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• Inheritance classes in Scheme

  - by DreamWalker

  Now I research OOP-part of Scheme. I can define class in Scheme like this: (define (create-queue) (let ((mpty #t) (the-list '())) (define (enque value) (set! the-list (append the-list (list value))) (set! mpty #f) the-list) (define (deque) (set! the-list (cdr the-list)) (if (= (length the-list) 0) (set! mpty #t)) the-list) (define (isEmpty) mpty) (define (ptl) the-list) (define (dispatch method) (cond ((eq? method 'enque) enque) ((eq? method 'deque) deque) ((eq? method 'isEmpty) isEmpty) ((eq? method 'print) ptl))) dispatch)) (Example from css.freetonik.com) Can I implement class inheritance in Scheme?

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• Silly Objective-C inheritance problem when using property

  - by Ben Packard

  I've been scratching my head with this for a couple of hours - I haven't used inheritance much. Here I have set up a simple Test B class that inherits from Test A, where an ivar is declared. But I get the compilation error that the variable is undeclared. This only happens when I add the property and synthesize declarations - works fine without them. TestA Header: #import <Cocoa/Cocoa.h> @interface TestA : NSObject { NSString *testString; } @end TestA Implementation is empty: #import "TestA.h" @implementation TestA @end TestB Header: #import <Cocoa/Cocoa.h> #import "TestA.h" @interface TestB : TestA { } @property NSString *testProp; @end TestB Implementation (Error - 'testString' is undeclared) #import "TestB.h" @implementation TestB @synthesize testProp; - (void)testing{ NSLog(@"test ivar is %@", testString); } @end

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• Understanding the concept of inheritance in Java

  - by Nirmal

  Hello All.... I am just refreshing the oops features of the java. So, I have a little confusion regarding inheritance concept. For that I have a following sample code : class Super{ int index = 5; public void printVal(){ System.out.println("Super"); } } class Sub extends Super{ int index = 2; public void printVal(){ System.out.println("Sub"); } } public class Runner { public static void main(String args[]){ Super sup = new Sub(); System.out.println(sup.index+","); sup.printVal(); } } Now above code is giving me output as : 5,Sub. Here, we are overriding printVal() method, so that is understandable that it is accessing child class method only. But I could not understand why it's accessing the value of x from Super class... Thanks in advance....

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• Inheritance in Ruby on Rails: setting the base class type

  - by Régis B.

  I am implementing a single table inheritance inside Rails. Here is the corresponding migration: class CreateA < ActiveRecord::Migration def self.up create_table :a do |t| t.string :type end end Class B inherits from A: class B < A end Now, it's easy to get all instances of class B: B.find(:all) or A.find_all_by_type("B") But how do I find all instances of class A (those that are not of type B)? Is this bad organization? I tried this: A.find_all_by_type("A") But instances of class A have a nil type. I could do A.find_all_by_type(nil) but this doesn't feel right, somehow. In particular, it would stop working if I decided to make A inherit from another class. Would it be more appropriate to define a default value for :type in the migration? Something like: t.string :type, :default => "A" Am I doing something wrong here?

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• C++ inheritance and member function pointers

  - by smh

  In C++, can member function pointers be used to point to derived (or even base) class members? EDIT: Perhaps an example will help. Suppose we have a hierarchy of three classes X, Y, Z in order of inheritance. Y therefore has a base class X and a derived class Z. Now we can define a member function pointer p for class Y. This is written as: void (Y::*p)(); (For simplicity, I'll assume we're only interested in functions with the signature void f() ) This pointer p can now be used to point to member functions of class Y. This question (two questions, really) is then: Can p be used to point to a function in the derived class Z? Can p be used to point to a function in the base class X?

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• Page inheritance in mixed asp.net Forms and MVC application

  - by Rising Star

  I'm working on a web application. One of my co-workers has written some asp.net forms pages. The page classes all inherit from BasePageClass, which of course inherits from the Page class. I wish to add some MVC controllers that I've been told need to use the same logic implemented in the BasePageClass. Ordinarily, I would want to inherit the functions in the BasePageClass in the controller classes, but this breaks the inheritance heirarchy. What is the best practice for solving this problem?

