You're welcome !

Don't forget to mark the thread as [solved] so other forum users may know a solution has been found :)

Thanks @SGaist for all the help and thank you @JKSH as well!

You are including mainwindow.h in behinglogin.h and behindlogin.h in mainwindow.h.

You can remove both includes: in behindlogin.h because you don't use it at all and in mainwindow.h because you can use forward declarations when declaring pointer data members. That also helps avoid including many headers to use your class.

In file included from /behindlogin.h
from behindlogin.cpp
behindlogin does not name a type.

and the line of code it's flagging is:

    behindlogin *behindLogin;

What's the full error message? Which line in which file is the error in?

#
# Project created by QtCreator 2015-03-25T23:10:30
#
#-------------------------------------------------

QT       += core gui sql

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = project
TEMPLATE = app


SOURCES += main.cpp\
        mainwindow.cpp \
    behindlogin.cpp \
    offlinebehind.cpp \
    savingwindow.cpp

HEADERS  += mainwindow.h \
    behindlogin.h \
    offlinebehind.h \
    savingwindow.h

FORMS    += mainwindow.ui \
    behindlogin.ui \
    offlinebehind.ui \
    savingwindow.ui

behindlogin.h

#ifndef BEHINDLOGIN_H
#define BEHINDLOGIN_H

#include <QMainWindow>
#include "mainwindow.h"

namespace Ui {
class behindlogin;
}

class behindlogin : public QMainWindow
{
    Q_OBJECT

public:
    explicit behindlogin(QWidget *parent = 0);
    ~behindlogin();

private slots:
    void on_actionExit_triggered();

    void on_actionNew_Data_triggered();

private:
    Ui::behindlogin *ui;
};

#endif // BEHINDLOGIN_H

mainwindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include "behindlogin.h"
#include <QMainWindow>
#include <QtSql>
#include <QtDebug>
#include <QFileInfo>


namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    QSqlDatabase mydb;
    void  connClose(){
        mydb.close();
        mydb.removeDatabase(QSqlDatabase::defaultConnection);
    }

    bool connOpen()
    {

        mydb=QSqlDatabase::addDatabase("QSQLITE");
        mydb.setDatabaseName("C:/Users/Jack/Desktop/ProjNEW/SQlite/dataset.db.sqlite");

        if(!mydb.open()){
           qDebug()<<("Disconnected..");
           return false;
        }
        else{
            qDebug()<<("Connected..");
            return true;
           }

    }

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private slots:

    void on_LoginButton_clicked();

    void on_actionOpen_triggered();

    void on_actionExit_triggered();

private:
    Ui::MainWindow *ui;
    behindlogin *behindLogin;
};

#endif // MAINWINDOW_H

@JKSH Hey, it is exactly the same as it is in the tutorial and the header file was the first thing I checked, behindlogin.h is included in my mainwindow.h which is where I'm trying to declare it.

Can you post:

1. The contents of your .pro file
2. The contents of behindlogin.h
3. The contents of mainwindow.h ?

@dejarked said:

in the main cpp file but it's saying that behindlogin does not name a type?

That means you forgot to #include the header that defines the behindlogin.

@dejarked said:

@SGaist Uh, honestly I'm not sure.. I'm very new to Qt and I've been following tutorials. Everything I've looked at makes it look like this should work but I can't figure out what I'm doing incorrectly.

Is your code exactly the same as what's in the tutorial?

                behindlogin *behindLogin;

in the header file and then

            behindLogin =new behindlogin(this);
            behindLogin->show();

in the main cpp file but it's saying that behindlogin does not name a type?

{
    behindlogin behindlogin;
    behindlogin.show();
    connClose();
} <- behindlogin is destroyed here

            behindlogin behindlogin;
            behindlogin.show();
            connClose();


Which causes the window to open and them immediately close after

setModal and exec are methods from QDialog. From what I understood you have a QMainWindow, no ?

            behindlogin behindlogin;
            behindlogin.setModal(true);
            behindlogin.exec();
            connClose();