QT -- 1. QT connection database

Video tutorial Links ：https://www.bilibili.com/video/BV1Li4y1f7ho?p=3.

class QSqlDatabase

The static member function in the above figure addDatabase The first parameter of type It indicates what type of database I want to connect to , Or what database software I'm connected to . The second parameter is the default parameter , Never mind .

class QSqlDatabase Common interface of

The function above setDatabaseName and removeDatabase The parameter of is to open or close the database file path .

The next step starts at QT Software to operate .

First, add the header file #include <QSqlDatabase>（ Here it is main.cpp in ）

As you can see from the figure, there is a yellow line under the header file and it shows that there is no such file or directory . Then why is it like this ？
It's about understanding QT The architecture of , Its whole framework is divided into modules . We usually use two modules , Namely Qt Core and QT GUI modular .

and QSqlDatabase It belongs to Qt SQL Of this module . So add this module to the project . How to add it ？ We need to work on the project xxx.pro Add a sentence to the file .

QT       += sql

As shown in the figure below .

here , And then main.c Middle view .

This indicates that the addition was successful .

Next , To create a QSqlDatabase The object of , But it cannot be created by a constructor , You need to use static functions addDatabase To create Sqlite3 Database connection .

The code is as follows ：

// Create a  QSqlDatabase  The object of 
// But it cannot be created by a constructor , You need to use static functions 
// establish Sqlite3 Database connection 
QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");

after , Specify which database file to open , Look at the code .

// Specify which database file to open （ Fill in the path of the database file ）
db.setDatabaseName("/home/chantui/work/code/sqlite3Pro/stu.db");
// Judge whether the opening is successful 
if(db.open()==false)
{
    
    qDebug()<<" Open the failure ";
}

If the database file of the specified path does not exist , Will automatically create a .
If you want to create it under the current project directory , You do not need to specify a path , Just write the file name .

db.setDatabaseName("stu.db");

Last , When we finish operating the database , And shut down the database , The code is as follows .

db.close();
db.removeDatabase("/home/chantui/work/code/sqlite3Pro/stu.db");

The code screenshot is as follows ：

Run code , Let's look at the effect . Go to the project directory （ Select the directory where you specify the builder ）,

Encapsulate the work of connecting to the database

The singleton pattern

Picture content source link : https://www.runoob.com/design-pattern/singleton-pattern.html.

（ Back of figure above , There is also a section on the implementation of the singleton pattern , You can adjust the past through the link to see , Since the focus of this chapter is not on the singleton mode , So don't go further here , If you have to distinguish between the implementation of the singleton pattern in this section , Should belong to the lazy type , Threads are not supported , Is the most basic implementation of singleton mode .）

good , Let's implement .

First , Create a file ,

The class name here is DBHelper （ Take at will ）,

And then click next , Until completion , When it's done , open DPHelper.h file

#ifndef DBHELPER_H
#define DBHELPER_H

#include <QSqlDatabase>

class DBHelper
{
    
public:
    // Open a... To the outside world “ Get the only available object ” Member function of 
    static DBHelper* getInstance();

private:
    // Create a  QSqlDatabase  The object of  db, After that, you need to  db  Object to initialize 
    QSqlDatabase db;
    static DBHelper* Instance ;
    // Because it's singleton mode , So let the constructor be  private, In this way, the class will not be instantiated 
    DBHelper();
};

#endif // DBHELPER_H

stay DPHelper.cpp in ,

#include "DBHelper.h"

DBHelper* DBHelper::Instance=0;

DBHelper* DBHelper::getInstance()
{
    
    if(Instance==0)
        Instance =new DBHelper();
    return Instance;
}

DBHelper::DBHelper()
{
    
    // In the constructor, right  db  Object to initialize 
    db = QSqlDatabase::addDatabase("QSQLITE");
}

void DBHelper::createConnection()
{
    
    // We need to specify the database file 
    db.setDatabaseName("tec.db");
    db.open();
}

void DBHelper::destoryConnection()
{
    
    db.close();
    db.removeDatabase("stu.db");
}

In this way, the singleton is implemented .

that , In this singleton process , We also hope to solve this problem through the object of a singleton db Object to create database files 、 Open database file and close database file . So we need to DPHelper Class, and then provide relevant interfaces .

stay main.cpp in , Let's first include the header file we created for this class #include “DBHelper.h”.

then , Call the interface to connect and disconnect the database .

#include "mainwindow.h"
#include <QApplication>
#include <QSqlDatabase>
#include <QDebug>
#include "DBHelper.h"

int main(int argc, char *argv[])
{
    
    QApplication a(argc, argv);

    DBHelper* helper = DBHelper::getInstance();

    helper->createConnection();
    helper->destoryConnection();

    MainWindow w;
    w.show();
    return a.exec();
}

Run code , As shown in the figure below , Database files tec.db It's created .