Inserting with a column that uses auto_increment

[Graham Reitz]

How do you prevent an insert as shown in the code below from putting  
"bogus" data in an auto_increment field?

It seems like the billing_id field, in the code below, inserts a  
number based on the un-initialized value of client_id, instead of  
letting the database auto_increment the value.  What's a good method  
to deal with this, that allows the database to auto_increment a  
database field?

Thanks,
Graham

/************ CODE ************/

#include <mysql++.h>
#include <custom.h>

using namespace mysqlpp;

sql_create_2(clients, 1, 2,
              unsigned int,     client_id, // auto_incremented field
              unsigned int,     billing_id)

void populate_client_info(clients& client_info)
{
     client_info.billing_id = 1;
}

int main (int argc, char* const argv[])
{
     Connection connection("db_name","ip_address","user",
                           "password",3306, false);

     Query client_query = connection.query();

     clients a_client;
     populate_client_info(a_client);

     client_query.insert(a_client);
     client_query.execute();

     return 0;
}