Abstract database implementation in C++ - c++

Question

I wanna create abstract Db class and implement Pg (postgresql) by inheriting from Db:
Db.h:
template <class T>
class Db
{
public:
Db(std::string, std::string, std::string, std::string);
virtual ~Db();
protected:
T* getConnection();
std::string getHost();
std::string getUsername();
std::string getDatabase();
std::string getPassword();
virtual std::string getConnectionString()=0;
private:
T* connection;
std::string host;
std::string username;
std::string database;
std::string password;
};
Db.cpp:
#include "Db.h"
using namespace std;
template <class T>
Db<T>::Db(string iHost, string iUsername, string iDatabase, string iPassword)
: host(iHost), username(iUsername), database(iDatabase), password(iPassword) {
T* conn(getConnectionString());
connection = conn;
}
template <class T>
T* Db<T>::getConnection() {
return connection;
}
... getters
Pg.h:
#include "../Db.h"
template <class T>
class Pg : public Db<T> {
public:
virtual std::string getConnectionString();
};
Pg.cpp:
#include "Pg.h"
template <class T>
std::string Pg<T>::getConnectionString() {
return "host=" + this->getHost() + " user=" + this->getUsername()
+ " password=" + this->getPassword() + " dbname=" + this->getDatabase();
}
main.cpp:
#include <iostream>
#include <pqxx/connection>
#include <pqxx/transaction>
#include "lib/db/pg/Pg.h"
using namespace std;
int main () {
string host = "localhost";
string username = "username";
string database = "db";
string password = "root";
try {
Pg<pqxx::connection> *db(host, username, database, password);
} catch (pqxx::broken_connection) {
cout << "Failed to establish connection." << endl;
}
return 0;
}
Sorry for long implementation. After compiling with g++ I get an error:
main.cpp: In function ‘int main()’:
main.cpp:15:62: error: expression list treated as compound expression in initializer [-fpermissive]
Pg<pqxx::connection> *db(host, username, database, password);
^
main.cpp:15:62: error: cannot convert ‘std::string {aka std::basic_string<char>}’ to ‘Pg<pqxx::basic_connection<pqxx::connect_direct> >*’ in initialization
I don't know if it is a good idea in the end, because my first version with only main() function looked more compact. I just try to hide connection details somewhere, such as connection string.

Answer 1

In main(), your db variable is declared as a pointer. You cannot pass constructor parameters to a pointer like you are doing.
You need to either:
drop the * from the declaration:
Pg<pqxx::connection> db(host, username, database, password);
keep the * and use new to construct the object:
Pg<pqxx::connection> *db = new Pg<pqxx::connection>(host, username, database, password);
...
delete db;