories[nLoop] != NULL) delete vFactories[nLoop](); } return true; } Before taking a look at the code below, let me help you in that my CGameObjectFactory creates pointers to functions creating particular object type. The pointers are stored within vFactories vector container. I have chosen this way because I parse an object map file. I have object type IDs (integer values) which I need to translate them into real objects. Because I have over 100 different object data types, I wished to avoid continuously traversing very long Switch() statement. Therefore, to create an object, I call vFactoriesnEnumObjectTypeID via CGameObjectFactory::create() to call stored function that generates desired object. The position of the appropriate function in the vFactories is identical to the nObjectTypeID, so I can use indexing to access the function. So the question remains, how to proceed with garbage collection and avoid reported memory leaks? #ifndef GAMEOBJECTFACTORY_H_UNIPIXELS #define GAMEOBJECTFACTORY_H_UNIPIXELS //#include "MemoryManager.h" #include <vector> template <typename BaseObject> class CGameObjectFactory { public: // cleanup and release registered object data types bool ReleaseClassTypes() { unsigned int nRecordCount = vFactories.size(); for (unsigned int nLoop = 0; nLoop < nRecordCount; nLoop++ ) { // if the object exists in the container and is valid, then render it if( vFactories[nLoop] != NULL) delete vFactories[nLoop](); } return true; } // register new object data type template <typename Object> bool RegisterClassType(unsigned int nObjectIDParam ) { if(vFactories.size() < nObjectIDParam) vFactories.resize(nObjectIDParam); vFactories[nObjectIDParam] = &CreateObjectFunc<Object>; return true; } // create new object by calling the pointer to the appropriate type function BaseObject* create(unsigned int nObjectIDParam) const { return vFactories[nObjectIDParam](); } // resize the vector array containing pointers to function calls bool resize(unsigned int nSizeParam) { vFactories.resize(nSizeParam); return true; } private: //DECLARE_HEAP; template <typename Object> static BaseObject * CreateObjectFunc() { return new Object(); } typedef BaseObject*(*factory)(); std::vector<factory> vFactories; }; //DEFINE_HEAP_T(CGameObjectFactory, "Game Object Factory"); #endif // GAMEOBJECTFACTORY_H_UNIPIXELS

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  • Query on object id in VQL

    - by Banang
    I'm currently working with the versant object database (using jvi), and have a case where I need to query the database based on an object id. What I'm trying to achieve is something along the lines of Employee employee = new Employee("Mr. Pickles"); session.commit(); FundVQLQuery q = new FundVQLQuery(session, "select * from Employee employee where employee = $1"); q.bind(employee); q.execute(); However, I'm finding out the hard way (I get an EVJ_NOT_A_VALID_KEY_TYPE error thrown at me) that this is infact not the way to do it. Anyone got any experience in working with VQL? Your help is much apreciated! A small clarification: The problem is I'm running some performance tests on the database using the pole position framework, and one of the tests in that framework requires me to fetch an object from the database using either an object reference or a low level object id. Thus, I'm not allowed to reference specific fields in the employee object, but must perform the query on the object in its entirety. So, it's not allowed for me to go "select * from Employee e where e.id = 4", I need it to use the entire object. Accepted answer: Since there is some sort of lunacy magic built into SO that prevents me to mark an accepted answer after a bounty has run out, readers should know that the answer posted by Chris Holmes solves this issue. Readers are encouraged to up-vote his post to further signalize the correctness of his answer to any future readers of this thread.

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  • Passing javascript object to webservice via Jquery ajax

    - by kralco626
    I have a webservice that returns an object [WebMethod] public List<User> ContractorApprovals() I also have a webservice that accepcts an object [WebMethod] public bool SaveContractor(Object u) When I make my webservice calls via Jquery: function ServiceCall(method, parameters, onSucess, onFailure) { var parms = "{" + (($.isArray(parameters)) ? parameters.join(',') : parameters) + "}"; // to json $.ajax({ type: "POST", url: "services/"+method, data: parms, contentType: "application/json; charset=utf-8", dataType: "json", success: function(msg) { if (typeof onSucess == 'function' || typeof onSucess == 'object') onSucess(msg.d); }, error: function(msg, err) { $("#dialog-error").dialog('open');} }); I can call the first one just fine. My onSucess function gets passed a javascript object exactly structured like my User object on the service. However, I am now having trouble getting the object back to the server. I'm accepting Object as a parameter on the server side so I can't inagine there is an issue there. So I'm thinking something is wrong with the parms on the client side but i'm not sure what... I am doing something to the effect ServiceCall("AuthorizationManagerWorkManagement.asmx/ContractorApprovals", "", function(data,args){$("#div").data('user',data[0])}, null) then ServiceCall("AuthorizationManagerWorkManagement.asmx/SaveContractor", $("#div").data('user'), //These also do not work: "{'u': ' + $("#div").data("user") + '}", NOR JSON.stringify({u: userObject}) function(data,args){(alert(data)}, null) I know the first service call works, I can get the data. The second one is causing the "onFailure" method to execute rather then "OnSuccess". Any ideas?

