Question

I’ve got a class outputInterface; that should handle the output (to files) of some data. The data is contained in objects of some custom classes, say dataClassA and dataClassB, that all derive from a common base class dataClassBase.

Now I want the data to be written to different files according to its type. So data of type dataClassA should go to fileA, data of type dataClassB should go to fileB and so on. As this output happens very often I would like the file handles (fileA and fileB) to stay open, i.e. I don’t want to open and close the files for the output of each piece of data. One outputInterface object can be expected to exist all the time.

So what I would like to achieve is something like this:

• Dynamically associate data of type dataClassA with the file handle fileA etc.
• When receiving data of type dataClassA check whether fileA is already connected to a file, if not, open the file.

How can I get this behavior (or least something similar / better)?
I’ve been thinking of making the file handles static members of dataClassA and dataClassB (or the base class dataClassBase?). But then, how do I take care of closing the files? I would have to somehow keep track of the data types that have actually been used (the files that have actually been opened).

Answer 1

Try something like this:

#ifndef OUTPUTINTERFACE?H   
#define OUTPUTINTERFACE?H

#include <string>
#include <fstream>
#include <map>

class DataClass
{
public:
    virtual bool WriteData(std::ofstream& FStream) = 0;
};
class DataClass1 :
    public DataClass
{    
    virtual bool WriteData(std::ofstream& FStream)
    {
        FStream << "teletubbies";
    }
};    
class DataClass2 :
    public DataClass
{
    virtual bool WriteData(std::ofstream& FStream)
    {
        FStream << "garbage";
    }
};


class OutputInterface
{
public:
    OutputInterface()
    {
    }
    ~OutputInterface()
    {
        //Release stream pointers
    }
    template<typename T>
    bool WriteData(T& Data)
    {
        std::string dClassUID = std::string(typeid(T).name);
        tFStreamMap::iterator it this->streamMap.find(dClassUID);
        std::ofstream* stream = NULL;

        if(it != streamMap.end())
        {
            stream = it->second;
        }
        else
        {
            stream = new std::ofstream();
            stream->open(dClassUID + ".txt");
            streamMap.insert(std::make_pair(dClassUID, stream));
        }

        Data.WriteData(stream);
    }
private:
    typedef std::map<std::string, std::ofstream*> tFStreamMap;
    tFStreamMap streamMap;
};    

#endif

This is just a prove of concept and can be optimized in many ways.
I would rather stick with overloaded functions than with runtime type checks.