object inicialization with constructor in QT, no matching function to call

Question

I'm using Mysql 5.6 with Qt5 and creating a class to encapsulate a database connection.
When I try to create an object in the main with dbConnect conn = new dbConnect("localhost", "test1","test","user"); I get and error telling me I need a conversion from dbConnect* to a non-scalar type.
Reading other posts I've seen this could be due to the type of inicialization I'm trying because it's not a good practice in C++ and should be something like dbConnect conn("localhost", "test1","test","user"); but, changing it like that, instead of getting the conversion error I get now a no matching function for call to QSqlDatabase::setHosTname(std::string&) and same with the other methods needed for connecting such as setDatabaseName, setUserName and setPassword
Could it be because of the std::string I'm using? Or how should I create the object?
This is m y header db.h:

#ifndef DB
#define DB
#include <QSqlDatabase>
#include <string>
class dbConnect:QSqlDatabase
{
  public:
      dbConnect(std::string host, std::string name,std::string user, std::string pass);
      bool createConnection(std::string host, std::string name,std::string user, std::string pass);
  private:
      QSqlDatabase dbObject;
 };
   #endif // DB_

Next here it is its implementation db.cpp:

#include<QSqlDatabase>
#include <QMessageBox>
#include <QSqlError>

class dbConnect:QSqlDatabase
{
private:
    QSqlDatabase dbObject;
public:
    dbConnect(std::string host, std::string name,std::string user, std::string pass)
    {
        if(createConnection(host, name, user, pass))
        {
            QMessageBox::information(0,"Connection Status","Connection established with database");
        }
        else
        {
             QMessageBox::critical(0,QObject::tr("Error connecting to database"),dbObject.lastError().text());
        }
    }
    bool createConnection(std::string host, std::string name,std::string user, std::string pass)
    {
        dbObject = QSqlDatabase::addDatabase("QMYSQL");
        dbObject.setHostName(host);
        dbObject.setDatabaseName(name);
        dbObject.setUserName(user);
        dbObject.setPassword(pass);
        if(!dbObject.open())
        {
            return false;
        }
        else
        {
            return true;
        }
    }
};

UPDATE

Took @gengisdave and @R Sahu solutions and now I'm trying to create an object in the main. If I try dbConnect conn(); it works fine even if the constructor takes for paremeter but, if I trye dbConnect conn("localhost","test1","test","user"); compiler gives me an error of undefined reference to dbConnect::dbConnect(std::string,std::string,std::string,std::string).

R Sahu · Accepted Answer · 2015-10-12 19:34:07Z

When you use:

dbConnect conn = new dbConnect("localhost", "test1","test","user");

the RHS is of type dbConnect* while the LHS is of type dbConnect. That is not correct. The compiler cannot take a pointer and assign it to an object.

You can use:

dbConnect* connPtr = new dbConnect("localhost", "test1","test","user");

or

dbConnect conn("localhost", "test1","test","user");

Other problems

bool createConnection(std::string host, std::string name,std::string user, std::string pass)
{
    dbObject = QSqlDatabase::addDatabase("QMYSQL");
    dbObject.setHostName(host);
    dbObject.setDatabaseName(name);
    dbObject.setUserName(user);
    dbObject.setPassword(pass);
    if(!dbObject.open())
    {
        return false;
    }
    else
    {
        return true;
    }
}

is not right. You probably need:

bool createConnection(std::string host, std::string name,std::string user, std::string pass)
{
    // Since this class is derived from QSqlDatabase,
    // you can use:
    this->addDatabase("QMYSQL");
    this->setHostName(QString::fromStdString(host));
    this->setDatabaseName(QString::fromStdString(name));
    this->setUserName(QString::fromStdString(user));
    this->setPassword(QString::fromStdString(pass));
    if(!this->open())
    {
        return false;
    }
    else
    {
        return true;
    }
}

Also, remove the member variable

  QSqlDatabase dbObject;

You don't need it since the class is already derived from QSqlDatabase. You'll need to use that only if you don't derive from QSqlDatabase

gengisdave · Accepted Answer · 2015-10-12 20:57:17Z

2

QSQlDatabase::setHostName requires a parameter of type QString, while you provide a std::string; the same with the next 3 lines

you can change that lines to

this->setHostName(QString::fromStdString(host));
this->setDatabaseName(QString::fromStdString(name));
this->setUserName(QString::fromStdString(user));
this->setPassword(QString::fromStdString(pass));

EDIT: it works fine

main.cpp

#include <QApplication>
#include "db.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    dbConnect connection("localhost", "test1", "test", "user");
    dbConnect *conn = new dbConnect("localhost", "test1", "test", "user");
}

db.cpp

#include <QSqlDatabase>
#include <QMessageBox>
#include <QSqlError>
#include "db.h"

dbConnect::dbConnect(std::string host, std::string name,std::string user, std::string pass)
{
    if ( createConnection(host, name, user, pass) )
        QMessageBox::information(0, "Connection Status", "Connection established with database");
    else
        QMessageBox::critical(0,QObject::tr("Error connecting to database"),dbObject.lastError().text());
}

bool dbConnect::createConnection(std::string host, std::string name,std::string user, std::string pass)
{
    this->dbObject = QSqlDatabase::addDatabase("QMYSQL");
    this->dbObject.setHostName(QString("localhost"));
    this->dbObject.setDatabaseName(QString("test.txt"));
    this->dbObject.setUserName(QString("test"));
    this->dbObject.setPassword(QString("test"));
    return this->dbObject.open();
}

db.h

#ifndef DB_H
#define DB_H

#include <QSqlDatabase>
#include <string>

class dbConnect:QSqlDatabase
{
  public:
    dbConnect(std::string host, std::string name,std::string user, std::string pass);
    bool createConnection(std::string host, std::string name,std::string user, std::string pass);
  private:
    QSqlDatabase dbObject;
};

#endif

compiled with: gcc db.cpp main.cpp -I/usr/include/qt5/QtSql -I/usr/include/qt5/QtWidgets -I/usr/include/qt5 -fPIC -o main -lstdc++ -lQt5Sql -lQt5Core -lQt5Widgets

edited Oct 12, 2015 at 20:57

answered Oct 12, 2015 at 19:31

gengisdave

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6 Comments

Edgar Sampere Over a year ago

Thanks dude! I thought from the very beginning I'd have problems with QString, that's why I used std::string but that was the cause of all this.

Edgar Sampere Over a year ago

Ok, not really, now if I create an object of dbConnect like dbConnect conn("localhost", "test1","test","user"); compiler says: `undefined reference to dbConnect::dbConnect(std::string,std::string,std::string,std::string) but, the constructor is declared as it should, isn't it?

gengisdave Over a year ago

you don't need to redefine your class in the .cpp file, change to dbConnect::dbConnect

Edgar Sampere Over a year ago

Changed, and then I get three error as stated: 1. dbConnect::dbConnect names the constructor, not the type 2. expected ';' before conn 3. statement cannot resolve the address of overloaded function. I think the first is the most important because my constructor is not overloaded. I don't really get what type means if I'm just calling the constructor

gengisdave Over a year ago

the first error means you're defining an object using the constructor dbConnect::dbConnect instead of dbConnect type

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