Cross-platform C++ libraries ACE,Boost,POCO,Qt
Sourcetrail a cross-platform source explorer, great and powerful tool to have it.
I personally recommend it for any junior or senior developers. A cross-platform source explorer for C/C++ and Java* deserves a blog post to reach more developers as well as serious commercial customers.
* Free for non-commercial use
SQLite operations in C++
This tutorial aimed to demonstrate how to use SQLite database operations such as CREATE, INSERT, UPDATE, SELECT and DELETE in C++. Since the appearance first smartphone and other mobile devices SQLite become most widely deployed and used database engine in the world. Precise numbers are difficult to obtain but SQLite is likely used more than all other database engines combined. We can find SQLite in every Android device, iPhone and iOS device, Mac, Windows10, Firefox, Chrome, Safari, Skype, Dropbox, TurboTax, QuickBooks, PHP, Python, television sets, set-top cable boxes and many automotive multimedia systems.
It’s an open-source SQL database engine. You can download the source code or precompiled binaries depending your requirements. You might also need to add [-lsqlite3] and [-std=c++17] to successfully compile, create an executable file. Each of the following source code available on my git repository.
github.com/lnrsoft/sqlite3_cpp_basics_updated
Use this link local_db.sqlite (173 downloads) if you would like to download the local_db.sqlite file that I used in this tutorial.
Step 1 – The first thing we need to do is to create the actual database file that we will use in this article. I called sqltdemo.db in our case.
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 |
#include <iostream> #include <sqlite3.h> #include <string> int main() { sqlite3* db; auto op = sqlite3_open("local_db.sqlite", &db); auto cl = sqlite3_close(db); if(op) { std::cerr << "SQLite ERROR: " << sqlite3_errmsg(db) << std::endl; return (0); } else { std::cout << "SQLite local db has been opened successfully." << std::endl; } if(cl) { std::cerr << "SQLite ERROR: " << sqlite3_errmsg(db) << std::endl; return (0); } else { std::cout << "SQLite local db has been closed successfully." << std::endl; } } |
Compile and run our source code to create the local_db.sqlite with the following command. Here we link our code with the sqlite3 library that will create our db file.
|
1 2 3 4 |
$ clang++ 01_openDB.cpp -Wall -Wextra -lsqlite3 -std=c++17 $ ./a.out SQLite local db has been opened successfully. SQLite local db has been closed successfully. |
Verify our result:
|
1 2 3 4 |
$ ls -la -rw-r--r-- 1 rolandihasz staff 605 Mar 1 21:05 01_openDB.cpp -rw-r--r-- 1 rolandihasz staff 21606 Mar 1 21:06 local_db.sqlite -rwxr-xr-x 1 rolandihasz staff 8660 Mar 1 21:06 a.out |
Step 2 – CREATE TABLE
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 |
#include <iostream> #include <sqlite3.h> #include <string> static int callback(void *NotUsed, int argc, char **argv, char **azColName) { int i; for(i = 0; i < argc; i++) { std::cerr << azColName[i] << argv[i] << argv[i] << "NULL"; } std::cout << std::endl; return 0; } int main() { sqlite3 *db; char *zErrMsg = nullptr; const char *sql; auto rc = sqlite3_open("local_db.sqlite", &db); if(rc) { std::cerr << "Failed open database: " << sqlite3_errmsg(db) << std::endl; return (0); } else { std::cerr << "Database successfully opened" << std::endl; } sql = "CREATE TABLE CANDIDATE(" "ID INT PRIMARY KEY NOT NULL," "NAME TEXT NOT NULL," "AGE INT NOT NULL," "ROLE TEXT NOT NULL," "SALARY TEXT NOT NULL);"; rc = sqlite3_exec(db, sql, callback, nullptr, &zErrMsg); if(rc != SQLITE_OK) { std::cerr << "SQL error: " << zErrMsg << std::endl; sqlite3_free(&zErrMsg); } else { std::cerr << "Table successfully created" << std::endl; } sqlite3_close(db); return 0; } |
Compile and run.