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  • Protected object this object on the rompager server is protected

    - by Sami-L
    I have a windows home server, when I connect to any web site in it I get an authentication window with the next message: "http://mydomain.com site requires user name and password, the site says: "SmartAX". Then when I close the window I get an error page saying: Protected object this object on the rompager server is protected Could you please have an idea on this, have it relation with ADSL router ?

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  • C# Reflection - Casting private Object field

    - by alhazen
    I have the following classes: public class MyEventArgs : EventArgs { public object State; public MyEventArgs (object state) { this.State = state; } } public class MyClass { // ... public List<string> ErrorMessages { get { return errorMessages; } } } When I raise my event, I set 'State' of the MyEventArgs object to an object of type MyClass. I'm trying to retrieve ErrorMessages by reflection in my event handler: public static void OnEventEnded(object sender, EventArgs args) { Type type = args.GetType(); FieldInfo stateInfo = type.GetField("State"); PropertyInfo errorMessagesInfo = stateInfo.FieldType.GetProperty("ErrorMessages"); object errorMessages = errorMessagesInfo.GetValue(null, null); } But this returns errorMessagesInfo as null (even though stateInfo is not null). Is it possible to retrieve ErrorMessages ? Thank you

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  • object representation and value representation

    - by FredOverflow
    3.9 §4 says: The object representation of an object of type T is the sequence of N unsigned char objects taken up by the object of type T, where N equals sizeof(T). The value representation of an object is the set of bits that hold the value of type T. For trivially copyable types, the value representation is a set of bits in the object representation that determines a value, which is one discrete element of an implementation-defined set of values. Does "The value representation of an object" imply that values are always stored in objects? What is the value representation of non-trivially copyable types?

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  • Does command/query separation apply to a method that creates an object and returns its ID?

    - by Gilles
    Let's pretend we have a service that calls a business process. This process will call on the data layer to create an object of type A in the database. Afterwards we need to call again on another class of the data layer to create an instance of type B in the database. We need to pass some information about A for a foreign key. In the first method we create an object (modify state) and return it's ID (query) in a single method. In the second method we have two methods, one (createA) for the save and the other (getId) for the query. public void FirstMethod(Info info) { var id = firstRepository.createA(info); secondRepository.createB(id); } public void SecondMethod(Info info) { firstRepository.createA(info); var key = firstRepository.getID(info); secondRepository.createB(key); } From my understanding the second method follows command query separation more fully. But I find it wasteful and counter-intuitive to query the database to get the object we have just created. How do you reconcile CQS with such a scenario? Does only the second method follow CQS and if so is it preferable to use it in this case?

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  • If an entity is composed, is it still a god object?

    - by Telastyn
    I am working on a system to configure hardware. Unfortunately, there is tons of variety in the hardware, which means there's a wide variety of capabilities and configurations depending on what specific hardware the software connects to. To deal with this, we're using a Component Based Entity design where the "hardware" class itself is a very thin container for components that are composed at runtime based on what capabilities/configuration are available. This works great, and the design itself has worked well elsewhere (particularly in games). The problem is that all this software does is configure the hardware. As such, almost all of the code is a component of the hardware instance. While the consumer only ever works against the strongly typed interfaces for the components, it could be argued that the class that represents an instance of the hardware is a God Object. If you want to do anything to/with the hardware, you query an interface and work with it. So, even if the components of an object are modular and decoupled well, is their container a God Object and the downsides associated with the anti-pattern?

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  • How to make this OO?

