Error Message

Error: C++ forbids declaration with no type

Problem

There is a circular reference somewhere.

This can happen if two classes include each other and at least one class declares a member instance of the other.

Solution

If possible, use a forward declaration instead of including the entire class.

class MyClass;          // Forward declaration
class OtherClass {
public:
    // ...

private:
    MyClass myClass;
}

If a forward declaration is not possible, then try moving the #include to the implementation file.