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• Inheritance and type parameters of Traversable

  - by Jesper

  I'm studying the source code of the Scala 2.8 collection classes. I have questions about the hierarchy of scala.collection.Traversable. Look at the following declarations: package scala.collection trait Traversable[+A] extends TraversableLike[A, Traversable[A]] with GenericTraversableTemplate[A, Traversable] trait TraversableLike[+A, +Repr] extends HasNewBuilder[A, Repr] with TraversableOnce[A] package scala.collection.generic trait HasNewBuilder[+A, +Repr] trait GenericTraversableTemplate[+A, +CC[X] <: Traversable[X]] extends HasNewBuilder[A, CC[A] @uncheckedVariance] Question: Why does Traversable extend GenericTraversableTemplate with type parameters [A, Traversable] - why not [A, Traversable[A]]? I tried some experimenting with a small program with the same structure and got a strange error message when I tried to change it to Traversable[A]: error: Traversable[A] takes no type parameters, expected: one I guess that the use of the @uncheckedVariance annotation in GenericTraversableTemplate also has to do with this? (That seems like a kind of potentially unsafe hack to force things to work...). Question: When you look at the hierarchy, you see that Traversable inherits HasNewBuilder twice (once via TraversableLike and once via GenericTraversableTemplate), but with slightly different type parameters. How does this work exactly? Why don't the different type parameters cause an error?

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• Request for advice about class design, inheritance/aggregation

  - by Lasse V. Karlsen

  I have started writing my own WebDAV server class in .NET, and the first class I'm starting with is a WebDAVListener class, modelled after how the HttpListener class works. Since I don't want to reimplement the core http protocol handling, I will use HttpListener for all its worth, and thus I have a question. What would the suggested way be to handle this: Implement all the methods and properties found inside HttpListener, just changing the class types where it matters (ie. the GetContext + EndGetContext methods would return a different class for WebDAV contexts), and storing and using a HttpListener object internally Construct WebDAVListener by passing it a HttpListener class to use? Create a wrapper for HttpListener with an interface, and constrct WebDAVListener by passing it an object implementing this interface? If going the route of passing a HttpListener (disguised or otherwise) to the WebDAVListener, would you expose the underlying listener object through a property, or would you expect the program that used the class to keep a reference to the underlying HttpListener? Also, in this case, would you expose some of the methods of HttpListener through the WebDAVListener, like Start and Stop, or would you again expect the program that used it to keep the HttpListener reference around for all those things? My initial reaction tells me that I want a combination. For one thing, I would like my WebDAVListener class to look like a complete implementation, hiding the fact that there is a HttpListener object beneath it. On the other hand, I would like to build unit-tests without actually spinning up a networked server, so some kind of mocking ability would be nice to have as well, which suggests I would like the interface-wrapper way. One way I could solve this would be this: public WebDAVListener() : WebDAVListener(new HttpListenerWrapper()) { } public WebDAVListener(IHttpListenerWrapper listener) { } And then I would implement all the methods of HttpListener (at least all those that makes sense) in my own class, by mostly just chaining the call to the underlying HttpListener object. What do you think? Final question: If I go the way of the interface, assuming the interface maps 1-to-1 onto the HttpListener class, and written just to add support for mocking, is such an interface called a wrapper or an adapter?

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• Rails STI validation inheritance

  - by KARASZI István

  Dear Rails users! I have STI models in my Rails application. The ancestor model has validations with the validates_... methods which are working fine. But I have custom validations as well, and I would like to add more different custom validations in the descendants. These custom validations would depend on the class. If I write class DescendantA < Ancestor protected def validate # ... end end It simply overwrites the original validations. Is there a convention to do this in Rails?

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• Inheritance in tables - structure problem

  - by Naor

  I have 3 types of users in my system. each type has different information I created the following tables: BaseUser(base_user_id, username, password, additional common data) base_user_id is PK and Identity UserType1(user_id, data related to type1 only) user_id is PK and FK to base_user_id UserType2(user_id, data related to type2 only) user_id is PK and FK to base_user_id UserType3(user_id, data related to type3 only) user_id is PK and FK to base_user_id Now I have relation from each type of user to warehouses table. Users from type1 and type2 should have only warehouse_id and users from type3 should have warehouse_id and customer_id. I thought about this structure: WarehouseOfUser(base_user_id,warehouse_id) base_user_id is FK to base_user_id in BaseUser WarehouseOfTyp3User(base_user_id,warehouse_id, customer_id) base_user_id is FK to base_user_id in BaseUser The problem is that such structure allows 2 things I want to prevent: 1. add to WarehouseOfTyp3User data of user from type2 or type1. 2. add to WarehouseOfUser data of user from type3. what is the best structure for such case?