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• Reducing template bloat with inheritance

  - by benoitj

  Does anyone have experience reducing template code bloat using inheritance? i hesitate rewriting our containers this way: class vectorBase { public: int size(); void clear(); int m_size; void *m_rawData; //.... }; template< typename T > class vector : public vectorBase { void push_back( const T& ); //... }; I should keep maximum performance while reducing compile time I'm also wondering why stl implementations do not uses this approach Thanks for your feedbacks

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• How is inheritance implemented at the memory level?

  - by cambr

  Suppose I have class A { public: void print(){cout<<"A"; }}; class B: public A { public: void print(){cout<<"B"; }}; class C: public C { }; How is inheritance implemented at the memory level? Does C copy print() code to itself or does it have a pointer to the it that points somewhere in A part of the code? How does the same thing happen when we override the previous definition, for example in B (at the memory level)?

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• Imitating Multiple Inheritance in PHP

  - by fabieno

  I am working on my own MVC framework and found myself stuck. I need the following construction: Controller -- Backend_Controller -- Backend_Crud_Controller -- Frontend_Controller -- Frontend_Crud_Controller Both 'Backend_Crud_Controller' and 'Frontend_Crud_Controller' have the same functionality and thus they should extend another class named 'Base_Crud_Controller', the only difference comes from the 'Backend/Frontend' Controllers which implement different mechanisms. Basically they should inherit both classes but my problem is that 'Backend/Frontend' controller doesn't necessarily extend 'Base_Crud_Controller'. I know multiple inheritance doesn't exist in PHP but I am looking for a solution, I choose to refrain Mixins (like in Symfony) as I don't consider that an elegant solution. Interfaces do not suit me as all of these end up as concrete classes that should implement methods.

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• Java Inheritance Concept Understanding

  - by Nirmal

  Hello All.... I am just refreshing the oops features of the java. So, I have a little confusion regarding inheritance concept. For that I have a following sample code : class Super{ int index = 5; public void printVal(){ System.out.println("Super"); } } class Sub extends Super{ int index = 2; public void printVal(){ System.out.println("Sub"); } } public class Runner { public static void main(String args[]){ Super sup = new Sub(); System.out.println(sup.index+","); sup.printVal(); } } Now above code is giving me output as : 5,Sub. Here, we are overriding printVal() method, so that is understandable that it is accessing child class method only. But I could not understand why it's accessing the value of x from Super class... Thanks in advance....

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• C++ inheritance question

  - by user233973

  Hi guys, I have a C++ inheritance related question. I have a set of classes like this (I have not given the complete class structure coz I am lazy :) ). I want to access chiComponent class public methods using com pointer. How should I go about it? Note that I am having to change the object which "com" is pointing to in a lot of places. So I do not think I can have another chiComponent *ccom = <some_cast> com; ccom.chiComponentMethod() How should I go about it? class Component{ }; class chiComponent : public Component { public: void chiComponentMethod() { cout << "Hi! Chi component function called!!"; } } class parent { protected: Component *com; }; class child : public parent{ public: child() { com = new chiComponent(); } } Regards Arun

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• Java combine parents of two large inheritance chains

  - by Soylent Green

  I have two parent classes in a huge project, let's say ClassA and ClassB. Each class has many subclasses, which in turn have many subclasses, which in turn have many subclasses, etc. My task is to "marry" these two "families" so that both inherit from a SINGLE parent. I need to essentially make ClassA and ClassB one class (parent) to both of their combined subclasses (children). ClassA and ClassB both currently implement Serializable. I am currently trying to make both inheritance chains inherit from ClassA, and then copy all functions and data members from ClassB into ClassA. This is tedious, and I think a terrible solution. What would be the CORRECT way to solve this problem?

