C++ variable type at runtime

Question

I am trying to store some data that I read from a file. The data is an array and can be one of a few types; short, float, etc. I am trying to abstract this data to a class, so I immediately thought:

class BaseData {
public:
    void *data;
    virtual void foo() = 0;
};

template <typename T>
class Data : public BaseData {
public:
    T *data;

    Data(const File *file) {
        data = (T*) file->data;
    }

    T operator()(int x, int y, int z) {
       return data[x + y*yd + z*zd];
    }

    void foo() {}
};

I use a switch statement when reading the file to instantiate the right object and then when I want to work with the different Data objects I do:

if (Data<short> *d = dynamic_cast<Data<short> *>(image->data)) {
   cout << (*d)(100, 50, 100) << endl;
}

where image->data is of type BaseData.

What are your thoughts on this? Is this the right way to go about things? Ideally I would like to grab the array (and the type) and do things with it, but I don't know what the type is going to be at compile time.

Just a quick point, by declaring T *data in class Data, you're hiding void *data defined in class BaseData - the void pointer will never be used. — Drew McGowen
– Drew McGowen, Commented Jul 12, 2013 at 21:26
I would recommend boost::variant. It allows you to make type safe unions. — David Brown
– David Brown, Commented Jul 12, 2013 at 21:34

Andrew Tomazos · Accepted Answer · 2013-07-12 21:57:19Z

1

What are your thoughts on this? Is this the right way to go about things? Ideally I would like to grab the array (and the type) and do things with it, but I don't know what the type is going to be at compile time.

Yes, generally speaking creating a parametric polymorphic wrapper like you are doing is a good approach. It is similar to the "Any" design pattern as exemplified by boost::any, and attaches run-time type information to otherwise unrelated non-polymorphic types such as int, float, etc.

You should also look at boost::variant which is a similar but slightly different approach. In variant the set of types become template parameters of the Base. As a pro this allows some additional compile-time checking and also a slightly more efficient implementation, as a con it has an arguably more complicated interface.

edited Jul 12, 2013 at 21:57

answered Jul 12, 2013 at 21:48

Andrew Tomazos

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