Username and Password authentication using pqxx library.

Hello,

            I am using pqxx API for postgresql.The following is the part of my code.The problem is in the query I am selecting the number of rows from the table since there is only one user name and password row it should return 1 and if there is no match then it should return 0.But here Even though I give wrong username or password its getting authenticated.please help me,what should I put in the "if ()" statement inorder to solve this issue .

result R(T.exec("select count(*) from dbtable where username="+username+" and password="+password+""));
   
    if (R.empty())
        {
            throw logic_error("No tables found");
            *shm = '5';
        }
    else
        {
            cout<<"Authenticated";
            *shm='0';
        }

Thank You,
Avinash.