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• Problem about C++ class (inheritance, variables scope and functions)

  - by Luigi Giaccari

  I have a class that contains some data: class DATA Now I would to create some functions that uses those data. I can do it easily by writing member functions like DATA::usedata(); Since there are hundreds of functions, I would to keep an order in my code, so I would like to have some "categories" (not sure of the correct name) like: DATA data; data.memory.free(); data.memory.allocate(); data.file.import(); data.whatever.foo(); where memory, file and whatever are the "categories" and free, allocate and foo are the functions. I tried the inheritance way, but I got lost since I can not declare inside DATA a memory or file object, error C2079 occurs: http://msdn.microsoft.com/en-us/library/9ekhdcxs%28VS.80%29.aspx Since I am not a programmer please don't be too complicated and if you have an easier way I am all ears.

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• Exception and Inheritance in JAVA

  - by user1759950

  Suppose we have this problem public class Father{ public void method1(){...} } public class Child1 extends Father{ public void method1() throws Exception{ super.method1(); ... } } Child1 extends Father and override method1 but given implementation Child1.method1 now throws a exception, this wont compile as override method can't throw new exceptions. What is the best solution? Propagate the required exception to the Father.. to me this is against encapsulation, inheritance and general OOP ( the father potentially throw and exception that will never happen ) Use a RuntimeException instead? This solution wont propagate the Exception to the father but I read In Oracle docs and others sources states class of exceptions should be used when "Client code cannot do anything" this is not that case, this exception will b useful to recover blablabla ( why is wrong to use RuntimeException instead? ) Other.. thanks, Federico

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• Sharing base object with inheritance

  - by max

  I have class Base. I'd like to extend its functionality in a class Derived. I was planning to write: class Derived(Base): def __init__(self, base_arg1, base_arg2, derived_arg1, derived_arg2): super().__init__(base_arg1, base_arg2) # ... def derived_method1(self): # ... Sometimes I already have a Base instance, and I want to create a Derived instance based on it, i.e., a Derived instance that shares the Base object (doesn't re-create it from scratch). I thought I could write a static method to do that: b = Base(arg1, arg2) # very large object, expensive to create or copy d = Derived.from_base(b, derived_arg1, derived_arg2) # reuses existing b object but it seems impossible. Either I'm missing a way to make this work, or (more likely) I'm missing a very big reason why it can't be allowed to work. Can someone explain which one it is? [Of course, if I used composition rather than inheritance, this would all be easy to do. But I was hoping to avoid the delegation of all the Base methods to Derived through __getattr__.]

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• Default class for SQLAlchemy single table inheritance

  - by eclaird

  I've set up a single table inheritance, but I need a "default" class to use when an unknown polymorphic identity is encountered. The database is not in my control and so the data can be pretty much anything. A working example setup: import sqlalchemy as sa from sqlalchemy import orm engine = sa.create_engine('sqlite://') metadata = sa.MetaData(bind=engine) table = sa.Table('example_types', metadata, sa.Column('id', sa.Integer, primary_key=True), sa.Column('type', sa.Integer), ) metadata.create_all() class BaseType(object): pass class TypeA(BaseType): pass class TypeB(BaseType): pass base_mapper = orm.mapper(BaseType, table, polymorphic_on=table.c.type, polymorphic_identity=None, ) orm.mapper(TypeA, inherits=base_mapper, polymorphic_identity='A', ) orm.mapper(TypeB, inherits=base_mapper, polymorphic_identity='B', ) Session = orm.sessionmaker(autocommit=False, autoflush=False) session = Session() Now, if I insert a new unmapped identity... engine.execute('INSERT INTO EXAMPLE_TYPES (TYPE) VALUES (\'C\')') session.query(BaseType).first() ...things break. Traceback (most recent call last): File "<stdin>", line 1, in <module> File ".../SQLAlchemy-0.6.5-py2.6.egg/sqlalchemy/orm/query.py", line 1619, in first ret = list(self[0:1]) File ".../SQLAlchemy-0.6.5-py2.6.egg/sqlalchemy/orm/query.py", line 1528, in __getitem__ return list(res) File ".../SQLAlchemy-0.6.5-py2.6.egg/sqlalchemy/orm/query.py", line 1797, in instances rows = [process[0](row, None) for row in fetch] File ".../SQLAlchemy-0.6.5-py2.6.egg/sqlalchemy/orm/mapper.py", line 2179, in _instance _instance = polymorphic_instances[discriminator] File ".../SQLAlchemy-0.6.5-py2.6.egg/sqlalchemy/util.py", line 83, in __missing__ self[key] = val = self.creator(key) File ".../SQLAlchemy-0.6.5-py2.6.egg/sqlalchemy/orm/mapper.py", line 2341, in configure_subclass_mapper discriminator) AssertionError: No such polymorphic_identity u'C' is defined What I expected: >>> result = session.query(BaseType).first() >>> result <BaseType object at 0x1c8db70> >>> result.type u'C' I think this used to work with some older version of SQLAlchemy, but I haven't been keeping up with the development lately. Any pointers on how to accomplish this?

