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 prob

Answer 1

Provide implementation for any pure virtual functions that the class has.

Answer 2

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.

Answer 3

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& bruh){
}

Answer 4

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 .

Answer 5

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<Abstract> pAbs(new Derived);

Answer 6

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.