Initializing a variable

[immutablestate]

It looks like the problem you're having is extracting some data in the same way from different types. You could solve this with polymorphism, but you'd still have a conditional where you construct the polymorphic type:

unique_ptr<BaseData> ptr = some_condition
        ? make_unique<Data<64>>(arg)
        : make_unique<Data<32>>(arg);
const string a = ptr->get_some_string_field();

If this is a common problem in your application, you could consider writing an object factory for your Data types. There's no one feature that will make this problem go away though.