    - by John
    Hello, Sorry for the poor title,I'm new to OOP so I don't know what is the term for what I need to do. I have, say, 10 different Objects that inherit one Object.They have different amount and type of class members,but all of them have one property in common - Visible. type TObj1=class(TObject) private a:integer; ...(More members) Visible:Boolean; end; TObj2=class(TObject) private b:String; ...(More members) Visible:Boolean; end; ...(Other 8 objects) For each of them I have a variable. var Obj1:TObj1; Obj2:TObj2; Obj3:TObj3; ....(Other 7 objects) Rule 1: Only one object can be initialized at a time(others have to be freed) to be visible. For this rule I have a global variable var CurrentVisibleObj:TObject; //Because they all inherit TObject Finally there is a procedure that changes visibility. procedure ChangeObjVisibility(newObj:TObject); begin CurrentVisibleObj.Free; //Free the old object CurrentVisibleObj:=newObj; //assign the new object CurrentVisibleObj:= ??? //Create new object CurrentVisibleObj.Visible:=true; //Set visibility to new object end; There is my problem,I don't know how to initialize it,because the derived class is unknown. How do I do this? I simplified the explanation,in the project there are TFrames each having different controls and I have to set visible/not visible the same way(By leaving only one frame initialized). Sorry again for the title,I'm very new to OOP.