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• Inheritance mapping with Fluent NHibernate

  - by Berryl

  Below is an example of how I currently use automapping overrides to set up a my db representation of inheritance. It gets the job done functionality wise BUT by using some internal default values. For example, the discriminator column name winds up being the literal value 'discriminator' instead of "ActivityType, and the discriminator values are the fully qualified type of each class, instead of "ACCOUNT" and "PROJECT". I am guessing that this is a bug that doesn't get much attention now that conventions are preferred, and that the convention approach works correctly. I am looking for a sample of usage. Cheers, Berryl public class ActivityBaseMap : IAutoMappingOverride<ActivityBase> { public void Override(AutoMapping<ActivityBase> mapping) { ... mapping.DiscriminateSubClassesOnColumn("ActivityType"); } } public class AccountingActivityMap : SubclassMap<AccountingActivity> { public AccountingActivityMap() { ... DiscriminatorValue("ACCOUNT"); } } public class ProjectActivityMap : SubclassMap<ProjectActivity> { public ProjectActivityMap() { ... DiscriminatorValue("PROJECT"); } }

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• Objective-C Simple Inheritance and OO Principles

  - by bleeckerj

  I have a subclass SubClass that inherits from baseclass BaseClass. BaseClass has an initializer, like so: -(id)init { self = [super init]; if(self) { [self commonInit]; } return self; } -(void)commonInit { self.goodStuff = [[NSMutableArray alloc]init]; } SubClass does its initializer, like so: -(id)init { self = [super init]; if(self) { [self commonInit]; } return self; } -(void)commonInit { self.extraGoodStuff = [[NSMutableArray alloc]init]; } Now, I've *never taken a proper Objective-C course, but I'm a programmer more from the Electrical Engineering side, so I make do. I've developed server-side applications mostly in Java though, so I may be seeing the OO world through Java principles. When SubClass is initialized, it calls the BaseClass init and my expectation would be — because inheritance to me implies that characteristics of a BaseClass pass through to SubClass — that the commonInit method in BaseClass would be called during BaseClass init. It is not. I can *sorta understand maybe-possibly-stretch-my-imagination why it wouldn't be. But, then — why wouldn't it be based on the principles of OOP? What does "self" represent if not the instance of the class of the running code? Okay, so — I'm not going to argue that what a well-developed edition of Objective-C is doing is wrong. So, then — what is the pattern I should be using in this case? I want SubClass to have two main bits — the goodStuff that BaseClass has as well as the extraGoodStuff that it deserves as well. Clearly, I've been using the wrong pattern in this type of situation. Am I meant to expose commonInit (which makes me wonder about encapsulation principles — why expose something that, in the Java world at least, would be considered "protected" and something that should only ever be called once for each instance)? I've run into a similar problem in the recent past and tried to muddle through it, but now — I'm really wondering if I've got my principles and concepts all straight in my head. Little help, please.

