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• C#.NET Generic Methods and Inheritance.

  - by ealgestorm

  Is it possible to do the following with generics in C#.NET public abstract class A { public abstract T MethodB<T>(string s); } public class C: A { public override DateTime MethodB(string s) { } } i.e. have a generic method in a base class and then use a specific type for that method in a sub class.

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• QMap inheritance with QMapIterator

  - by gregseth

  Hi, I made a personnal class which inherits QMap: class CfgMgr : public QMap<QString, CfgSet*> {...} I'm trying to iterate over all its elements like that: CfgMgr* m_pDefaults = new CfgMgr; // .../... QMapIterator<QString, CfgSet*> ics(*m_pDefaults); while (ics.hasNext()) { // doing my stuff } And I get the compile error: Can't convert parameter 1 from 'CfgMgr' to 'const QMap< Key,T &' with [ Key=QString, T=CfgSet * ] I tried with a dynamic_cast: QMapIterator<QString, CfgSet*> ics( *dynamic_cast< QMap<QString,CfgSet*>* >(m_pDefaults) ); it compiles, but always returns NULL. What's wrong? How can I solve this?

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• C++ Storing variables and inheritance

  - by Kaa

  Hello Everyone, Here is my situation: I have an event driven system, where all my handlers are derived from IHandler class, and implement an onEvent(const Event &event) method. Now, Event is a base class for all events and contains only the enumerated event type. All actual events are derived from it, including the EventKey event, which has 2 fields: (uchar) keyCode and (bool)isDown. Here's the interesting part: I generate an EventKey event using the following syntax: Event evt = EventKey(15, true); and I ship it to the handlers: EventDispatch::sendEvent(evt); // void EventDispatch::sendEvent(const Event &event); (EventDispatch contains a linked list of IHandlers and calls their onEvent(const Event &event) method with the parameter containing the sent event. Now the actual question: Say I want my handlers to poll the events in a queue of type Event, how do I do that? x Dynamic pointers with reference counting sound like too big of a solution. x Making copies is more difficult than it sounds, since I'm only receiving a reference to a base type, therefore each time I would need to check the type of event, upcast to EventKey and then make a copy to store in a queue. Sounds like the only solution - but is unpleasant since I would need to know every single type of event and would have to check that for every event received - sounds like a bad plan. x I could allocate the events dynamically and then send around pointers to those events, enqueue them in the array if wanted - but other than having reference counting - how would I be able to keep track of that memory? Do you know any way to implement a very light reference counter that wouldn't interfere with the user? What do you think would be a good solution to this design? I thank everyone in advance for your time. Sincerely, Kaa

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• Problem with inheritance and List<>

  - by Jagd

  I have an abstract class called Grouping. I have a subclass called GroupingNNA. public class GroupingNNA : Grouping { // blah blah blah } I have a List that contains items of type GroupingNNA, but is actually declared to contain items of type Grouping. List<Grouping> lstGroupings = new List<Grouping>(); lstGroupings.Add( new GroupingNNA { fName = "Joe" }); lstGroupings.Add( new GroupingNNA { fName = "Jane" }); The Problem: The following LINQ query blows up on me because of the fact that lstGroupings is declared as List< Grouping and fName is a property of GroupingNNA, not Grouping. var results = from g in lstGroupings where r.fName == "Jane" select r; Oh, and this is a compiler error, not a runtime error. Thanks in advance for any help on this one! More Info: Here is the actual method that won't compile. The OfType() fixed the LINQ query, but the compiler doesn't like the fact that I'm trying to return the anonymous type as a List< Grouping. private List<Grouping> ApplyFilterSens(List<Grouping> lstGroupings, string fSens) { // This works now! Thanks @Lasse var filtered = from r in lstGroupings.OfType<GroupingNNA>() where r.QASensitivity == fSens select r; if (filtered != null) { **// Compiler doesn't like this now** return filtered.ToList<Grouping>(); } else return new List<Grouping>(); }

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• C++ function-pointer and inheritance

  - by pingvinus

  In parent class I have function, that operates under an array of functions, declared in child-class, number of functions for every child-class may vary. But since every function uses some object-variables, I can't declare them as static. I've try to do something like this: class A { public: typedef int (A::*func)(); func * fs; void f() { /*call functions from this->fs*/ } }; class B : public A { public: int smth; B(int smth) { this->smth = smth; this->fs = new func[1]; fs[0] = &B::f; } int f() { return smth + 1; } }; But, obviously it doesn't work. Any suggestions?