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• Transfer ADOBE licence after inheritance

  - by Vilius

  Hello there! After my aunts death, I have inherited an iMac with several Adobe software on it (= Photoshop, Illustrator, InDesign, etc.). Is ist possible to sell (or better to overwrite the licence) the software to other persons in the name of my aunt? Or whom does the software belong now? Thanks in advice! :)

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• Why `is_base_of` works with private inheritance?

  - by Alexey Malistov

  Why the following code works? typedef char (&yes)[1]; typedef char (&no)[2]; template <typename B, typename D> struct Host { operator B*() const; operator D*(); }; template <typename B, typename D> struct is_base_of { template <typename T> static yes check(D*, T); static no check(B*, int); static const bool value = sizeof(check(Host<B,D>(), int())) == sizeof(yes); }; //Test sample class B {}; class D : private B {}; //Exspression is true. int test[is_base_of<B,D>::value && !is_base_of<D,B>::value]; Note that B is private base. Note that operator B*() is const. How does this work? Why this works? Why static yes check(D*, T); is better than static yes check(B*, int); ?

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• CSS LESS class inheritance

  - by Haradzieniec

  Here is a css description of properties for my #myform1 .btn1 class: #myform1 .btn1 { ... } #myform1 .btn1:hover { ... } #myform1 .btn1.active { ... } #myform1 .btn1.disabled { ... } Is it possible to add absolutely the same properties for my #myform2 .btn2 class using LESS (any way is OK) without writing #myform1 .btn1 ,#myform2 .btn2 { ... } #myform1 .btn1:hover, #myform2 .btn2:hover { ... } #myform1 .btn1.active, #myform2 .btn2.active { ... } #myform1 .btn1.disabled, #myform2 .btn2.disabled { ... } Is it possible?

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• Inheritance - initialization problem

  - by dumbquestion

  I have a c++ class derived from a base class in a framework. The derived class doesn't have any data members because I need it to be freely convertible into a base class and back - the framework is responsible for loading and saving the objects and I can't change it. My derived class just has functions for accessing the data. But there are a couple of places where I need to store some temporary local variables to speed up access to data in the base class. mydata* MyClass::getData() { if ( !m_mydata ) { // set to NULL in the constructor m_mydata = some_long_and complex_operation_to_get_the_data_in_the_base() } return m_mydata; } The problem is if I just access the object by casting the base class pointer returned from the framework to MyClass* the ctor for MyClass is never called and m_mydata is junk. Is there a way of only initializing the m_mydata pointer once?

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• JavaScript inheritance extend function

  - by Zach

  I'm having some trouble understanding the IF clause at the end of this function from Pro JavaScript Design Patterns: function extend(subClass, superClass) { var F = function() {}; F.prototype = superClass.prototype; subClass.prototype = new F(); subClass.prototype.constructor = subClass; subClass.superclass = superClass.prototype; if(superClass.prototype.constructor == Object.prototype.constructor) { superClass.prototype.constructor = superClass; } } The book explains that these lines ensure that the superclass's constructor attribute is correctly set, even if the superclass is the Object class itself. Yet, if I omit those three lines and do the following: function SubClass() {}; extend(SubClass, Object); alert(Object.prototype.constructor == Object); The alert says 'true', which means the superclass's constructor is set correctly even without those last three lines. Under what conditions, then, does this IF statement do something useful? Thanks.

