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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++ Multiple Inheritance Question

- by John

The scenario generating this is quite complex so I'll strip out a few pieces and give an accurate representation of the classes involved. /* This is inherited using SI by many classes, as normal */ class IBase { virtual string toString()=0; }; /* Base2 can't inherit IBase due to other methods on IBase which aren't appropriate */ class Base2 { string toString() { ... } }; /* a special class, is this valid? */ class Class1 : public IBase, public Base2 { }; So, is this valid? Will there be conflicts on the toString? Or can Class1 use Base2::toString to satisfy IBase? Like I say there are lots of other things not shown, so suggestions for major design changes on this example are probably not that helpful... though any general suggestions/advice are welcome.

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Inheritance of TCollectionItem

- by JamesB

I'm planning to have collection of items stored in a TCollection. Each item will derive from TBaseItem which in turn derives from TCollectionItem, With this in mind the Collection will return TBaseItem when an item is requested. Now each TBaseItem will have a Calculate function, in the the TBaseItem this will just return an internal variable, but in each of the derivations of TBaseItem the Calculate function requires a different set of parameters. The Collection will have a Calculate All function which iterates through the collection items and calls each Calculate function, obviously it would need to pass the correct parameters to each function I can think of three ways of doing this: Create a virtual/abstract method for each calculate function in the base class and override it in the derrived class, This would mean no type casting was required when using the object but it would also mean I have to create lots of virtual methods and have a large if...else statement detecting the type and calling the correct "calculate" method, it also means that calling the calculate method is prone to error as you would have to know when writing the code which one to call for which type with the correct parameters to avoid an Error/EAbstractError. Create a record structure with all the possible parameters in and use this as the parameter for the "calculate" function. This has the added benefit of passing this to the "calculate all" function as it can contain all the parameters required and avoid a potentially very long parameter list. Just type casting the TBaseItem to access the correct calculate method. This would tidy up the TBaseItem quite alot compared to the first method. What would be the best way to handle this collection?

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c++ inheritance

- by Meloun

Hi, i am trouble with this.. Is there some solution or i have to keep exactly class types? //header file Class Car { public: Car(); virtual ~Car(); }; class Bmw:Car { public: Bmw(); virtual ~Bmw(); }; void Start(Car& mycar) {}; //cpp file Car::Car(){} Car::~Car() {} Bmw::Bmw() :Car::Car(){} Bmw::~Bmw() {} int main() { Car myCar; Bmw myBmw; Start(myCar); //works Start(myBmw); //!! doesnt work return 0; }

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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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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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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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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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C# Function Inheritance--Use Child Class Vars with Base Class Function

- by Sean O'Connor

Good day, I have a fairly simple question to experienced C# programmers. Basically, I would like to have an abstract base class that contains a function that relies on the values of child classes. I have tried code similar to the following, but the compiler complains that SomeVariable is null when SomeFunction() attempts to use it. Base class: public abstract class BaseClass { protected virtual SomeType SomeVariable; public BaseClass() { this.SomeFunction(); } protected void SomeFunction() { //DO SOMETHING WITH SomeVariable } } A child class: public class ChildClass:BaseClass { protected override SomeType SomeVariable=SomeValue; } Now I would expect that when I do: ChildClass CC=new ChildClass(); A new instance of ChildClass should be made and CC would run its inherited SomeFunction using SomeValue. However, this is not what happens. The compiler complains that SomeVariable is null in BaseClass. Is what I want to do even possible in C#? I have used other managed languages that allow me to do such things, so I certain I am just making a simple mistake here. Any help is greatly appreciated, thank you.

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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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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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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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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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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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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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