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• C++: inheritance problem

  - by Helltone

  It's quite hard to explain what I'm trying to do, I'll try: Imagine a base class A which contains some variables, and a set of classes deriving from A which all implement some method bool test() that operates on the variables inherited from A. class A { protected: int somevar; // ... }; class B : public A { public: bool test() { return (somevar == 42); } }; class C : public A { public: bool test() { return (somevar > 23); } }; // ... more classes deriving from A Now I have an instance of class A and I have set the value of somevar. int main(int, char* []) { A a; a.somevar = 42; Now, I need some kind of container that allows me to iterate over the elements i of this container, calling i::test() in the context of a... that is: std::vector<...> vec; // push B and C into vec, this is pseudo-code vec.push_back(&B); vec.push_back(&C); bool ret = true; for(i = vec.begin(); i != vec.end(); ++i) { // call B::test(), C::test(), setting *this to a ret &= ( a .* (&(*i)::test) )(); } return ret; } How can I do this? I've tried two methods: forcing a cast from B::* to A::*, adapting a pointer to call a method of a type on an object of a different type (works, but seems to be bad); using std::bind + the solution above, ugly hack; changing the signature of bool test() so that it takes an argument of type const A& instead of inheriting from A, I don't really like this solution because somevar must be public.

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• Java Inheritance doubt in parameterised collection

  - by Gala101

  It's obvious that a parent class's object can hold a reference to a child, but does this not hold true in case of parameterised collection ?? eg: Car class is parent of Sedan So public void doSomething(Car c){ ... } public void caller(){ Sedan s = new Sedan(); doSomething(s); } is obviously valid But public void doSomething(Collection<Car> c){ ... } public void caller(){ Collection<Sedan> s = new ArrayList<Sedan>(); doSomething(s); } Fails to compile Can someone please point out why? and also, how to implement such a scenario where a function needs to iterate through a Collection of parent objects, modifying only the fields present in parent class, using parent class methods, but the calling methods (say 3 different methods) pass the collection of three different subtypes.. Ofcourse it compiles fine if I do as below: public void doSomething(Collection<Car> c){ ... } public void caller(){ Collection s = new ArrayList<Sedan>(); doSomething(s); }

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• Use multiple inheritance to discriminate useage roles?

  - by Arne

  Hi fellows, it's my flight simulation application again. I am leaving the mere prototyping phase now and start fleshing out the software design now. At least I try.. Each of the aircraft in the simulation have got a flight plan associated to them, the exact nature of which is of no interest for this question. Sufficient to say that the operator way edit the flight plan while the simulation is running. The aircraft model most of the time only needs to read-acess the flight plan object which at first thought calls for simply passing a const reference. But ocassionally the aircraft will need to call AdvanceActiveWayPoint() to indicate a way point has been reached. This will affect the Iterator returned by function ActiveWayPoint(). This implies that the aircraft model indeed needs a non-const reference which in turn would also expose functions like AppendWayPoint() to the aircraft model. I would like to avoid this because I would like to enforce the useage rule described above at compile time. Note that class WayPointIter is equivalent to a STL const iterator, that is the way point can not be mutated by the iterator. class FlightPlan { public: void AppendWayPoint(const WayPointIter& at, WayPoint new_wp); void ReplaceWayPoint(const WayPointIter& ar, WayPoint new_wp); void RemoveWayPoint(WayPointIter at); (...) WayPointIter First() const; WayPointIter Last() const; WayPointIter Active() const; void AdvanceActiveWayPoint() const; (...) }; My idea to overcome the issue is this: define an abstract interface class for each usage role and inherit FlightPlan from both. Each user then only gets passed a reference of the appropriate useage role. class IFlightPlanActiveWayPoint { public: WayPointIter Active() const =0; void AdvanceActiveWayPoint() const =0; }; class IFlightPlanEditable { public: void AppendWayPoint(const WayPointIter& at, WayPoint new_wp); void ReplaceWayPoint(const WayPointIter& ar, WayPoint new_wp); void RemoveWayPoint(WayPointIter at); (...) }; Thus the declaration of FlightPlan would only need to be changed to: class FlightPlan : public IFlightPlanActiveWayPoint, IFlightPlanEditable { (...) }; What do you think? Are there any cavecats I might be missing? Is this design clear or should I come up with somethink different for the sake of clarity? Alternatively I could also define a special ActiveWayPoint class which would contain the function AdvanceActiveWayPoint() but feel that this might be unnecessary. Thanks in advance!

