如何将 libpq 中的空值与C++绑定

我无法将 NULL 值绑定到与 libpq 库C++。我使用这个描述如何在Postgresql中为C++准备语句和绑定参数，它工作正常。

但是我找不到如何为数值类型绑定 NULL 值。

 pqxx::prepare::invocation& prep_dynamic(std::vector<std::string>& data, pqxx::prepare::invocation& inv) 
{ 
    std::vector<std::string>::const_iterator it; 
    for(it=data.begin();it!=data.end();it++) 
    { 
        inv((*it)); 
    } 
    return inv; 
}


try 
     {
        pqxx::connection C("dbname = postgres user = postgres password=postgres hostaddr = 127.0.0.1 port = 5432");
        if (C.is_open()) 
        {
            std::string dbname = C.dbname();
            DIAG(DETAILED_INFO, "Opened database successfully:[%s]", dbname.c_str());           
        } 
        else 
        {
           DIAG(DETAILED_INFO, "Can't open database!");       
        }
        work W(C);
        C.prepare("example", "insert into ipls_troubleticketerrorreport (CUSTOMER, NETWORK,SELECTABLEMONTH, ERRORCODE,ERRORMESSAGE, TTID ) " 
                             "values ($1, $2, to_date($3,'YYYY-MM-DD'), $4, $5, $6);"); 

        pqxx::prepare::invocation w_invocation = W.prepared("example");

        std::vector< std::pair<std::string,std::string> > sqlParameters2;
        sqlParameters2.push_back(std::pair<std::string, std::string>("CUSTOMER", ""));
        sqlParameters2.push_back(std::pair<std::string, std::string>("NETWORK", ""));
        sqlParameters2.push_back(std::pair<std::string, std::string>("SELECTABLEMONTH", ""));
        sqlParameters2.push_back(std::pair<std::string, std::string>("ERRORCODE",""));
        sqlParameters2.push_back(std::pair<std::string, std::string>("ERRORMESSAGE", ""));
        sqlParameters2.push_back(std::pair<std::string, std::string>("TTID",""));

       for (int i = 0; i < sqlParameters2.size(); i++)
       {
            std::string value = "";                   
            value = i_get(sqlParameters2[i].first.c_str());            
            sqlParameters2[i].second = value;                         
       }
       std::vector<std::string > vect;
       for(int i=0; i<sqlParameters2.size();i++)
       {
           vect.push_back(sqlParameters2[i].second);
       }           
       prep_dynamic(vect, w_invocation);
       pqxx::result r = w_invocation.exec();
       W.commit();
      C.disconnect ();
     }
    catch(const std::exception &e)
    {
       DIAG(DETAILED_INFO, "exception occured.[%s]",e.what());   
       throw e;

    }