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  • Using Stub Objects

    - by user9154181
    Having told the long and winding tale of where stub objects came from and how we use them to build Solaris, I'd like to focus now on the the nuts and bolts of building and using them. The following new features were added to the Solaris link-editor (ld) to support the production and use of stub objects: -z stub This new command line option informs ld that it is to build a stub object rather than a normal object. In this mode, it accepts the same command line arguments as usual, but will quietly ignore any objects and sharable object dependencies. STUB_OBJECT Mapfile Directive In order to build a stub version of an object, its mapfile must specify the STUB_OBJECT directive. When producing a non-stub object, the presence of STUB_OBJECT causes the link-editor to perform extra validation to ensure that the stub and non-stub objects will be compatible. ASSERT Mapfile Directive All data symbols exported from the object must have an ASSERT symbol directive in the mapfile that declares them as data and supplies the size, binding, bss attributes, and symbol aliasing details. When building the stub objects, the information in these ASSERT directives is used to create the data symbols. When building the real object, these ASSERT directives will ensure that the real object matches the linking interface presented by the stub. Although ASSERT was added to the link-editor in order to support stub objects, they are a general purpose feature that can be used independently of stub objects. For instance you might choose to use an ASSERT directive if you have a symbol that must have a specific address in order for the object to operate properly and you want to automatically ensure that this will always be the case. The material presented here is derived from a document I originally wrote during the development effort, which had the dual goals of providing supplemental materials for the stub object PSARC case, and as a set of edits that were eventually applied to the Oracle Solaris Linker and Libraries Manual (LLM). The Solaris 11 LLM contains this information in a more polished form. Stub Objects A stub object is a shared object, built entirely from mapfiles, that supplies the same linking interface as the real object, while containing no code or data. Stub objects cannot be used at runtime. However, an application can be built against a stub object, where the stub object provides the real object name to be used at runtime, and then use the real object at runtime. When building a stub object, the link-editor ignores any object or library files specified on the command line, and these files need not exist in order to build a stub. Since the compilation step can be omitted, and because the link-editor has relatively little work to do, stub objects can be built very quickly. Stub objects can be used to solve a variety of build problems: Speed Modern machines, using a version of make with the ability to parallelize operations, are capable of compiling and linking many objects simultaneously, and doing so offers significant speedups. However, it is typical that a given object will depend on other objects, and that there will be a core set of objects that nearly everything else depends on. It is necessary to impose an ordering that builds each object before any other object that requires it. This ordering creates bottlenecks that reduce the amount of parallelization that is possible and limits the overall speed at which the code can be built. Complexity/Correctness In a large body of code, there can be a large number of dependencies between the various objects. The makefiles or other build descriptions for these objects can become very complex and difficult to understand or maintain. The dependencies can change as the system evolves. This can cause a given set of makefiles to become slightly incorrect over time, leading to race conditions and mysterious rare build failures. Dependency Cycles It might be desirable to organize code as cooperating shared objects, each of which draw on the resources provided by the other. Such cycles cannot be supported in an environment where objects must be built before the objects that use them, even though the runtime linker is fully capable of loading and using such objects if they could be built. Stub shared objects offer an alternative method for building code that sidesteps the above issues. Stub objects can be quickly built for all the shared objects produced by the build. Then, all the real shared objects and executables can be built in parallel, in any order, using the stub objects to stand in for the real objects at link-time. Afterwards, the executables and real shared objects are kept, and the stub shared objects are discarded. Stub objects are built from a mapfile, which must satisfy the following requirements. The mapfile must specify the STUB_OBJECT directive. This directive informs the link-editor that the object can be built as a stub object, and as such causes the link-editor to perform validation and sanity checking intended to guarantee that an object and its stub will always provide identical linking interfaces. All function and data symbols that make up the external interface to the object must be explicitly listed in the mapfile. The mapfile must use symbol scope reduction ('*'), to remove any symbols not explicitly listed from the external interface. All global data exported from the object must have an ASSERT symbol attribute in the mapfile to specify the symbol type, size, and bss attributes. In the case where there are multiple symbols that reference the same data, the ASSERT for one of these symbols must specify the TYPE and SIZE attributes, while the others must use the ALIAS attribute to reference this primary symbol. Given such a mapfile, the stub and real versions of the shared object can be built using the same command line for each, adding the '-z stub' option to the link for the stub object, and omiting the option from the link for the real object. To demonstrate these ideas, the following code implements a shared object named idx5, which exports data from a 5 element array of integers, with each element initialized to contain its zero-based array index. This data is available as a global array, via an alternative alias data symbol with weak binding, and via a functional interface. % cat idx5.c int _idx5[5] = { 0, 1, 2, 3, 4 }; #pragma weak idx5 = _idx5 int idx5_func(int index) { if ((index 4)) return (-1); return (_idx5[index]); } A mapfile is required to describe the interface provided by this shared object. % cat mapfile $mapfile_version 2 STUB_OBJECT; SYMBOL_SCOPE { _idx5 { ASSERT { TYPE=data; SIZE=4[5] }; }; idx5 { ASSERT { BINDING=weak; ALIAS=_idx5 }; }; idx5_func; local: *; }; The following main program is used to print all the index values available from the idx5 shared object. % cat main.c #include <stdio.h> extern int _idx5[5], idx5[5], idx5_func(int); int main(int argc, char **argv) { int i; for (i = 0; i The following commands create a stub version of this shared object in a subdirectory named stublib. elfdump is used to verify that the resulting object is a stub. The command used to build the stub differs from that of the real object only in the addition of the -z stub option, and the use of a different output file name. This demonstrates the ease with which stub generation can be added to an existing makefile. % cc -Kpic -G -M mapfile -h libidx5.so.1 idx5.c -o stublib/libidx5.so.1 -zstub % ln -s libidx5.so.1 stublib/libidx5.so % elfdump -d stublib/libidx5.so | grep STUB [11] FLAGS_1 0x4000000 [ STUB ] The main program can now be built, using the stub object to stand in for the real shared object, and setting a runpath that will find the real object at runtime. However, as we have not yet built the real object, this program cannot yet be run. Attempts to cause the system to load the stub object are rejected, as the runtime linker knows that stub objects lack the actual code and data found in the real object, and cannot execute. % cc main.c -L stublib -R '$ORIGIN/lib' -lidx5 -lc % ./a.out ld.so.1: a.out: fatal: libidx5.so.1: open failed: No such file or directory Killed % LD_PRELOAD=stublib/libidx5.so.1 ./a.out ld.so.1: a.out: fatal: stublib/libidx5.so.1: stub shared object cannot be used at runtime Killed We build the real object using the same command as we used to build the stub, omitting the -z stub option, and writing the results to a different file. % cc -Kpic -G -M mapfile -h libidx5.so.1 idx5.c -o lib/libidx5.so.1 Once the real object has been built in the lib subdirectory, the program can be run. % ./a.out [0] 0 0 0 [1] 1 1 1 [2] 2 2 2 [3] 3 3 3 [4] 4 4 4 Mapfile Changes The version 2 mapfile syntax was extended in a number of places to accommodate stub objects. Conditional Input The version 2 mapfile syntax has the ability conditionalize mapfile input using the $if control directive. As you might