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• Inheritance of jQuery's prototype partially fails

  - by user1065745

  I want to use Coffeescript to create an UIObject class. This class should inherit from jQuery, so that instances of UIObject can be used as if they where created with jQuery. class UIObject isObject: (val) -> typeof val is "object" constructor: (tag, attributes) -> @merge jQuery(tag, attributes), this @UIObjectProperties = {} merge: (source, destination) -> for key of source if destination[key] is undefined destination[key] = source[key] else if @isObject(source[key]) @merge(source[key], destination[key]) return It partially works. Consider the Foobar class below: class Foobar extends UIObject constructor: -> super("<h1/>", html: "Foobar") $("body").append(new Foobar) works fine. BUT: (new Foobar).appendTo("body") places the tag, but also raises RangeError: Maximum call stack size exceeded. Was it just a bad idea to inherit from jQuery? Or is there a solurion? For those who don't know CoffeeScript, the JavaScript source is: var Foobar, UIObject; var __hasProp = Object.prototype.hasOwnProperty, __extends = function(child, parent) { for (var key in parent) { if (__hasProp.call(parent, key)) child[key] = parent[key]; } function ctor() { this.constructor = child; } ctor.prototype = parent.prototype; child.prototype = new ctor; child.__super__ = parent.prototype; return child; }; UIObject = (function () { UIObject.prototype.isObject = function (val) { return typeof val === "object"; }; function UIObject(tag, attributes) { this.merge(jQuery(tag, attributes), this); this.UIObjectProperties = {}; } UIObject.prototype.merge = function (source, destination) { var key; for (key in source) { if (destination[key] === void 0) { destination[key] = source[key]; } else if (this.isObject(source[key])) { this.merge(source[key], destination[key]); } } }; return UIObject; })(); Foobar = (function () { __extends(Foobar, UIObject); function Foobar() { Foobar.__super__.constructor.call(this, "<h1/>", { html: "Foobar" }); } return Foobar; })();

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• Variables variable and inheritance

  - by Xack

  I made code like this: class Object { function __get($name){ if(isset($this->{$name})){ return $this->{$name}; } else { return null; } } function __set($name, $value){ $this->{$name} = $value; } } If I extend this class (I don't want to repeat this code every time), it says "Undefined variable". Is there any way to do it?

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• Comparing objects and inheritance

  - by ereOn

  Hi, In my program I have the following class hierarchy: class Base // Base is an abstract class { }; class A : public Base { }; class B : public Base { }; I would like to do the following: foo(const Base& one, const Base& two) { if (one == two) { // Do something } else { // Do something else } } I have issues regarding the operator==() here. Of course comparing an instance A and an instance of B makes no sense but comparing two instances of Base should be possible. (You can't compare a Dog and a Cat however you can compare two Animals) I would like the following results: A == B = false A == A = true or false, depending on the effective value of the two instances B == B = true or false, depending on the effective value of the two instances My question is: is this a good design/idea ? Is this even possible ? What functions should I write/overload ? My apologies if the question is obviously stupid or easy, I have some serious fever right now and my thinking abilities are somewhat limited :/ Thank you.

