This question is about the C++ Boost program_options library.
All the tutorials are very clear that I should call
notify() is a member function of value_semantic. It is a hook that is provided so that, once the final value of an option is determined, any action that should be taken with that option can be done automatically and be encapsulated in its own function. This prevents code from having one long function that acts on each of the options. As the possible options grow, that kind of procedural code can get unwieldy.
options_description desc; desc.add_options() ("compression", value<int>()->default_value(10), "compression level") ("email", value< vector<string> >() ->composing()->notifier(&your_function), "email") ;
These declarations specify that default value of the first option is 10, that the second option can appear several times and all instances should be merged, and that after parsing is done, the library will call function &your_function, passing the value of the "email" option as argument.