imagine, these directives are used frequently with ASSERT directives for data, because a given data symbol will frequently have a different size in 32 or 64-bit code, or on differing hardware such as x86 versus sparc. The link-editor maintains an internal table of names that can be used in the logical expressions evaluated by $if and $elif. At startup, this table is initialized with items that describe the class of object (_ELF32 or _ELF64) and the type of the target machine (_sparc or _x86). We found that there were a small number of cases in the Solaris code base in which we needed to know what kind of object we were producing, so we added the following new predefined items in order to address that need: NameMeaning ...... _ET_DYNshared object _ET_EXECexecutable object _ET_RELrelocatable object ...... STUB_OBJECT Directive The new STUB_OBJECT directive informs the link-editor that the object described by the mapfile can be built as a stub object. STUB_OBJECT; A stub shared object is built entirely from the information in the mapfiles supplied on the command line. When the -z stub option is specified to build a stub object, the presence of the STUB_OBJECT directive in a mapfile is required, and the link-editor uses the information in symbol ASSERT attributes to create global symbols that match those of the real object. When the real object is built, the presence of STUB_OBJECT causes the link-editor to verify that the mapfiles accurately describe the real object interface, and that a stub object built from them will provide the same linking interface as the real object it represents. All function and data symbols that make up the external interface to the object must be explicitly listed in the mapfile. The mapfile must use symbol scope reduction ('*'), to remove any symbols not explicitly listed from the external interface. All global data in the object is required to have an ASSERT attribute that specifies the symbol type and size. If the ASSERT BIND attribute is not present, the link-editor provides a default assertion that the symbol must be GLOBAL. If the ASSERT SH_ATTR attribute is not present, or does not specify that the section is one of BITS or NOBITS, the link-editor provides a default assertion that the associated section is BITS. All data symbols that describe the same address and size are required to have ASSERT ALIAS attributes specified in the mapfile. If aliased symbols are discovered that do not have an ASSERT ALIAS specified, the link fails and no object is produced. These rules ensure that the mapfiles contain a description of the real shared object's linking interface that is sufficient to produce a stub object with a completely compatible linking interface. SYMBOL_SCOPE/SYMBOL_VERSION ASSERT Attribute The SYMBOL_SCOPE and SYMBOL_VERSION mapfile directives were extended with a symbol attribute named ASSERT. The syntax for the ASSERT attribute is as follows: ASSERT { ALIAS = symbol_name; BINDING = symbol_binding; TYPE = symbol_type; SH_ATTR = section_attributes; SIZE = size_value; SIZE = size_value[count]; }; The ASSERT attribute is used to specify the expected characteristics of the symbol. The link-editor compares the symbol characteristics that result from the link to those given by ASSERT attributes. If the real and asserted attributes do not agree, a fatal error is issued and the output object is not created. In normal use, the link editor evaluates the ASSERT attribute when present, but does not require them, or provide default values for them. The presence of the STUB_OBJECT directive in a mapfile alters the interpretation of ASSERT to require them under some circumstances, and to supply default assertions if explicit ones are not present. See the definition of the STUB_OBJECT Directive for the details. When the -z stub command line option is specified to build a stub object, the information provided by ASSERT attributes is used to define the attributes of the global symbols provided by the object. ASSERT accepts the following: ALIAS Name of a previously defined symbol that this symbol is an alias for. An alias symbol has the same type, value, and size as the main symbol. The ALIAS attribute is mutually exclusive to the TYPE, SIZE, and SH_ATTR attributes, and cannot be used with them. When ALIAS is specified, the type, size, and section attributes are obtained from the alias symbol. BIND Specifies an ELF symbol binding, which can be any of the STB_ constants defined in <sys/elf.h>, with the STB_ prefix removed (e.g. GLOBAL, WEAK). TYPE Specifies an ELF symbol type, which can be any of the STT_ constants defined in <sys/elf.h>, with the STT_ prefix removed (e.g. OBJECT, COMMON, FUNC). In addition, for compatibility with other mapfile usage, FUNCTION and DATA can be specified, for STT_FUNC and STT_OBJECT, respectively. TYPE is mutually exclusive to ALIAS, and cannot be used in conjunction with it. SH_ATTR Specifies attributes of the section associated with the symbol. The section_attributes that can be specified are given in the following table: Section AttributeMeaning BITSSection is not of type SHT_NOBITS NOBITSSection is of type SHT_NOBITS SH_ATTR is mutually exclusive to ALIAS, and cannot be used in conjunction with it. SIZE Specifies the expected symbol size. SIZE is mutually exclusive to ALIAS, and cannot be used in conjunction with it. The syntax for the size_value argument is as described in the discussion of the SIZE attribute below. SIZE The SIZE symbol attribute existed before support for stub objects was introduced. It is used to set the size attribute of a given symbol. This attribute results in the creation of a symbol definition. Prior to the introduction of the ASSERT SIZE attribute, the value of a SIZE attribute was always numeric. While attempting to apply ASSERT SIZE to the objects in the Solaris ON consolidation, I found that many data symbols have a size based on the natural machine wordsize for the class of object being produced. Variables declared as long, or as a pointer, will be 4 bytes in size in a 32-bit object, and 8 bytes in a 64-bit object. Initially, I employed the conditional $if directive to handle these cases as follows: $if _ELF32 foo { ASSERT { TYPE=data; SIZE=4 } }; bar { ASSERT { TYPE=data; SIZE=20 } }; $elif _ELF64 foo { ASSERT { TYPE=data; SIZE=8 } }; bar { ASSERT { TYPE=data; SIZE=40 } }; $else $error UNKNOWN ELFCLASS $endif I found that the situation occurs frequently enough that this is cumbersome. To simplify this case, I introduced the idea of the addrsize symbolic name, and of a repeat count, which together make it simple to specify machine word scalar or array symbols. Both the SIZE, and ASSERT SIZE attributes support this syntax: The size_value argument can be a numeric value, or it can be the symbolic name addrsize. addrsize represents the size of a machine word capable of holding a memory address. The link-editor substitutes the value 4 for addrsize when building 32-bit objects, and the value 8 when building 64-bit objects. addrsize is useful for representing the size of pointer variables and C variables of type long, as it automatically adjusts for 32 and 64-bit objects without requiring the use of conditional input. The size_value argument can be optionally suffixed with a count value, enclosed in square brackets. If count is present, size_value and count are multiplied together to obtain the final size value. Using this feature, the example above can be written more naturally as: foo { ASSERT { TYPE=data; SIZE=addrsize } }; bar { ASSERT { TYPE=data; SIZE=addrsize[5] } }; Exported Global Data Is Still A Bad Idea As you can see, the additional plumbing added to the Solaris link-editor to support stub objects is minimal. Furthermore, about 90% of that plumbing is dedicated to handling global data. We have long advised against global data exported from shared objects. There are many ways in which global data does not fit well with dynamic linking. Stub objects simply provide one more reason to avoid this practice. It is always better to export all data via a functional interface. You should always hide your data, and make it available to your users via a function that they can call to acquire the address of the data item. However, If you do have to support global data for a stub, perhaps because you are working with an already existing object, it is still easilily done, as shown above. Oracle does not like us to discuss hypothetical new features that don't exist in shipping product, so I'll end this section with a speculation. It might be possible to do more in this area to ease the difficulty of dealing with objects that have global data that the users of the library don't need. Perhaps someday... Conclusions It is easy to create stub objects for most objects. If your library only exports function symbols, all you have to do to build a faithful stub object is to add STUB_OBJECT; and then to use the same link command you're currently using, with the addition of the -z stub option. Happy Stubbing!