|
1 2 3 4 |
$ clang++ 02_sqlite3_create_table.cpp -Wall -Wextra -lsqlite3 -std=c++17 $ ./a.out Database successfully opened Table successfully created |
Step 3 – INSERT INTO our CANDIDATE TABLE
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 |
#include <iostream> #include <sqlite3.h> #include <string> static int callback(void *NotUsed, int argc, char **argv, char **azColName) { int i; for(i = 0; i < argc; i++) { std::cerr << azColName[i] << argv[i] << argv[i] << "NULL"; } std::cout << std::endl; return 0; } int main() { sqlite3 *db; char *zErrMsg = nullptr; int rc; const char *sql; rc = sqlite3_open("local_db.sqlite", &db); if(rc) { std::cerr << "Failed open database: " << sqlite3_errmsg(db) << std::endl; return (0); } else { std::cerr << "Database successfully opened" << std::endl; } sql = "INSERT INTO CANDIDATE (ID,NAME,AGE,ROLE,SALARY) " "VALUES (1, 'Daniel', 29, 'CCO', 130000 ); " "INSERT INTO CANDIDATE (ID,NAME,AGE,ROLE,SALARY) " "VALUES (2, 'Roland', 35, 'CTO', 175000 ); " "INSERT INTO CANDIDATE (ID,NAME,AGE,ROLE,SALARY)" "VALUES (3, 'Leslie', 33, 'CEO', 200000 );"; rc = sqlite3_exec(db, sql, callback, nullptr, &zErrMsg); if(rc != SQLITE_OK) { std::cerr << "SQL error: " << zErrMsg << std::endl; sqlite3_free(zErrMsg); } else { std::cerr << "Records created successfully" << std::endl; } sqlite3_close(db); return 0; } |
Compile and run.
|
1 2 3 4 |
$ clang++ 03_sqlite3_insert.cpp -Wall -Wextra -lsqlite3 -std=c++17 $ ./a.out Database successfully opened Records created successfully |
Step 4 – Fetch and display records from the company CANDIDATE TABLE. In this scenario we use a callback function that obtains results from SELECT statement.
|
1 2 3 4 5 6 7 |
//callback function declaration typedef int (*sqlite3_callback)( void*, /* Data provided in the 4th argument of sqlite3_exec() */ int, /* The number of columns in row */ char**, /* An array of strings representing fields in the row */ char** /* An array of strings representing column names */ ); |
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 |
#include <iostream> #include <sqlite3.h> #include <stdio.h> #include <stdlib.h> #include <string> static int callback(void *data, int argc, char **argv, char **azColName) { fprintf(stderr, "\nCallback function call: ", &data); for(auto i = 0; i < argc; i++) { printf("%s = %s\n", azColName[i], argv[i] ? argv[i] : "NULL"); } return 0; } int main() { sqlite3 *db; char *zErrMsg = nullptr; int rc; const char *sql; const char *data = "Callback function call"; rc = sqlite3_open("local_db.sqlite", &db); if(rc) { std::cerr << "Failed open database: " << sqlite3_errmsg(db) << std::endl; return (0); } else { std::cerr << "Database successfully opened" << std::endl; } sql = "SELECT * from CANDIDATE"; rc = sqlite3_exec(db, sql, callback, &data, &zErrMsg); if(rc != SQLITE_OK) { std::cerr << "SQL error: " << zErrMsg << std::endl; sqlite3_free(zErrMsg); } else { std::cout << "\nExecution completed successfully" << std::endl; } sqlite3_close(db); return 0; } |
Compile and run.
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 |
$ clang++ 04_sqlite3_select_operation.cpp -Wall -Wextra -lsqlite3 -std=c++17 $ ./a.out Database successfully opened Callback function call: ID = 1 NAME = Daniel AGE = 29 ROLE = CCO SALARY = 130000 Callback function call: ID = 2 NAME = Roland AGE = 35 ROLE = CTO SALARY = 175000 Callback function call: ID = 3 NAME = Leslie AGE = 33 ROLE = CEO SALARY = 200000 Execution completed successfully |
Step 5 – Using UPDATE statement to update any records in our CANDIDATE table.