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• Constructors with inheritance in c++

  - by Crystal

  If you have 3 classes, with the parent class listed first shape- 2d shapes, 3d shapes - circle, sphere When you write your constructor for the circle class, would you ever just initialize the parent Shape object and then your current object, skipping the middle class. It seems to me you could have x,y coordinates for Shape and initialize those in the constructor, and initialize a radius in the circle or sphere class, but in 2d or 3d shape classes, I wouldn't know what to put in the constructor since it seems like it would be identical to shape. So is something like this valid Circle::Circle(int x, int y, int r) : Shape(x, y), r(r) {} I get a compile error of: illegal member initialization: 'Shape' is not a base or member So I wasn't sure if my code was legal or best practice even. Or if instead you'd have the middle class just do what the top level Shape class does TwoDimensionalShape::TwoDimensionalShape(int x, int y) : Shape (x, y) {} and then in the Circle class Circle::Circle(int x, int y, int r) : TwoDimensionalShape(x, y), r(r) {}

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• Inheritance policy when designing the base class

  - by Xaqron

  I have a base class and a derived class both in design phase. The base class will remain one but many derived class will inherit from it. So it's very costly to make change to derived classes in the future and I'm looking for the best design to prevent this. In fact derived class only needs a few methods to override (if needed) but it's tempting to reveal more details to it. My question is about the policy which is extensible in future. Can I minimize the inherited methods/properties to derived class and reveal more in the next versions if needed without any change to derived classes ? Or I should reveal anything that maybe used by derived classes in the future and let them to choose if they need them or not ? Thanks

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• How to map inheritance with property returned other inheritance?

  - by dario-g

  Hi I have abstract class Vehicle and two classes that derive from: Car and ForkLift. public abstract class Vehicle { public EngineBase Engine { get; set; } } public class Car : Vehicle { public GasEngine Engine { get; set; } } public class ForkLift : Vehicle { public ElectricEngine Engine { get; set; } } and Engine clasess: public abstract class Engine { } public class GasEngine : Engine { } public class ElectricEngine : Engine { } Engines are mapped with "table per class hierarchy". With Vehicles I want to use the same pattern. How to map Engine class and derived with that Engine property?

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• C#, Generic Lists and Inheritance

  - by Andy

  I have a class called Foo that defines a list of objects of type A: class Foo { List<A> Items = new List<A>(); } I have a class called Bar that can save and load lists of objects of type B: class Bar { void Save(List<B> ComplexItems); List<B> Load(); } B is a child of A. Foo, Bar, A and B are in a library and the user can create children of any of the classes. What I would like to do is something like the following: Foo MyFoo = new Foo(); Bar MyBar = new Bar(); MyFoo.Items = MyBar.Load(); MyBar.Save(MyFoo.Items); Obviously this won't work. Is there a clever way to do this that avoids creating intermediate lists? thanks, Andy

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• Simultaneous private and public inheritance in C++

  - by gspr

  Suppose a class Y publicly inherits a class X. Is it possible for a class Z to privately inherit Y while publicly inheriting X? To make this clearer, suppose X defines public methods x1 and x2. Y inherits X, overrides x1 and provides a method y. Does C++ allow for a third class Z to subclass Y in such a way that Y's implementation of x1 and y are privately available to it, while the outside world only sees it inheriting X publicly, i.e. having only a single public method x2?

