How do I forward declare an inner class? [duplicate]

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This is simply not possible. You cannot forward declare a nested structure outside the container. You can only forward declare it within the container.

You’ll need to do one of the following

  • Make the class non-nested
  • Change your declaration order so that the nested class is fully defined first
  • Create a common base class that can be both used in the function and implemented by the nested class.

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