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

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.

I don't believe forward declaring inner class of on an incomplete class works (because without the class definition, there is no way of knowing if there actually is an inner class). So you'll have to include the definition of Container, with a forward declared inner class:

class Container {
public:
    class Iterator;
};

Then in a separate header, implement Container::Iterator:

class Container::Iterator {
};

Then #include only the container header (or not worry about forward declaring and just include both)