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  • constructor function's object literal returns toString() method but no other method

    - by JohnMerlino
    I'm very confused with javascript methods defined in objects and the "this" keyword. In the below example, the toString() method is invoked when Mammal object instantiated: function Mammal(name){ this.name=name; this.toString = function(){ return '[Mammal "'+this.name+'"]'; } } var someAnimal = new Mammal('Mr. Biggles'); alert('someAnimal is '+someAnimal); Despite the fact that the toString() method is not invoked on the object someAnimal like this: alert('someAnimal is '+someAnimal.toString()); It still returns 'someAnimal is [Mammal "Mr. Biggles"]' . That doesn't make sense to me because the toString() function is not being called anywhere. Then to add even more confusion, if I change the toString() method to a method I make up such as random(): function Mammal(name){ this.name=name; this.random = function(){ return Math.floor(Math.random() * 15); } } var someAnimal = new Mammal('Mr. Biggles'); alert(someAnimal); It completely ignores the random method (despite the fact that it is defined the same way was the toString() method was) and returns: [object object] Another issue I'm having trouble understanding with inheritance is the value of "this". For example, in the below example function person(w,h){ width.width = w; width.height = h; } function man(w,h,s) { person.call(this, w, h); this.sex = s; } "this" keyword is being send to the person object clearly. However, does "this" refer to the subclass (man) or the super class (person) when the person object receives it? Thanks for clearing up any of the confusion I have with inheritance and object literals in javascript.