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• C# Property inheritance question

  - by mcxiand

  Hi all, How do i inherit a property in c# from an interface and give that property other name on the class? For example: public interface IFoo { int Num {get;set;} } public class IFooCls : IFoo { int Ifoo.Num{get;set} } In this case, what the property name in the interface is also the same in the class. What i want is to give other property name on the class but still pointing to "Num" in the interface in this case. In VB, we can do it like this: Public ReadOnly Property UserId() As String Implements System.Security.Principal.IIdentity.Name Get Return _userId End Get End Property

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• Adding an inheritance to an Qt designed object

  - by Jaguar

  Hello, I have a problem and I want to implement the MVC pattern to my QT application, that's why I need for example to inherite in my QTableWidget about another class like myClass { myMethod(); } but our QTableWidget is contained by our Mainwidows that is designed by the QT designer and generate an ui_MainWindow class ! Do you know how to do that ? Is that a method inside the Qt Designer to do that ? or another method ? Thank you for your answer !

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• dynamic inheritance without touching classes

  - by Jasper

  I feel like the answer to this question is really simple, but I really am having trouble finding it. So here goes: Suppose you have the following classes: class Base; class Child : public Base; class Displayer { public: Displayer(Base* element); Displayer(Child* element); } Additionally, I have a Base* object which might point to either an instance of the class Base or an instance of the class Child. Now I want to create a Displayer based on the element pointed to by object, however, I want to pick the right version of the constructor. As I currently have it, this would accomplish just that (I am being a bit fuzzy with my C++ here, but I think this the clearest way) object->createDisplayer(); virtual void Base::createDisplayer() { new Displayer(this); } virtual void Child::createDisplayer() { new Displayer(this); } This works, however, there is a problem with this: Base and Child are part of the application system, while Displayer is part of the GUI system. I want to build the GUI system independently of the Application system, so that it is easy to replace the GUI. This means that Base and Child should not know about Displayer. However, I do not know how I can achieve this without letting the Application classes know about the GUI. Am I missing something very obvious or am I trying something that is not possible?

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• Inheritance and Implicit Type Casting

  - by Josué Molina

  Suppose I have the following three classes: class Animal {}; class Human : public Animal {}; class Dog : public Animal { public: void setOwner(Animal* owner) { this->owner = owner; } private: Animal* owner; }; Why is the following allowed, and what exactly is happening? Dog d; Human h; d.setOwner(&h); // ? At first, I tried to cast it like this d.setOwner(&(Animal)h), but the compiler gave me a warning, and I hit a run-time error. Edit: the warning the compiler gave me was "taking address of temporary". Why is this so?

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• operator overloading and inheritance

  - by user168715

  I was given the following code: class FibHeapNode { //... // These all have trivial implementation virtual void operator =(FibHeapNode& RHS); virtual int operator ==(FibHeapNode& RHS); virtual int operator <(FibHeapNode& RHS); }; class Event : public FibHeapNode { // These have nontrivial implementation virtual void operator=(FibHeapNode& RHS); virtual int operator==(FibHeapNode& RHS); virtual int operator<(FibHeapNode& RHS); }; class FibHeap { //... int DecreaseKey(FibHeapNode *theNode, FibHeapNode& NewKey) { FibHeapNode *theParent; // Some code if (theParent != NULL && *theNode < *theParent) { //... } //... return 1; } }; Much of FibHeap's implementation is similar: FibHeapNode pointers are dereferenced and then compared. Why does this code work? (or is it buggy?) I would think that the virtuals here would have no effect: since *theNode and *theParent aren't pointer or reference types, no dynamic dispatch occurs and FibHeapNode::operator< gets called no matter what's written in Event.

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• multiple inheritance

  - by hitech

  when we say "a member declated as protected is accessible to any class imediately derived from it" what does this mean. in the follwing example get_number function can be accessible by the result class , as per the statement it sould only be accessile to test class. class student { protected: int roll_number; public: void get_number(int){ cout<< "hello"; } void put_number(void) {cout<< "hello"; } }; class test : public student { protected : float sub1; float sub2; public: void get_marks(float, float) {cout<< "hello"; roll_number = 10; } void put_marks(void) {cout<< "hello"; cout << "roll_number = " << roll_number ; } }; class result :public test { float total; public: void display(){cout<< "hello"; roll_number = 10; } }; int main() { result student; student.get_marks(2.2, 2.2); student.put_marks(); return 0; } i changed the code as per the first statement the protected variable roll_number not be accessible upto the result class ?

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• Not sure I am using inheritance/polymorphism issue?

