C++ – How to forward declare an inner class?

c++forward-declaration

I have a class like so…

class Container {
public:
    class Iterator {
        ...
    };

    ...
};

Elsewhere, I want to pass a Container::Iterator by reference, but I don't want to include the header file. If I try to forward declare the class, I get compile errors.

class Container::Iterator;

class Foo {
    void Read(Container::Iterator& it);
};

Compiling the above code gives…

test.h:3: error: ‘Iterator’ in class ‘Container’ does not name a type
test.h:5: error: variable or field ‘Foo’ declared void
test.h:5: error: incomplete type ‘Container’ used in nested name specifier
test.h:5: error: ‘it’ was not declared in this scope

How can I forward declare this class so I don't have to include the header file that declares the Iterator class?

Best Solution

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.