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• Javascript: prototypeal inheritance and the prototype proprity

  - by JanD

  Hi, I have a simple code fragment in JS working with prototype inheritance. function object(o) { function F() {} F.prototype = o; return new F(); } //the following code block has a alternate version var mammal={ color: "brown", getColor: function(){ return this.color; } } var myCat = object(mammal); myCat.meow = function(){return "meow";} that worked fine but adding this: mammal.prototype.kindOf = "predator"; does not. ("mammal.prototype is undefined") Since I guessed that object maybe have no prototype I rewrote it, replacing the var mammal={... block with: function mammal(){ this.color="brown"; this.getColor = function(){return this.color;} } which gave me a bunch of other errors: "Function.prototype.toString called on incompatible object" and if I try to call _myCat.getColor() "myCat.getColor is not a function" Now I am totally confused. After reading Crockford, and Flanagan I did not get the solution for the errors. So it would be great if somebody knows... - why is the prototype undefined in the first example (which is foremost concern; I thought the prototype of explicitly set in the object() function) - why get I these strange errors trying to use the mammal function as prototype object in the object() function?

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• Generics vs inheritance (whenh no collection classes are involved)

  - by Ram

  This is an extension of this questionand probably might even be a duplicate of some other question(If so, please forgive me). I see from MSDN that generics are usually used with collections The most common use for generic classes is with collections like linked lists, hash tables, stacks, queues, trees and so on where operations such as adding and removing items from the collection are performed in much the same way regardless of the type of data being stored. The examples I have seen also validate the above statement. Can someone give a valid use of generics in a real-life scenario which does not involve any collections ? Pedantically, I was thinking about making an example which does not involve collections public class Animal<T> { public void Speak() { Console.WriteLine("I am an Animal and my type is " + typeof(T).ToString()); } public void Eat() { //Eat food } } public class Dog { public void WhoAmI() { Console.WriteLine(this.GetType().ToString()); } } and "An Animal of type Dog" will be Animal<Dog> magic = new Animal<Dog>(); It is entirely possible to have Dog getting inherited from Animal (Assuming a non-generic version of Animal)Dog:Animal Therefore Dog is an Animal Another example I was thinking was a BankAccount. It can be BankAccount<Checking>,BankAccount<Savings>. This can very well be Checking:BankAccount and Savings:BankAccount. Are there any best practices to determine if we should go with generics or with inheritance ?

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• Javascript: prototypeal inheritance and the prototype property

  - by JanD

  Hi, I have a simple code fragment in JS working with prototype inheritance. function object(o) { function F() {} F.prototype = o; return new F(); } //the following code block has a alternate version var mammal = { color: "brown", getColor: function() { return this.color; } } var myCat = object(mammal); myCat.meow = function(){return "meow";} that worked fine but adding this: mammal.prototype.kindOf = "predator"; does not. ("mammal.prototype is undefined") Since I guessed that object maybe have no prototype I rewrote it, replacing the var mammal={... block with: function mammal() { this.color = "brown"; this.getColor = function() { return this.color; } } which gave me a bunch of other errors: "Function.prototype.toString called on incompatible object" and if I try to call _myCat.getColor() "myCat.getColor is not a function" Now I am totally confused. After reading Crockford, and Flanagan I did not get the solution for the errors. So it would be great if somebody knows... - why is the prototype undefined in the first example (which is foremost concern; I thought the prototype of explicitly set in the object() function) - why get I these strange errors trying to use the mammal function as prototype object in the object() function?

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• Inheritance inside a template - public members become invisible?

  - by Juliano

  I'm trying to use inheritance among classes defined inside a class template (inner classes). However, the compiler (GCC) is refusing to give me access to public members in the base class. Example code: template <int D> struct Space { struct Plane { Plane(Space& b); virtual int& at(int y, int z) = 0; Space& space; /* <= this member is public */ }; struct PlaneX: public Plane { /* using Plane::space; */ PlaneX(Space& b, int x); int& at(int y, int z); const int cx; }; int& at(int x, int y, int z); }; template <int D> int& Space<D>::PlaneX::at(int y, int z) { return space.at(cx, y, z); /* <= but it fails here */ }; Space<4> sp4; The compiler says: file.cpp: In member function ‘int& Space::PlaneX::at(int, int)’: file.cpp:21: error: ‘space’ was not declared in this scope If using Plane::space; is added to the definition of class PlaneX, or if the base class member is accessed through the this pointer, or if class Space is changed to a non-template class, then the compiler is fine with it. I don't know if this is either some obscure restriction of C++, or a bug in GCC (GCC versions 4.4.1 and 4.4.3 tested). Does anyone have an idea?

