c++ class member initialization (poco)

Question

i am making a wrapper for my use for database, using Poco::Database::ODBC

the normal code is supposed to be like :

Poco::Data::ODBC::Connector::registerConnector();
Session ses("ODBC", "DSN=mytest;Uid=mytest;Pwd=mytest");
bool bConnected = ses.isConnected();
Statement select(ses);
select << "SELECT firstname FROM Patients", range(0, 10);
RecordSet rs(select);
while (!select.done())
{
    select.execute();
    bool more = rs.moveFirst();
    while (more)
    {
        for (std::size_t col = 0; col < rs.columnCount(); ++col)
        {
            std::cout << rs[col].convert<std::string>() << " ";
        }
        std::cout << std::endl;
        more = rs.moveNext();
    }
}

Poco::Data::ODBC::Connector::unregisterConnector();

This works just fine.

Now for my class

class database{
     Session ses; //Since this is needed all for all the queries.
   public:
     database():ses("ODBC", "DSN=name;uid=user;pwd=pass"){
     }
};//end class

how do i call Poco::Data::ODBC::Connector::registerConnector() before initialization call of ses

I tried

database():Poco::Data::ODBC::Connector::registerConnector(),ses("ODBC", "DSN=name;uid=user;pwd=pass"){
}

but this doesnt works. it gives error

'registerConnector' : is not a member of 'Poco::Data::ODBC::Connector'

how should i do it ??

Answer 1

Either do it outside database::database(), or use a pointer to a Session instead of a member, and allocate it on the heap inside the constructor. Something like:

database::database()
{
    Poco::Data::ODBC::Connector::registerConnector();
    ses = new Session("ODBC", "DSN=name;uid=user;pwd=pass");
}

database::~database()
{
    delete ses;
}

Answer 2

To enable this kind of functionality directly, you write a RAII-style wrapper object around the registerConnector function:

class wrapper {
public:
    wrapper() {
        Poco::Data::ODBC::Connector::registerConnector();
    }
    ~wrapper() {
        Poco::Data::ODBC::Connector::unregisterConnector();
    }
}

So you can now do:

class database{
     Wrapper wrap;
     Session ses; //Since this is needed all for all the queries.
   public:
     database() : wrap(), ses("ODBC", "DSN=name;uid=user;pwd=pass"){
     }
};

However, I don't think this is a good design; it leaves unanswered questions:

1. Why are you initializing the database subsystem in the database wrapper? Maybe it should be done as part of the program's init routine?
2. What happens if multiple database objects get used at the same time? Registering/unregistering would cause problems; doing the init as above would not.

Answer 3

It looks like registerConnector is a free function. To call other functions within the initialization list, you can use the comma operator:

 database()
    : ses(
        ( Poco::Data::ODBC::Connector::registerConnector(), "ODBC" )
        , "DSN=name;uid=user;pwd=pass"
      )
 {...}

The builtin comma operator evaluates two expressions and returns the result of the second one.

Alternatively, you may be able to create a custom class that handles the registerConnector() class, and place it within the class before ses so that its construction will happen before that of ses.