Reify values of optional arguments into std::optional

[zhihaoy]

Semantically, an optional argument is "optional" to present. But currently, if default_value isn't set, an optional argument's "not presented" state is not representable. Attempting to access the state (value) will get a logic_error. Representing the "not presented" state is often necessary because the option's type may leave no value for a good default_value. For example, an empty string may be an valid argument to an option.

Python argparse has no such problem because any option may be None. Universally Nullable is a problem, but allowing None for specific types is adopted by many languages. In C++ it's std::optional<T>.

I suggest to add a program.present<T>("--option") API in addition to the existing program.get<T>("--option") API. The new API returns std::optional<T> by reifying T's possible values and the "not presented" state. Its usage looks like the following:

if (auto v = program.present<int>("-n")) {
  do_something_with(*v);
}

The name "present" is taken from Java's Optional.ifPresent. Bike-shedding is mildly welcome.

[p-ranav]

Sounds good to me. Stands to reason.