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• C++ allocate objects on heap of base class with protected constructors via inheritance

  - by KRao

  I have a class with protected constructor: class B { protected: B(){}; }; Now I derive from it and define two static functions and I manage to actually create objects of the class B, but not on the heap: class A : public B { public: static B createOnStack() {return B();} //static B* createOnHeap() {return new B;} //Compile time Error on VS2010 }; B b = A::createOnStack(); //This works on VS2010! The question is: 1) Is VS2010 wrong in allowing the first case? 2) Is it possible to create objects of B without modifying B in any way (no friendship and no extra functions). I am asking, because it is possible to make something similar when dealing with instances of B and its member functions, see: http://accu.org/index.php/journals/296 Thank you in advance for any suggestion! Kind regards

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• defualt parameter values in arguments and inheritance

  - by sil3nt

  Hello there, Im having trouble with some Java, How do I give in default parameter values in java?. for example I have this in c++ DVD(int i, string t, int y, string d="Unknown"): Items(i,t,y),director(d){} and in Java I tried public Dvd(int i, String t,int y, String d="Unknown"){ super(i,t,y); director = d; } which fails to build. So how do I go about giving in default values? also In my main testing class I tried giving in 3 arguments insead of 4 but this fails also. How do I get around this problem?.

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• C++ inheritance: scoping and visibility of members

  - by Poiuyt

  Can you explain why this is not allowed, #include <stdio.h> class B { private: int a; public: int a; }; int main() { return 0; } while this is? #include <stdio.h> class A { public: int a; }; class B : public A{ private: int a; }; int main() { return 0; } In both the cases, we have one public and one private variable named a in class B. edited now!

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• Objective C - Custom Getter with inheritance

  - by anhdat

  Recently I have worked with Core Data. When I want to set a default value for some fields, I came up with this problem: So I made a simple represent: We have 2 class Parent and Child, in which Child inherit from Parent. // Parent.h @interface Parent : NSObject @property (strong, nonatomic) NSString *lastName; // Child.h @interface Child : Parent In Parent class, I made a custom getter to set a default value when nothing is set: // Parent.h - (NSString *)lastName { if (_lastName) { return _lastName; } else { return @"Parent Default Name"; } } But I cannot make a custom default value for the field "name" which Child inherits from its Parent. // Child.h @implementation Child - (NSString *)lastname { if (super.lastName) { return super.lastName; } else { return @"Child Default Name"; } } Apparently, this method is never called. So my question here is: How can I set a custom getter for the field the Child class inherits from Parent without define an overriding property?

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• Javascript Inheritance and Arrays

  - by Inespe

  Hi all! I am trying to define a javascript class with an array property, and its subclass. The problem is that all instances of the subclass somehow "share" the array property: // class Test function Test() { this.array = []; this.number = 0; } Test.prototype.push = function() { this.array.push('hello'); this.number = 100; } // class Test2 : Test function Test2() { } Test2.prototype = new Test(); var a = new Test2(); a.push(); // push 'hello' into a.array var b = new Test2(); alert(b.number); // b.number is 0 - that's OK alert(b.array); // but b.array is containing 'hello' instead of being empty. why? As you can see I don't have this problem with primitive data types... Any suggestions?

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• C++ Iterators and inheritance

  - by jomnis

  Have a quick question about what would be the best way to implement iterators in the following: Say I have a templated base class 'List' and two subclasses "ListImpl1" and "ListImpl2". The basic requirement of the base class is to be iterable i.e. I can do: for(List<T>::iterator it = list->begin(); it != list->end(); it++){ ... } I also want to allow iterator addition e.g.: for(List<T>::iterator it = list->begin()+5; it != list->end(); it++){ ... } So the problem is that the implementation of the iterator for ListImpl1 will be different to that for ListImpl2. I got around this by using a wrapper ListIterator containing a pointer to a ListIteratorImpl with subclasses ListIteratorImpl2 and ListIteratorImpl2, but it's all getting pretty messy, especially when you need to implement operator+ in the ListIterator. Any thoughts on a better design to get around these issues?

