How do you handle a cannot instantiate abstract class error in C

Question

How do you handle a  cannot instantiate abstract class  error in C    I have looked at some of the similar errors here and none of them seem to be exactly the same or problem that I am having   But  then again  I will admit that there are several to go over   Here is the compile error     This leads me to this page  http   msdn microsoft com query dev10 query appId Dev10IDEF1 amp l EN-US amp k k C2259  k VS ERRORLIST  amp rd true Compile Error C2259 is from a C   program but the page calls the abstract class an  interface       Whenever you derive from an interface and implement the interface methods in the derived class with access permissions other than public  you may receive C2259  This occurs because the compiler expects the interface methods implemented in the derived class to have public access  When you implement the member functions for an interface with more restrictive access permissions  the compiler does not consider them to be implementations for the interface methods defined in the interface  which in turn makes the derived class an abstract class       There are two possible workarounds for the problem        Make the access permissions public for the implemented methods       Use the scope resolution operator for the interface methods implemented in the derived class to qualify the implemented method name with the name of the interface    The bad news is that I have already made all of the methods public in the class   class AmbientOccluder  public Light       public           AmbientOccluder void

User · Answer

If anyone is getting this error from a function  try using a reference to the abstract class in the parameters instead  void something Abstract bruh      to void something Abstract amp  bruh

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I have answered this question here  Covariant virtual functions return type problem  See if it helps for some one

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Provide implementation for any pure virtual functions that the class has

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Why can t we create Object of Abstract Class   When we create a pure virtual function in Abstract class  we reserve a slot for a function in the VTABLE studied in last topic   but doesn t put any address in that slot  Hence the VTABLE will be incomplete  As the VTABLE for Abstract class is incomplete  hence the compiler will not let the creation of object for such class and will display an errror message whenever you try to do so   Pure Virtual definitions  Pure Virtual functions can be given a small definition in the Abstract class  which you want all the derived classes to have  Still you cannot create object of Abstract class  Also  the Pure Virtual function must be defined outside the class definition  If you will define it inside the class definition  complier will give an error  Inline pure virtual definition is Illegal

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Visual Studio s Error List pane only shows you the first line of the error   Invoke View Output and I bet you ll see something like   c  path to your code cpp 42   error C2259   AmbientOccluder    cannot instantiate abstract class           due to following members             ULONG MysteryUnimplementedMethod void     is abstract           c  path to some include h 8    see declaration of  MysteryUnimplementedMethod

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In my case i declared a function in COM Control  idl file like    id 1   HRESULT MyMethod  in INT param     but not declared in my interface  h file like this  STDMETHOD MyMethod  INT param     Problem solved by adding above line into my interface  h file  this might help some one

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The error means there are some methods of the class that aren t implemented  You cannot instantiate such a class  so there isn t anything you can do  other than implement all of the methods of the class   On the other hand  a common pattern is to instantiate a concrete class and assign it to a pointer of an abstrate base class   class Abstract      stuff    4   class Derived   virtual public Abstract      implement Abstract s methods        Abstract  pAbs   new Derived     OK   Just an aside  to avoid memory management issues with the above line  you could consider using a smart pointer  such as an  std  unique ptr   std  unique ptr lt Abstract gt  pAbs new Derived

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An abstract class cannot be instantiated by definition  In order to use this class  you must create a concrete subclass which implements all virtual functions of the class  In this case  you most likely have not implemented all the virtual functions declared in Light  This means that AmbientOccluder defaults to an abstract class  For us to further help you  you should include the details of the Light class

[c++] How do you handle a "cannot instantiate abstract class" error in C++?

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