Question

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:

    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."
    }
};

Answer 1

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.