C++ policy based design… base class as either sigleton or not based on template parametere.

Question

The OutputPolicy of my class sometimes goes to a static/singleton object and sometime goes to a 1-to-1 object.

Not sure if I can explain in english.. so here is the desired behavior in pseudo code:

template< 
    class Algo,
    template<class> OutputPolicy
>
class AlgoBase : Algo, OutputPolicy<Algo>
{
};

class AlgoImp
{
public:
    AlgoImp(string topic) {} 
    void OnData(
    {
        cout << " AlgoImp::OnData";
        if ( .... ) 
            NewMsg(msg);
    }
};

class AlgoImp2
{
public:
    AlgoImp2(string topic) {} 
    void OnData(
    {
        cout << " AlgoImp2::OnData";
        if ( .... ) 
            NewMsg(msg);
    }
};

AlgoImp::OnData, does some processing and calls NewMsg. I am trying to decouple the implimentation of the NewMsg() call.

template < class T>
class OutMessageQueueTcp
{
    void NewMsg(string in)
    {
        //Should be a signgleton or static
        cout << " OutMessageQueueTcp::NewMsg";
    }
};

If OutputPolicy is OutMessageQueueTcp (above) then there should only be one instance of OutputPolicy. So I would either have to derive from a Singleton or use a static variable somewhere.

template <class T>
class OutMessageAsInput : AlgoBase<AlgoImp2, OutMessageQueueTcp>
{ 
    void NewMsg(string in)
    {
        cout << " OutMessageAsInput::OnData";
        AlgoBase<AlgoImp2, OutMessageQueueTcp>::NewMsg(in);
    }
};

If OutputPolicy is OutMessageAsInput (above) then the out becomes input to a sister object and then will be sent to OutMessageQueueTcp.

Here is the main.

main 
{
    AlgoBase<AlgoImp, OutMessageAsInput> myalgo1;
    myalgo1.OnData("hi");
}

And my desired output:

AlgoImp::OnData
OutMessageAsInput::NewMsg 
AlgoImp2::OnData
OutMessageQueueTcp::NewMsg

What I really have is a collection of AlgoImp objects, each with its own topic and data. AlgoImp will generate outout by calling NewMsg. Sometimes these messages should be sent to a Socket, and sometimes they should be processed by AlgoImp2.

This decision will happen at compile time. There will be many Algo's each doing diff things. Seems like I am trying to do some kind of Aspect oriented stuff.

Question:

Is it possible to have a Base class be either a singleton or a normal object by making it a policy? Is there a design pattern for something like this? Maybe some kind of factory?

Thanks for the help.

Answer 1

To keep things simple - OutputPolicy at your algo classes level should be a simple class with simple method. Of course you can implement it with singleton or static methods from the real implementation class:

template < class T>
class OutMessageQueueTcp
{
public:
    void NewMsg(string in)
    {
       impl.NewMsg(in);
    }
private:
   static OutMessageQueueTcpImpl<T> impl;
};

Or:

template < class T>
class OutMessageQueueTcp
{
public:
    void NewMsg(string in)
    {
       OutMessageQueueTcpImpl<T>::NewMsg(in);
    }
};

For your Algo* classes: if output policy is given at compile time - make all these classes template clasess, and treat OutputPolicy as a Strategy - keeo it aggregated (as member variable) in your algo classes:

template <template <typename> class OutputPolicy>
class AlgoImp
{
public:
    AlgoImp(string topic) {} 
    void OnData(
    {
        cout << " AlgoImp::OnData";
        if ( .... ) 
            outputPolicy.NewMsg(msg);
    }
private:
   OutputPolicy<AlgoImp> outputPolicy;
};