Boost :: program_options - 如果没有提供输入，则打印使用情况

I am trying to use Boost program_options in order to parse my program inputs. In general the docs provide the necessary info for parsing. However, I would like to make my program print the usage instructions when no inputs are provided and I can't seem to figure it out. There does not seem to be a "default" options nor can I find how to count the number of inputs provided (to test).I am trying to use Boost program_options in ord