Hello, recently I was faced with a situation of writing a custom validator for a certain command line option I wanted to handle. The validator was supposed to check whether the value on the command line was in a certain range. After some head scratching, I realized there's no elegant way to do this with the current validation overload function. In the end I just ended up doing the validation after the options had been parsed. I was thinking about a good way to support this, and I thought that maybe the best way is to add an additional value semantic so that the validate function could instead be: template<class T, class validation_data> void validate(boost::any& v, const std::vector<std::string>& values, const validation_data& data, T* target_type, int); and we could write add_options() ("test", value<int>()->validate(boost::interval<int>(1, 10)) ); this would require the user to provide the following overload: void validate(boost::any& v, std::vector<std::string>& values, const boost::interval<int>& data, int* target_type, int); the default implementation of validate would all use some dummy type for validation_data, this way compilation would fail if user calls validate but does not provide the appropriate overload. And if the user does not want to provide additional validation data he can simply use the current syntax, but instead would provide the following validate() overload void validate(boost::any& v, const std::vector<std::string>& values, const no_validation_data& data, T* target_type, int); Thoughts? Zach