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.