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  • java: assigning object reference IDs for custom serialization

    - by Jason S
    For various reasons I have a custom serialization where I am dumping some fairly simple objects to a data file. There are maybe 5-10 classes, and the object graphs that result are acyclic and pretty simple (each serialized object has 1 or 2 references to another that are serialized). For example: class Foo { final private long id; public Foo(long id, /* other stuff */) { ... } } class Bar { final private long id; final private Foo foo; public Bar(long id, Foo foo, /* other stuff */) { ... } } class Baz { final private long id; final private List<Bar> barList; public Baz(long id, List<Bar> barList, /* other stuff */) { ... } } The id field is just for the serialization, so that when I am serializing to a file, I can write objects by keeping a record of which IDs have been serialized so far, then for each object checking whether its child objects have been serialized and writing the ones that haven't, finally writing the object itself by writing its data fields and the IDs corresponding to its child objects. What's puzzling me is how to assign id's. I thought about it, and it seems like there are three cases for assigning an ID: dynamically-created objects -- id is assigned from a counter that increments reading objects from disk -- id is assigned from the number stored in the disk file singleton objects -- object is created prior to any dynamically-created object, to represent a singleton object that is always present. How can I handle these properly? I feel like I'm reinventing the wheel and there must be a well-established technique for handling all the cases.

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  • Deriving an HTMLElement Object from jQuery Object

    - by Jasconius
    I'm doing a fairly exhaustive series of DOM manipulations where a few elements (specifically form elements) have some events. I am dynamically creating (actually cloning from a source element) several boxes and assigning a change() event to them. The change event executes, and within the context of the event, "this" is the HTML Element Object. What I need to do at this point however is determine a contact for this HTML Element Object. I have these objects stored already as jQuery entities in assorted arrays, but obviously [HTMLElement Object] != [Object Object] And the trick is that I cannot cast $(this) and make a valid comparison since that would create a new object and the pointer would be different. So... I've been banging my head against this for a while. In the past I've been able to circumvent this problem by doing an innerHTML comparison, but in this case the objects I am comparing are 100% identical, just there's lots of them. Therefore I need a solid comparison. This would be easy if I could somehow derive the HTMLElement object from my originating jQuery object. Thoughts, other ideas? Help. :(

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  • Convert JSON flattened for forms back to an object

    - by George Jempty
    I am required (please therefore no nit-picking the requirement, I've already nit-picked it, and this is the req) to convert certain form fields that have "object nesting" embedded in the field names, back to the object(s) themselves. Below are some typical form field names: phones_0_patientPhoneTypeId phones_0_phone phones_1_patientPhoneTypeId phones_1_phone The form fields above were derived from an object such as the one toward the bottom (see "Data"), and that is the format of the object I need to reassemble. It can be assumed that any form field with a name that contains the underscore _ character needs to undergo this conversion. Also that the segment of the form field between underscores, if numeric, signifies a Javascript array, otherwise an object. I found it easy to devise a (somewhat naive) implementation for the "flattening" of the original object for use by the form, but am struggling going in the other direction; below the object/data below I'm pasting my current attempt. One problem (perhaps the only one?) with it is that it does not currently properly account for array indexes, but this might be tricky because the object will subsequently be encoded as JSON, which will not account for sparse arrays. So if "phones_1" exists, but "phones_0" does not, I would nevertheless like to ensure that a slot exists for phones[0] even if that value is null. Implementations that tweak what I have begun, or are entirely different, encouraged. If interested let me know if you'd like to see my code for the "flattening" part that is working. Thanks in advance Data: var obj = { phones: [{ "patientPhoneTypeId": 4, "phone": "8005551212" }, { "patientPhoneTypeId": 2, "phone": "8885551212" }]}; Code to date: var unflattened = {}; for (var prop in values) { if (prop.indexOf('_') > -1) { var lastUnderbarPos = prop.lastIndexOf('_'); var nestedProp = prop.substr(lastUnderbarPos + 1); var nesting = prop.substr(0, lastUnderbarPos).split("_"); var nestedRef, isArray, isObject; for (var i=0, n=nesting.length; i<n; i++) { if (i===0) { nestedRef = unflattened; } if (i < (n-1)) { // not last if (/^\d+$/.test(nesting[i+1])) { isArray = true; isObject = false; } else { isArray = true; isObject = false; } var currProp = nesting[i]; if (!nestedRef[currProp]) { if (isArray) { nestedRef[currProp] = []; } else if (isObject) { nestedRef[currProp] = {}; } } nestedRef = nestedRef[currProp]; } else { nestedRef[nestedProp] = values[prop]; } } }

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