constructors calling subclass

I have a (partally implemented) class hierarchy where

template<typename T> {
    class data { 
        data ( string s ) {}; // loads from file
        ...
    }
    class image: public data <T> { 
        image ( string s ) {}; // loads from file
        ...
    }
    class jpgimage : public image<T> {
        jpgimage ( string s ) {}; // loads from file 
        ...
    }
    // other image types
}

Now in the rest of my code I would like to be able to abstract from whether something is a jpeg image or even an image, so I would like to work with data. But at the same time I would like to pass commands specific to jpeg images to those functions.

So if I call data<int> img("lena.jpg"); which turns out to be an image, even a jpeg image, I would like the data constructor to call the image constructor, which in turn calls the jpgimage constructor.

There is no way I can get it to work, and people warn about slicing, virtual constructors, etc. But is this such a strange way to set it up?

Solution

Inheritance is to be used for is a relationships. So, an image<T> is a data<T>, but not the other way around! It makes no sense to call a method specific to image<T> for a data<T> object, which after all may not be an image<T>. The fact that you want to do that shows that your code design is flawed. Rethink your code design.