how_do_i_do_x.md

How do I do X with this library

How do I marshal and unmarshal markable int types through a framework that only understands int?

Such framework doesn't have to know that -1 is a special value, so use the representation interface:

markable<mark_int<int, -1>> oi = /* ... */;
lib::write_int(ot.representation_value()); // from markable
oi.assign_representation(lib::read_int()); // to markable

How do I model "true or no-value"?

This is often needed to represent "flags" that turn on optional features. You can use mark_int:

using flag = markable<mark_int<bool, false>>;
flag off_{}, false_{false}, true_{true};

assert (!off_.has_value());
assert (!false_.has_value());
assert (true_.has_value());

Later, when you need to pass this value to an interface that understands bools but no optional values, you can use:

bool convert_to_bool(flag f) {
  return f.representation_value();
}

Or when you need to convert it to boost::opitonal<bool> where value of false is never used:

boost::optional<bool> convert_to_opt_bool(flag f) {
  return boost::make_optional(f, true);
}