rfl::Size

rfl::Size can be used to impose constraints on any container that contains the method .size(), which includes std::string.

For instance, you can a require a vector to be non-empty as follows:

using NonEmptyStringVector =
    rfl::Validator<std::vector<std::string>, rfl::Size<rfl::Minimum<1>>>;

Or you could require it to contain exactly zero or three elements, using validator composition:

using NoneOrThree =
    rfl::Validator<std::vector<std::string>,
                   rfl::Size<rfl::AnyOf<rfl::EqualTo<0>, rfl::EqualTo<3>>>>;