{ return hppbard{}; }

Variadic templates appeared in C++11 to cover such cases when you would have a template functions that could have a numerous members of different types. Doesn't it remind you of variadic functions that uses va_start, va_arg, va_end and so others? Cause it should be.

Defining a function you might want to make some of it's arguments to be constant to avoid unnecessary copying or have a bit more strict interface. There is another way to use constness - there we meet constant class members.