void addFlag ( const std::string opt, const std::string opt_full, const std::string info )
add an boolean flag. The default value is always ‘false’.
Parameters:
| opt | the short opt name, must start with “-“, or being “” if not used. |
| opt_full | the full opt name, must start with “–”, pass “” if not used. |
| info | the info to be shown in usage. |