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• Objective-C protocol vs inheritance vs extending?

  - by ryanjm.mp

  I have a couple classes that have nearly identical code. Only a string or two is different between them. What I would like to do is to make them "x" from another class that defines those functions and then uses constants or something else to define those strings that are different. I'm not sure if "x" is inheritance or extending or what. That is what I need help with. For example: objectA.m: -(void)helloWorld { NSLog("Hello %@",child.name); } objectBob.m: #define name @"Bob" objectJoe.m #define name @"Joe" (I'm not sure if it's legal to define strings, but this gets the point across) It would be ideal if objectBob.m and objectJoe.m didn't have to even define the methods, just their relationship to objectA.m. Is there any way to do something like this? It is kind of like protocol, except in reverse, I want the "protocol" to actually define the functions. If all else fails I'll just make objectA.m: -(void)helloWorld:(NSString *name) { NSLog("Hello %@",name); } And have the other files call that function (and just #import objectA.m).

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• SQL n:m Inheritance join

  - by Nightmares

  I want to join a table which contains n:m relationship between groups. (Groups are defined in a separate table). This table only has entries listing a member_group_id and a parent_group_id. Given this structure: id(int) | member_group_id(int) | parent_group_id(int) The "base" query looks like this: select p1.group_id, p2.group_id, p1.member_group_id, p2.member_group_id from group_member_group as p1 join group_member_group as p2 on p2.member_group_id = p1.member_group_id The "base" query correctly shows all relationships (I checked by doing it manually.) The problem is when I try to apply a where clause to this query to filter for a specific group as "point of origin" (the first group for which I want all parent groups) it returns only the closest parents. For example like this: select p1.group_id, p2.group_id, p1.member_group_id, p2.member_group_id from group_member_group as p1 join group_member_group as p2 on p2.member_group_id = p1.member_group_id where p1.group_id = 1 Can anyone give a clue how I can fix this? Or a different approach to realize this. (I suppose I could always do this in my C++ source code on the server side but I would have to transfer a entire table which has a high growth potential to the application server.) UPDATE: select p1.group_id, p2.group_id, p1.member_group_id, p2.member_group_id from group_member_group as p1 join group_member_group as p2 on p2.group_id = p1.member_group_id Typing mistake confirmed. Now I don't get past first level of inheritance period. Thanks at denied for pointing that out.

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• Generics vs inheritance (when no collection classes are involved)

  - by Ram

  This is an extension of this questionand probably might even be a duplicate of some other question(If so, please forgive me). I see from MSDN that generics are usually used with collections The most common use for generic classes is with collections like linked lists, hash tables, stacks, queues, trees and so on where operations such as adding and removing items from the collection are performed in much the same way regardless of the type of data being stored. The examples I have seen also validate the above statement. Can someone give a valid use of generics in a real-life scenario which does not involve any collections ? Pedantically, I was thinking about making an example which does not involve collections public class Animal<T> { public void Speak() { Console.WriteLine("I am an Animal and my type is " + typeof(T).ToString()); } public void Eat() { //Eat food } } public class Dog { public void WhoAmI() { Console.WriteLine(this.GetType().ToString()); } } and "An Animal of type Dog" will be Animal<Dog> magic = new Animal<Dog>(); It is entirely possible to have Dog getting inherited from Animal (Assuming a non-generic version of Animal)Dog:Animal Therefore Dog is an Animal Another example I was thinking was a BankAccount. It can be BankAccount<Checking>,BankAccount<Savings>. This can very well be Checking:BankAccount and Savings:BankAccount. Are there any best practices to determine if we should go with generics or with inheritance ?

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