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• Pass Variables In Inheritance (Obj - C)

  - by Marmik Shah

  I working on a project in Obj-C where i have a base class (ViewController) and a Derived Class (MultiPlayer). Now i have declared certain variables and properties in the base class. My properties are getting accessed from the derived class but im not able to access the variables (int,char and bool type). I'm completely new to Obj-C so i have no clue whats wrong. I have used the data types which are used in C and C++. Is there some specific way to declare variables in Obj-C?? If so, How? Here are my files ViewController.h #import <UIKit/UIKit.h> @interface ViewController : UIViewController @property (weak,nonatomic) IBOutlet UIImageView* backGroungImage; @property (strong,nonatomic) IBOutlet UIImageView *blockView1; @property (strong,nonatomic) IBOutlet UIImageView *blockView2; @property (strong,nonatomic) IBOutlet UIImageView *blockView3; @property (strong,nonatomic) IBOutlet UIImageView *blockView4; @property (strong,nonatomic) IBOutlet UIImageView *blockView5; @property (strong,nonatomic) IBOutlet UIImageView *blockView6; @property (strong,nonatomic) IBOutlet UIImageView *blockView7; @property (strong,nonatomic) IBOutlet UIImageView *blockView8; @property (strong,nonatomic) IBOutlet UIImageView *blockView9; @property (strong,nonatomic) UIImage *x; @property (strong,nonatomic) UIImage *O; @property (strong,nonatomic) IBOutlet UIImageView* back1; @property (strong,nonatomic) IBOutlet UIImageView* back2; @end ViewController.m #import "ViewController.h" @interface ViewController () @end @implementation ViewController int chooseTheBackground = 0; int movesToDecideXorO = 0; int winningArrayX[3]; int winningArrayO[3]; int blocksTotal[9] = {8,3,4,1,5,9,6,7,2}; int checkIfContentInBlocks[9] = {0,0,0,0,0,0,0,0,0}; char determineContentInBlocks[9] = {' ',' ',' ',' ',' ',' ',' ',' ',' '}; bool player1Win = false; bool player2Win = false; bool playerWin = false; bool computerWin = false; - (void)viewDidLoad { [super viewDidLoad]; if(chooseTheBackground==0) { UIImage* backImage = [UIImage imageNamed:@"MainBack1.png"]; _backGroungImage.image=backImage; } if(chooseTheBackground==1) { UIImage* backImage = [UIImage imageNamed:@"MainBack2.png"]; _backGroungImage.image=backImage; } } - (void)didReceiveMemoryWarning { [super didReceiveMemoryWarning]; // Dispose of any resources that can be recreated. } @end I am not able to use the above declared variables in my derived classes!

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• Python Attributes and Inheritance

  - by user368186

  Say I have the folowing code: class Class1(object): def __init__(self): self.my_attr = 1 self.my_other_attr = 2 class Class2(Class1): def __init__(self): super(Class1,self).__init__() Why does Class2 not inherit the attributes of Class1?

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• C++ Inheritance and Constructors

  - by DizzyDoo

  Hello, trying to work out how to use constructors with an inherited class. I know this is very much wrong, I've been writing C++ for about three days now, but here's my code anyway: clientData.h, two classes, ClientData extends Entity : #pragma once class Entity { public: int x, y, width, height, leftX, rightX, topY, bottomY; Entity(int x, int y, int width, int height); ~Entity(); }; class ClientData : public Entity { public: ClientData(); ~ClientData(); }; and clientData.cpp, which contains the functions: #include <iostream> #include "clientData.h" using namespace std; Entity::Entity(int x, int y, int width, int height) { this->x = x; this->y = y; this->width = width; this->height = height; this->leftX = x - (width/2); this->rightX = x + (width/2); this->topY = y - (height/2); this-bottomY = y + (height/2); } Entity::~Entity() { cout << "Destructing.\n"; } ClientData::ClientData() { cout << "Client constructed."; } ClientData::~ClientData() { cout << "Destructing.\n"; } and finally, I'm creating a new ClientData with: ClientData * Data = new ClientData(32,32,32,16); Now, I'm not surprised my compiler shouts errors at me, so how do I pass the arguments to the right classes? The first error (from MVC2008) is error C2661: 'ClientData::ClientData' : no overloaded function takes 4 arguments and the second, which pops up whatever changes I seem to make is error C2512: 'Entity' : no appropriate default constructor available Thanks.

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