I recently discovered an undocumented feature of `boost::program_options`, namely that it accepts "*" as a special wildcard that allows declaration of a group of options with the same prefix, like this: configOptions.add_options() ("item_*", value<int>(), "items"); This declaration worked as expected and recognized `item_1`, `item_3`, etc, while rejecting unknown options. Well now the question is, how can I write a custom validate function that will populate a map with options key and its value, like this: map<string, int> itemsMap; options_description items("items options"); items.add_options() ("item_*",value<map<string, int>>(&itemsMap)->multitoken(), "items") ; My question is - how do I get the key of the option being validated from within validate() function? template <typename T> void validate(boost::any& v, const std::vector<std::string>& values, map<string, T> *, int) Thank you, Simone