Design Issue (C++)

Question

My apologies in advance if I'm not asking this in a right Stack Overflow forum. I'm trying to come up with a design which is easily expandable and also easy to use. So here is my problem:

In C++, I have two classes, called Class One and Class Two which both supports an operation called doSomething(T& data). T is a struct which for example has two members for Class One and has four members for Class Two. Since I want to enforce doSomething(T& data) into those classes, I have a base class called Base.

template <class T>
class Base
{
    typedef T    data_type;
    virtual bool doSomething(data_type& data) const = 0;
};

struct OneData { int a; int b; };
class One : public Base<OneData>
{
    bool doSomething(data_type& data) const { ... }
};

struct TwoData { int a; float b; int c; char d; };
class Two : public Base<TwoData>
{
    bool doSomething(data_type& data) const { ... }
};

One big problem I have with this approach is, I cannot defined a Base type variable. I'm forced to use One or Two as type which forces me to defined 2 variables (what I really want is to create Base type variable and allocate memory or initialize it to One or Two based on the given T).

Furthermore, I was thinking to create class called OneTwo which defines doSomething(T& data) which basically hides the headache of which object to create from clients.

// This is just idea of what I want but don't know how??
template<class OneT, class TwoT>
struct OneTwo
{
    bool doSomething(T& data) const // Here I don't know how to get to T
    {
        if (OneT::owns(data))      // call OneT::doSomething
        else if (TwoT::owns(data)) // call TwoT::doSomething
        else                       // Maybe exception, error, ...
    }
}; 

I'm not sure even if I did right approach on class One and Two. Also, I tried to explain it as clear as possible but please ask if anyone need more information.

Please help me to understand on how to design this?

Thank you,

---

UPDATE:

In nutshell, I want to have minimum of 2 classes (class One and Two in above example) and possibly more that follow same interface such implement same operation (doSomething(...) in above example), but the given data to that operation is different for each class.

Moreover, I want to provide a single interface to all those classes classes (class OneTwo in above example) for ease of use.

Answer 1

Here is a design that comes close to doing what you've asked to do, and hopefully when you see it, you'll understand that what you are asking for is logically inconsistent, like asking for a circle in euclidean space in which the ratio between the radius and the diameter is 3.

#include <stdexcept>

struct OneData;
struct TwoData;

class Base {
  public:
   virtual bool doSomething(OneData &) const { throw ::std::runtime_error("I don't know how to operate on a OneData!"); }
   virtual bool doSomething(TwoData &) const { throw ::std::runtime_error("I don't know how to operate on a TwoData!"); }

   virtual ~Base() = 0 { } // Make this an abstract base class that can't be instantiated.
};

struct OneData { int a; int b; };
class One : public Base
{
    virtual bool doSomething(OneData& data) const { return; }
};

struct TwoData { int a; float b; int c; char d; };
class Two : public Base
{
    virtual bool doSomething(TwoData& data) const { return; }
};

You can now have your Base * and call doSomething on it to your heart's content giving it a OneData or a TwoData as you please. But woe betide you if you give it the wrong type for the class it actually is because it'll throw an exception.

Now, how do you fix that? Well, you can't. doSomething for a One class must have a OneData and for a Two class must have a TwoData but you don't know which you have if you have a Base *.

Perhaps you can instead ask a different question. Because the train of thought that led you to ask this nonsense question that has no answer left somewhere. And that's where it needs to be put back on track. What led you to believe you wanted to do this thing that makes no sense?