  - by planker1010

  So for this assignment I have to create a car class(parent) and a certifiedpreowned (child) and I need to have the parent class have a method to check if it is still under warranty. *checkWarrantyStatus(). that method calls the boolean isCoveredUnderWarranty() to veryify if the car still has warranty. My issue is in the certifiedpreowned class I have to call the isCoveredUnderWarranty() as well to see if it is covered under the extended warranty and then have it be called via the checkWarrantyStatus() in the car method. I hope this makes sense. So to sum it up I need to in the child class have it check the isCoveredUnderWarranty with extended warranty info. Then it has to move to the parent class so it can be called via checkWarrantyStatus. Here is my code, I have 1 error. public class Car { public int year; public String make; public String model; public int currentMiles; public int warrantyMiles; public int warrantyYears; int currentYear =java.util.Calendar.getInstance().get(java.util.Calendar.YEAR); /** construct car object with specific parameters*/ public Car (int y, String m, String mod, int mi){ this.year = y; this.make = m; this.model = mod; this.currentMiles = mi; } public int getWarrantyMiles() { return warrantyMiles; } public void setWarrantyMiles(int warrantyMiles) { this.warrantyMiles = warrantyMiles; } public int getWarrantyYears() { return warrantyYears; } public void setWarrantyYears(int warrantyYears) { this.warrantyYears = warrantyYears; } public boolean isCoveredUnderWarranty(){ if (currentMiles < warrantyMiles){ if (currentYear < (year+ warrantyYears)) return true; } return false; } public void checkWarrantyStatus(){ if (isCoveredUnderWarranty()){ System.out.println("Your car " + year+ " " + make+ " "+ model+ " With "+ currentMiles +" is still covered under warranty"); } else System.out.println("Your car " + year+ " " + make+ " "+ model+ " With "+ currentMiles +" is out of warranty"); } } public class CertifiedPreOwnCar extends Car{ public CertifiedPreOwnCar(int y, String m, String mod, int mi) { super(mi, m, mod, y); } public int extendedWarrantyYears; public int extendedWarrantyMiles; public int getExtendedWarrantyYears() { return extendedWarrantyYears; } public void setExtendedWarrantyYears(int extendedWarrantyYears) { this.extendedWarrantyYears = extendedWarrantyYears; } public int getExtendedWarrantyMiles() { return extendedWarrantyMiles; } public void setExtendedWarrantyMiles(int extendedWarrantyMiles) { this.extendedWarrantyMiles = extendedWarrantyMiles; } public boolean isCoveredUnderWarranty() { if (currentMiles < extendedWarrantyMiles){ if (currentYear < (year+ extendedWarrantyYears)) return true; } return false; } } public class TestCar { public static void main(String[] args) { Car car1 = new Car(2014, "Honda", "Civic", 255); car1.setWarrantyMiles(60000); car1.setWarrantyYears(5); car1.checkWarrantyStatus(); Car car2 = new Car(2000, "Ferrari", "F355", 8500); car2.setWarrantyMiles(20000); car2.setWarrantyYears(7); car2.checkWarrantyStatus(); CertifiedPreOwnCar car3 = new CertifiedPreOwnCar(2000, "Honda", "Accord", 65000); car3.setWarrantyYears(3); car3.setWarrantyMiles(30000); car3.setExtendedWarrantyMiles(100000); car3.setExtendedWarrantyYears(7); car3.checkWarrantyStatus(); } }

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• inheritance from the django user model results in error when changing password

  - by Jerome

  I inherited form the django user model like so: from django.db import models from django.contrib.auth.models import User, UserManager from django.utils.translation import ugettext_lazy as _ class NewUserModel(User): custom_field_1 = models.CharField(_('custom field 1'), max_length=250, null=True, blank=True) custom_field_2 = models.CharField(_('custom field 2'), max_length=250, null=True, blank=True) objects = UserManager() When i go to the admin and add an entry into this model, it saves fine, but below the "Password" field where it has this text "Use '[algo]$[salt]$[hexdigest]' or use the change password form.", if i click on the "change password form' link, it produces this error Truncated incorrect DOUBLE value: '7/password' What can i do to fix this?

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• Why does C# not support multiple inheritance? [closed]

  - by Qutbuddin Kamaal

  Please provide insights and opinions.

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• Adding inheritance to a class programmatically in python?

  - by Timmy

  Can I make a class inherit a class "in-program" in Python?

