Constructor of an abstract class in C

Question

Why is it possible to write constructor for an abstract class in C   As far as I know we can t instantiate an abstract class   so what is it for  You can t instantiate the class  right

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You are absolutely correct  We cannot instantiate an abstract class because abstract methods don t have any body i e  implementation is not possible for abstract methods  But there may be some scenarios where you want to initialize some variables of base class   You can do that by using base keyword as suggested by  Rodrick  In such cases  we need to use constructors in our abstract class

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Because there might be a standard way you want to instantiate data in the abstract class   That way you can have classes that inherit from that class call the base constructor   public abstract class A       private string data       protected A string myString         data   myString            public class B   A         B string myString    base myString

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Key Points About Abstract Class  An abstract class cannot be instantiated  An abstract class can have constructor and destructor  An abstract class cannot be a sealed class because the sealed modifier prevents a class from being inherited  An abstract class contains abstract as well as non-abstract members  An abstract class members can be private  protected and internal  Abstract members cannot have a private access modifier  Abstract members are implicitly virtual and must be implemented by a non-abstract derived class

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Defining a constructor with public or internal storage class in an inheritable concrete class Thing effectively defines two methods    A method  which I ll call InitializeThing  which acts upon this  has no return value  and can only be called from Thing s CreateThing and InitializeThing methods  and subclasses  InitializeXXX methods  A method  which I ll call CreateThing  which returns an object of the constructor s designated type  and essentially behaves as   Thing CreateThing int whatever        Thing result   AllocateObject lt Thing gt         Thing initializeThing whatever        Abstract classes effectively create methods of only the first form   Conceptually  there s no reason why the two  methods  described above should need to have the same access specifiers  in practice  however  there s no way to specify their accessibility differently   Note that in terms of actual implementation  at least in  NET  CreateThing isn t really implemented as a callable method  but instead represents a code sequence which gets inserted at a newThing   new Thing 23   statement

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I too want to make some shine on abstract surface   All answer has covered almost all the things  Still my 2 cents   abstract classes are normal classes with A few exceptions   You any client Consumer of that class can t create object of that class  It never means that It s constructor will never call  Its derived class can choose which constructor to call  as depicted in some answer  It may has abstract function

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Far as I know we can t instantiate an abstract class   There s your error right there  Of course you can instantiate an abstract class    abstract class Animal    class Giraffe   Animal        Animal animal   new Giraffe      There s an instance of Animal right there  You instantiate an abstract class by making a concrete class derived from it  and instantiating that  Remember  an instance of a derived concrete class is also an instance of its abstract base class   An instance of Giraffe is also an instance of Animal even if Animal is abstract   Given that you can instantiate an abstract class  it needs to have a constructor like any other class  to ensure that its invariants are met    Now  a static class is a class you actually cannot instantiate  and you ll notice that it is not legal to make an instance constructor in a static class

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It s there to enforce some initialization logic required by all implementations of your abstract class  or any methods you have implemented on your abstract class  not all the methods on your abstract class have to be abstract  some can be implemented    Any class which inherits from your abstract base class will be obliged to call the base constructor

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an abstract class can have member variables that needs to be initialized so they can be initialized in the abstract class constructor and this constructor is called when derived class object is initialized

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From https   msdn microsoft com en-us library ms182126 aspx     Constructors on abstract types can be called only by derived types  Because public constructors create instances of a type  and you cannot create instances of an abstract type  an abstract type that has a public constructor is incorrectly designed    Since only derived classes can use an abstract class constructor then an abstract class constructor  if needed  must be declared as protected   However  that said VS compiler will not complain  with default rules  when declaring public constructors in abstract classes however it will not allow creating a new instance

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It s a way to enforce a set of invariants of the abstract class  That is  no matter what the subclass does  you want to make sure some things are always true of the base class    example   abstract class Foo       public DateTime TimeCreated  get  private set        protected Foo                  this TimeCreated   DateTime Now           abstract class Bar   Foo       public Bar     base     Bar s constructor s must call Foo s parameterless constructor              Don t think of a constructor as the dual of the new operator  The constructor s only purpose is to ensure that you have an object in a valid state before you start using it  It just happens to be that we usually call it through a new operator

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Normally constructors involve initializing the members of an object being created  In concept of inheritance  typically each class constructor in the inheritance hierarchy  is responsible for instantiating its own member variables  This makes sense because instantiation has to be done where the variables are defined    Since an abstract class is not completely abstract  unlike interfaces   it is mix of both abstract and concrete members  and the members which are not abstract are needed to be initialized  which is done in abstract class s constructors  it is necessary to have constructors in the abstract class  Off course the abstract class s constructors can only be called from the constructors of derived class

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There are two  following important features that prevent to inherit Abstract class   Abstract class must have an abstract method otherwise it is not a abstract class Abstract class must be inherited by derived class So if a class inherited by other class than what is use of to create object of that class

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You can instantiate it after you implemented all the methods   Then the constructor will be called

[c#] Constructor of an abstract class in C#

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