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

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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:

[IMG]http://i67.photobucket.com/albums/h292/Athono/cannotinstantiateabstractclass.png[/IMG]

This leads me to this page: http://msdn.microsoft.com/query/dev10.query?appId=Dev10IDEF1&l=EN-US&k=k(C2259);k(VS.ERRORLIST)&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);
abstract-class c++
8 Answers

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){
}

Can't construct an abstract class, even from a subclass. Abstract classes are basically templates for other classes to be built on and don't have constructors themselves. It's kind of another way to do an interface that involves inheritance.

There's a lot of stuff online about polymorphism, and it's all dumb. C++ makes it so that you can inherit from multiple classes at the same time, so abstract classes and interfaces are the same there. I think the reason why abstract classes exist is because some languages, like Java, can only inherit/extend from one class only.

Abstract classes are quite literally an abstract concept. They are parents of classes, but only in our minds. They are an answer to a question that doesn't have a right answer. Like, "What is the predecessor to GTA?" and you'd say, "Well, there's Race n' Chase" but that's not even a correct answer because they're the same game, just with the name changed. Abstract classes are the answer to "what's the parent of that class"? Like you could say "Oh, a String is just a character array" and "a character array is just a couple iterators" and "iterators are just pointers". Abstract classes allow us to formalize these stupid answers into inheritance.

user_101415280
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