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• Using inheritance with multiple files in Ruby

  - by Preethi Jain

  I am new to Ruby . I have a question with respect to using Inheritence in Ruby . I have a class called as Doggy inside a file named Doggy.rb class Doggy def bark puts "Vicky is barking" end end I have written another class named Puppy in another file named puppy.rb class Puppy < Doggy end puts Doggy.new.bark I am getting this Error: Puppy.rb:1:in `<main>': uninitialized constant Doggy (NameError) Is it mandatory to have these classes (Doggy and Puppy ) inside a single file only? Edited As per the suggestions , i have tried using require and require_relative as shown , but still i am getting below Error Puppy.rb:1:in `<main>': uninitialized constant Doggy (NameError) class Puppy < Doggy end require_relative 'Doggy.rb' puts Doggy.new.bark

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• ASP.NET configuration inheritance

  - by NowYouHaveTwoProblems

  I have an ASP.NET application that defines a custom configuration section in web.config. Recently I had a customer who wanted to deploy two instances of the application (for testing in addition to an existing production application). The configuration chosen by the customer was: foo.com - production application foo.com/Testing - test application In this case, the ASP.NET configuration engine decided to apply the settings at foo.com/web.config to foo.com/Testing/web.config. Thankfully this caused a configuration error because the section was redefined at the second level rather than giving the false impression that the two web applications were isolated. What I would like to do is to specify that my configuration section is not inherited and must be re-defined for any web application that requires it but I haven't been able to find a way to do this. My web.config ends up something like this <configuration> <configSections> <section name="MyApp" type="MyApp.ConfigurationSection"/> </configSections> <MyApp setting="value" /> <NestedSettingCollection> <Item key="SomeKey" value="SomeValue" /> <Item key="SomeOtherKey" value="SomeOtherValue" /> </NestedSettingCollection> </MyApp> </configuration>

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• NSString inheritance

  - by Stef

  Hi, I'm doing an useless thing for my first step in Obj-C @interface String : NSString { int m_isnull; } - (id) init; - (int) isNull; @end @implementation String - (id) init { self = [super init]; m_isnull=1; return self; } - (int) isNull { return m_isnull; } @end test : String *a; a=@"ok"; Works fine, but just 2 little questions 1) When I'm compiling I have this warning warning: incompatible Objective-C types assigning 'struct NSString *', expected 'struct String *' I don't know how to avoid it !? 2) a=@"ok" is a fastest way to initialize a string, but when I'm debugging, I don't stop by at my init constructor why ?

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• enable_shared_from_this and inheritance

  - by DeadMG

  I've got a type which inherits from enable_shared_from_this<type>, and another type that inherits from this type. Now I can't use the shared_from_this method because it returns the base type and in a specific derived class method I need the derived type. Is it valid to just construct a shared_ptr from this directly? Edit: In a related question, how can I move from an rvalue of type shared_ptr<base> to a type of shared_ptr<derived>? I used dynamic_cast to verify that it really was the correct type, but now I can't seem to accomplish the actual move.

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• Objects with inheritance memory storage

  - by nikitas350

  Say that i have some classes like this example. class A { int k, m; public: A(int a, int b) { k = a; m = b; } }; class B { int k, m; public: B() { k = 2; m = 3; } }; class C : private A, private B { int k, m; public: C(int a, int b) : A(a, b) { k = b; m = a; } }; Now, in a class C object, are the variables stored in a specific way? I know what happens in a POD object, but this is not a POD object...

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• C# Generics Multiple Inheritance Problem

  - by Ciemnl

  Can any one help me with this syntax issue with C#? I have no idea how to do it. class SomeClass<T> : SomeOtherClass<T> where T : ISomeInterface , IAnotherInterface { ... } I want SomeClass to inherit from SomeOtherClass and IAnotherInterface and for T to inherit ISomeInterface only It seems the problem is that the where keyword screws everything up so that the compiler thinks both ISomeInterface and IAnotherInterface should both be inherited by T. This problem is very annoying and I think the solution is some kind of parenthesis but I have tried and failed finding one that works. Also, switching around the order of the two items inherited from SomeClass does not work because the class inherited always has to come before any interfaces. I couldn't find any solutions on the MSDN C# generics pages and I can't beleive I'm the first person to have this problem. Thanks, any help is much appreciated!

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