char delimiter = ',';

boost::program_options::options_description desc("Options");
desc.add_options()
    ("delimiter,d",
     boost::program_options::value<char>(&delimiter)->default_value(','),
     "specify printable ASCII delimiter (comma/CSV by default)");

if (delimiter != ',' && !isprint(delimiter) && delimiter != 't') {
    std::cerr << "Delimiter must be printable ASCII character.n";
    return EXIT_FAILURE;
}

ERROR: the argument ('t') for option '--delimiter' is invalid