Error: unreal engine cannot instantiate abstract class

What's Causing This Error

When Unreal Engine reports that it "cannot instantiate abstract class," this typically means that you're trying to create an instance of a class that contains one or more 'pure virtual' functions that have not been implemented. In C++ and Unreal Engine, an abstract class is a class that is designed to be specifically used as a base class. An abstract class contains at least one pure virtual function.

Here are common causes of this error:

  1. Inheriting from an Abstract Base Class: You may have created a subclass from an abstract base class but failed to provide concrete implementations for all of its pure virtual functions.
  2. UFUNCTION() Macro: Unreal Engine uses UFUNCTION() macros to expose C++ functions to the engine. If a function within a UObject is marked as pure virtual (=0) and doesn't have a definition, this can lead to instantiation issues.
  3. Incomplete Definitions: Sometimes, a function might be declared but not defined properly in your class, which could result in it being treated as abstract.

Solution - Here's How To Resolve It

To resolve the issue, ensure that all pure virtual functions in the base class have overriden implementations in the derived class. Follow these steps:

  1. Review Abstract Base Class: Look at the base class to identify any pure virtual functions (functions ending with =0;).
  2. Implement the Functions: In your derived class, provide definitions for all of the pure virtual functions from your base class.
  3. Check for Overrides: Use the override keyword in the derived class's function declarations to ensure that you're actually overriding the intended functions from the base class.
  4. UCLASS Declaration: Check the UCLASS macro in your class header to make sure it's not marked as Abstract. If it is, remove this unless you intend for the class to be abstract.
  5. Compile and Test: After making changes, recompile your code to make sure there are no compilation errors and test to see if the error has been resolved.

If after following these steps you still encounter the error, double-check that all classes you're attempting to instantiate are indeed meant to be instantiated and are not just there to provide a polymorphic interface for other subclasses.

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