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 |
#include <iostream> #include <sqlite3.h> #include <stdio.h> #include <stdlib.h> #include <string> static int callback(void *data, int argc, char **argv, char **azColName) { fprintf(stderr, "\nCallback function call: ", &data); for(auto i = 0; i < argc; i++) { printf("%s = %s\n", azColName[i], argv[i] ? argv[i] : "NULL"); } return 0; } int main() { sqlite3 *db; char *zErrMsg = nullptr; int rc; const char *sql; const char *data = "Callback function call"; rc = sqlite3_open("local_db.sqlite", &db); if(rc) { std::cerr << "Failed open database: " << sqlite3_errmsg(db) << std::endl; return (0); } else { std::cout << "Database successfully opened" << std::endl; } sql = "UPDATE CANDIDATE set SALARY = 150000 where ID=1; " "SELECT * from CANDIDATE"; rc = sqlite3_exec(db, sql, callback, &data, &zErrMsg); if(rc != SQLITE_OK) { std::cerr << "SQL error: " << zErrMsg << std::endl; sqlite3_free(zErrMsg); } else { std::cout << "Execution completed successfully" << std::endl; } sqlite3_close(db); return 0; } |
Compile and run.
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 |
$ clang++ 05_sqlite3_update_operation.cpp -Wall -Wextra -lsqlite3 -std=c++17 $ ./a.out Database successfully opened Callback function call: ID = 1 NAME = Daniel AGE = 29 ROLE = CCO SALARY = 150000 Callback function call: ID = 2 NAME = Roland AGE = 35 ROLE = CTO SALARY = 175000 Callback function call: ID = 3 NAME = Leslie AGE = 33 ROLE = CEO SALARY = 200000 Execution completed successfully |
Step 6 – DELETE statement can delete any records from the CANDIDATE table.
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 |
#include <iostream> #include <sqlite3.h> #include <stdio.h> #include <stdlib.h> #include <string> static int callback(void *data, int argc, char **argv, char **azColName) { fprintf(stderr, "\nCallback function call: ", &data); for(auto i = 0; i < argc; i++) { printf("%s = %s\n", azColName[i], argv[i] ? argv[i] : "NULL"); } return 0; } int main() { sqlite3 *db; char *zErrMsg = nullptr; int rc; const char *sql; const char *data = "Callback function call"; rc = sqlite3_open("local_db.sqlite", &db); if(rc) { std::cerr << "Failed open database: " << sqlite3_errmsg(db) << std::endl; return (0); } else { std::cerr << "Database successfully opened" << std::endl; } sql = "DELETE from CANDIDATE where ID=2; " "SELECT * from CANDIDATE"; rc = sqlite3_exec(db, sql, callback, &data, &zErrMsg); if(rc != SQLITE_OK) { std::cerr << "SQL error: " << zErrMsg << std::endl; sqlite3_free(zErrMsg); } else { std::cout << "\nExecution completed successfully" << std::endl; } sqlite3_close(db); return 0; } |
Compile and run.
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 |
$ clang++ 06_sqlite3_delete_operation.cpp -Wall -Wextra -lsqlite3 -std=c++17 $ ./a.out Database successfully opened Callback function call: ID = 1 NAME = Daniel AGE = 29 ROLE = CCO SALARY = 150000 Callback function call: ID = 3 NAME = Leslie AGE = 33 ROLE = CEO SALARY = 200000 Execution completed successfully |
I recommend you to check the official SQLite documentation at sqlite.org for more advance SQLite features and options and support. For further info about using sql with c++ you might found it useful to read me previous post of a simple Qt tool to test QSqlDatabase access connection to a MySQL database. I created this simple Qt command line tool to test connection between Qt client applications and a remote Mysql Server. The code includes Secure Sockets Layer (SSL) support to test secure connection. The QSqlDatabase class provides an interface for accessing a database through a connection where an instance of QSqlDatabase represents the connection. The connection provides access to the database via one of the supported database drivers, which are derived from QSqlDriver.
