Accessing Values in a Class Similar to boost::any

I'm making a simple boost::any-like class for educational purposes, but I can't figure out how to access the stored value. I can set the value perfectly, but when I try to access any member in the "holder" class the compiler just complains that the member wasn't found in the class it was derived from. I can't declare the members as virtual because of the templates.

Here's the relevant code:

class Element
{
    struct ValueStorageBase
    {
    };

    template <typename Datatype>
    struct ValueStorage: public ValueStorageBase
    {
        Datatype Value;

        ValueStorage(Datatype InitialValue)
        {
            Value = InitialValue;
        }
    };

    ValueStorageBase* StoredValue;

public:

    开发者_开发技巧template <typename Datatype>
    Element(Datatype InitialValue)
    {
        StoredValue = new ValueStorage<Datatype>(InitialValue);
    }

    template <typename Datatype>
    Datatype Get()
    {
        return StoredValue->Value; // Error: "struct Element::ValueStorageBase" has no member named "Value."
    }
};

It's fine to add virtual functions to templates- just the functions themselves cannot be templates. A templated class or struct can still have virtual functions just fine. You need to use the magic of dynamic_cast.

class Element
{
    struct ValueStorageBase
    {
        virtual ~ValueStorageBase() {}
    };

    template <typename Datatype>
    struct ValueStorage: public ValueStorageBase
    {
        Datatype Value;

        ValueStorage(Datatype InitialValue)
        {
            Value = InitialValue;
        }
    };

    ValueStorageBase* StoredValue;

public:

    template <typename Datatype>
    Element(Datatype InitialValue)
    {
        StoredValue = new ValueStorage<Datatype>(InitialValue);
    }

    template <typename Datatype>
    Datatype Get()
    {
        if(ValueStorage<DataType>* ptr = dynamic_cast<ValueStorage<DataType>*>(StoredValue)) {
            return ptr->Value;
        else
            throw std::runtime_error("Incorrect type!"); // Error: "struct Element::ValueStorageBase" has no member named "Value."
    }
};

If you change Get to return a Datatype* you can return NULL instead of throwing. You also haven't handled the memory of the previous value of StoredValue, but I'm leaving that up to you.

You need to cast it to ValueStorage first. Also add virtual destructur to ValueStorageBase class, to have polymorphic class. Without it you can't runtime check if your casting is OK :).

After it you can write:

template <typename Datatype>
    Datatype Element_cast()
    {
        //throw exception for a wrong cast
        if(typeid(*StoredValue)!=typeid(ValueStorage<Datatype>) )
               throw exception;

        //we already checked, that our type casting is OK,
        //So no need for dynamic_cast anymore
        return static_cast<ValueStorage*> (StoredValue)->value;
    }