How to build this c++ typelist into a variant?

Question

Here,

how do I fix this c++ typelist template compile error?

we built a typelist, using the code from modern c++ design.

Question is now -- how do I take

Answer 1

The proper, but more advanced, approach would be actually store the values in a holder type that knows how to manage the actual type it contains.

A simpler approach, for learning purposes, would be to map types to numbers (i.e. their position in the typelist). With that you can remember what type you are currently storing in the variant.

To get that a working version you'll probably want to have templated constructors and setters and also an accessor function that use that type-number mapping.

Quite simplified it could look something like this:

template
class variant {
    unsigned type;
    void* value;
public:

    // ...

    template
    void set_to(const T& t) {
        STATIC_ASSERT(contains::value);
        // ... clean up previous value
        type  = index_of::value;
        value = static_cast(new T(t));
    }

    template
    const T& get_as() {
        STATIC_ASSERT(contains::value);
        if(type == index_of::value)
            return *static_cast(value);
        else
            throw type_error();
    }
};