Alternatively, we can subclass your own database driver from QSqlDriver. My next post will be an article how to write your own database driver in Qt for Unix, OSX and Windows platforms.
List of build-specific variants to specify your Qt project.
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 |
QMAKE_CC = gcc QMAKE_LEX = flex QMAKE_LEXFLAGS = QMAKE_YACC = byacc QMAKE_YACCFLAGS = -d QMAKE_CFLAGS = QMAKE_CFLAGS_DEPS = -M QMAKE_CFLAGS_WARN_ON = -Wall QMAKE_CFLAGS_WARN_OFF = -w QMAKE_CFLAGS_RELEASE = -O2 QMAKE_CFLAGS_DEBUG = -g QMAKE_CFLAGS_YACC = -Wno-unused -Wno-parentheses QMAKE_CXX = g++ QMAKE_CXXFLAGS = $$QMAKE_CFLAGS QMAKE_CXXFLAGS_DEPS = $$QMAKE_CFLAGS_DEPS QMAKE_CXXFLAGS_WARN_ON = $$QMAKE_CFLAGS_WARN_ON QMAKE_CXXFLAGS_WARN_OFF = $$QMAKE_CFLAGS_WARN_OFF QMAKE_CXXFLAGS_RELEASE = $$QMAKE_CFLAGS_RELEASE QMAKE_CXXFLAGS_DEBUG = $$QMAKE_CFLAGS_DEBUG QMAKE_CXXFLAGS_YACC = $$QMAKE_CFLAGS_YACC QMAKE_CXXFLAGS_THREAD = $$QMAKE_CFLAGS_THREAD QMAKE_CXXFLAGS_RTTI_ON = -frtti QMAKE_CXXFLAGS_RTTI_OFF = -fno-rtti QMAKE_CXXFLAGS_EXCEPTIONS_ON = -fexceptions -mthreads QMAKE_CXXFLAGS_EXCEPTIONS_OFF = -fno-exceptions |
A simple command line tool to test connection between Qt client applications and a remote Mysql Server. The code includes Secure Sockets Layer (SSL) support to test secure connection. The QSqlDatabase class provides an interface for accessing a database through a connection. An instance of QSqlDatabase represents the connection. The connection provides access to the database via one of the supported database drivers (MySQL in our case), which are derived from QSqlDriver. Alternatively, we can subclass your own database driver from QSqlDriver. I’ll post an another article about how to write your own database driver in Qt for Unix, OSX and Windows platforms. It’s recommend to write your own database driver especially when you planning to deploy your application in different platforms. It’s simply because there could be surprises especially on client deploying your application that using one of the supported database drivers due to missing dependencies or environmental variables.
Software prerequisites:
Remote MySQL server version: 5.7 or above
Qt version: Qt_5_7_1 or above
OpenSSL 1.0.x or above the for SSL feature
How to use it:
1. I simply run the mysql_conn_test_database.sql SQL script file to create a test database on the remote server.
2. Set the hostname or IP, port number, username and password. You migth need to create a new database user for this purpose.
[You will need to create SSL certificates for the database server and client in order to test SSL Feature on the MySQL Server.]
3. If everthing is set just simple build and run the project in your QtCreator.
If everything goes well you will recieve the following feedback from your terminal:
QSqlDatabase connection test tool was created and written by Roland Ihasz and it is licensed under the GNU General Public License (GPL) Version 2 or above.
