Quick A: Separate definition and implementation.

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Templated Function results in Circular Inclusion

Define registerEvent after IApp.

class IApp;

class Component
{
    IApp* app;
    template<typename T>
    void registerEvent(const int& evtId, Status (T::*func) (int));
};

class IApp : public Component {
  ...
};

template <typename T>
Component::registerEvent(const int& evtId, Status (T::*func) (int)) {
  auto res = std::bind(func, (T*)this, std::placeholders::_1);
  app->registerForEvent(evtId);
}

If need be, also define A::registerEvent after Component::registerEvent.