recursive c++ template problem

Question

Say I have a template class that takes msgs from source, does something smart to them, and then sends them to a sink:

template <typename Source, typename Sink>
class MsgHandler
{
MsgHandler(Source* pSource)
: m_pSource(pSource)
{
  m_pSource->setHandler(this);
}
};

//Now the definition of the Source:

template <typename Handler>
class Source
{
void setHandler(Handler* pHandler)
{
  m_pHandler = pHandler;
}
};

All fine, but now I can't really make a Source or Handler. Eg:

MsgHandler<FileSource<MsgHandler<FileSource.... recursing parameters...
FileSource<MsgHandler<FileSource<MsgHandler.... same problem when trying to build a source

Is there a way to solve this problem without using a virtual base class for the Handler?

Virtual base class solution:

class MyHandler
{
virtual ~MyHandler() {};
virtual void handleSomething() = 0;
};

template <typename Source, typename Sink>
class MsgHandler : public MyHandler
{
  MsgHandler(Source* pSource)
  : m_pSource(pSource)
  {
    m_pSource->setHandler(this);
  }
  void handleSomething() {}
 };

class Source
{
void setHandler(MyHandler* pHandler)
{
m_pHandler = pHandler;
}
};

Answer 1

I don't understand why your Source needs to be parameterized on its handler. If Source and Handler really do need to be tightly coupled in the way you describe, it does not seem like templates buy you very much beyond interface definition. Seems to me like you could just have a non-template Source class that encapsulates Handler<Source, Sink>.

Answer 2

It looks like the Handler shouldn't know anything about the Source. How about simple linear dependency:

template <typename Sink>
class Handler {
private:
    Sink* sink; // get this pointer in the constructor?
public:
    void handle( const Msg& m ) {
        // processing
        sink->accept( m );
    }
};

template <typename Handler>
class Source {
private:
    Handler* handler; 
public:
    void genMessage() {
        Msg m;
        // get message off the wire?
        handler->handle( m );
    }
};

Could also be twisted to have "handling" and "sinking" as policies.

Answer 3

You could use a templated parameter for the source parameter of your handler:

class MySink;
template <template<typename Handler> class Source, typename Sink>
class MsgHandler
{
    Source<MsgHandler>* m_pSource;

    MsgHandler(Source<MsgHandler>* pSource)
    : m_pSource(pSource)
    {
      m_pSource->setHandler(this);
    }
};

//Now the definition of the Source:

template <typename Handler>
class Source
{
    void setHandler(Handler* pHandler)
    {
      m_pHandler = pHandler;
    }
};

//Now you can define variables like this
MsgHandler<Source, MySink> myHandler;

Of course that requires the Source parameter of MsgHandler to be a template with exactly one parameter (the handler), but if you can live with that constraint this would solve your definition problem (otherwise you might (or might not depending on what exactly you would be trying) be able to use some extra template foo to circumvent this restriction (creating another template which takes the handler as parameter and has a typedef for the corresponding SourcesType comes to mind).

In this scenario it might also be a good idea to add an typedef Source<MsgHandler> SourceType to MsgHandler to make the Source-Instantiation visible to the caller (instead of requiring the programmer to guess that MsgHandler will instantiate Source.