Check out on GitHub. https://github.com/lnrsoft/QSqlDatabase-connection-test-tool
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 |
# (c) Roland Ihasz - https://github.com/lnrsoft QT += core QT -= gui QT += sql QT += sql widgets CONFIG += c++11 CONFIG += console #CONFIG -= console CONFIG -= app_bundle TEMPLATE = app TARGET = mysql_db_connection_test SOURCES += main.cpp QTPLUGIN += sqldrivers QMAKE_CXXFLAGS += -std=c++11 |
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 |
// (c) Roland Ihasz - https://github.com/lnrsoft #include <QSqlDatabase> #include <QApplication> #include <QSqlRecord> #include <QSqlError> #include <QSqlQuery> #include <QDebug> #include <QtCore> #include <QtSql> int main(int argc, char* argv[]) { QApplication a(argc, argv); { QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL"); db.setHostName("192.168.1.100"); // your host or ip name db.setPort(3306); // port number db.setDatabaseName("mysql_conn_test"); // db name db.setUserName("username"); // db username db.setPassword("password"); // db password //----------Additional-SSL-Feature-for-secure-database-connection---------- // db.setConnectOptions( // "SSL_KEY=/Users/lnrsoft/repository/mysql_db_conn_test/client-key.pem;" // "SSL_CERT=/Users/lnrsoft/repository/mysql_db_conn_test/client-cert.pem;" // "SSL_CA=/Users/lnrsoft/repository/mysql_db_conn_test/ca-cert.pem;" // "SSL_CIPHER=DHE-RSA-AES256-SHA;"); //----------Additional-SSL-Feature-for-secure-database-connection---------- if(!db.open()) { qDebug() << "\nDATABASE CONNECTION UNSUCCESSFUL." << "\nlastError: " << db.lastError().text() <<"\n"; return 0; } if(db.open()) { qDebug() << "\nSUCCESSFULLY CONNECTED TO THE DATABASE SERVER."; QSqlQuery query( "SELECT " "dataTable.ID, " "dataTable.x1, " "dataTable.y1, " "dataTable.z1, " "dataTable.t " "FROM mysql_conn_test.dataTable"); QSqlRecord rec1 = query.record(); QSqlRecord rec2 = query.record(); QSqlRecord rec3 = query.record(); QSqlRecord rec4 = query.record(); QSqlRecord rec5 = query.record(); qDebug() << "NUMBER OF ROWS: " << query.size(); // Return number of rows int productCode1 = rec1.indexOf("ID"); int productCode2 = rec2.indexOf("x1"); int productCode3 = rec3.indexOf("y1"); int productCode4 = rec4.indexOf("z1"); int productCode5 = rec5.indexOf("t"); qDebug() << "===================================================================================="; while(query.next()) { qDebug() << "ROW NUMBER" << query.value(productCode1).toString() << "" << query.value(productCode2).toString() << "" << query.value(productCode3).toString() << "" << query.value(productCode4).toString() << "" << query.value(productCode5).toString(); } qDebug() << "===================================================================================="; qDebug() << "COMPLETE..."; return 0; } } } |
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 |
/* (c) Roland Ihasz - https://github.com/lnrsoft */ DROP DATABASE IF EXISTS `mysql_conn_test`; CREATE DATABASE IF NOT EXISTS `mysql_conn_test` /*!40100 DEFAULT CHARACTER SET utf8 */; USE `mysql_conn_test`; /*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */; /*!40101 SET @OLD_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS */; /*!40101 SET @OLD_COLLATION_CONNECTION=@@COLLATION_CONNECTION */; /*!40101 SET NAMES utf8 */; /*!40103 SET @OLD_TIME_ZONE=@@TIME_ZONE */; /*!40103 SET TIME_ZONE='+00:00' */; /*!40014 SET @OLD_UNIQUE_CHECKS=@@UNIQUE_CHECKS, UNIQUE_CHECKS=0 */; /*!40014 SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0 */; /*!40101 SET @OLD_SQL_MODE=@@SQL_MODE, SQL_MODE='NO_AUTO_VALUE_ON_ZERO' */; /*!40111 SET @OLD_SQL_NOTES=@@SQL_NOTES, SQL_NOTES=0 */; DROP TABLE IF EXISTS `dataTable`; /*!40101 SET @saved_cs_client = @@character_set_client */; /*!40101 SET character_set_client = utf8 */; CREATE TABLE `dataTable` ( `ID` int(1) NOT NULL, `x1` double NOT NULL, `y1` double NOT NULL, `z1` double NOT NULL, `t` int(16) NOT NULL ) ENGINE=MyISAM DEFAULT CHARSET=utf8; /*!40101 SET character_set_client = @saved_cs_client */; LOCK TABLES `dataTable` WRITE; /*!40000 ALTER TABLE `dataTable` DISABLE KEYS */; INSERT INTO `dataTable` VALUES (1,0.3352925219759345,0.336742962207645,0.3117983819916844,2),(2,0.3351856799423695,0.334428142439574,0.3127960397861898,5),(3,0.3592007360234857,0.314406356215476,0.3792905477248132,9); /*!40000 ALTER TABLE `dataTable` ENABLE KEYS */; UNLOCK TABLES; /*!40103 SET TIME_ZONE=@OLD_TIME_ZONE */; /*!40101 SET SQL_MODE=@OLD_SQL_MODE */; /*!40014 SET FOREIGN_KEY_CHECKS=@OLD_FOREIGN_KEY_CHECKS */; /*!40014 SET UNIQUE_CHECKS=@OLD_UNIQUE_CHECKS */; /*!40101 SET CHARACTER_SET_CLIENT=@OLD_CHARACTER_SET_CLIENT */; /*!40101 SET CHARACTER_SET_RESULTS=@OLD_CHARACTER_SET_RESULTS */; /*!40101 SET COLLATION_CONNECTION=@OLD_COLLATION_CONNECTION */; /*!40111 SET SQL_NOTES=@OLD_SQL_NOTES */; |
Alternatively, use the following shorter source code to check the connection without any sql query.
QSqlDatabase connection test tool without query
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 |
// (c) Roland Ihasz - https://github.com/lnrsoft #include <QCoreApplication> #include <QSqlDatabase> #include <QSqlError> #include <QSqlQuery> #include <QDebug> #include <QDebug> #include <QtCore> #include <QtSql> int main(int argc, char* argv[]) { QCoreApplication a(argc, argv); QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL"); db.setHostName("192.168.1.100"); // host or ip name db.setPort(3306); // port number db.setDatabaseName("mysql_conn_test"); // db name db.setUserName("username"); // db username db.setPassword("password"); // db password if(!db.open()) { qDebug() << "\nDATABASE CONNECTION UNSUCCESSFUL." << "\nlastError: " << db.lastError().text() <<"\n"; return 0; } if(db.open()) { qDebug() << "\nSUCCESSFULLY CONNECTED TO THE DATABASE SERVER.\n"; return 0; } } |
After spending a quite a bit of time to find the solution of this often-occurring
QMYSQL driver not loaded problem mainly after installing Qt installation in a new software environment, So I thought I am going to share my best practice to fix this issue.
I wasn’t able to establish database connection even if I received the list of available drivers:
QSqlDatabase: available drivers: QSQLITE QMYSQL QMYSQL3 QODBC QPSQL
QSqlDatabase: QMYSQL driver not loaded
The following simple method helps to fix the problem.
On OSX system just simple install or re-install mysql using the flowing command. I usually prefer Homebrew as it fulfils most my requirements and using MacPort when I really no other options available.
|
1 |
$ brew install mysql |
then place the following libmysqlclient.18.dylib.zip (223 downloads) file to the path below:
|
1 |
/opt/local/lib/mysql55/mysql/ |
|
1 |
/opt/local/lib/mysql55/mysql/libmysqlclient.18.dylib |
On different Linux distributions I usually apt install or yum install the MySQL package basically the only thing I need to make sure that place the libmysqlclient.18.dylib.zip (223 downloads) file to the following path:
|
1 |
/opt/local/lib/mysql55/mysql/ |
This method solves the QMYSQL driver not loaded issue and allows me to establish database connection, it might work on Windows system as well, however I had not opportunity to try before.
Please, let me know if this method also working for you by dropping a line.
QString QString::number(double n, char format = ‘g’, int precision = 6) returns a string equivalent of the number n, formatted according to the specified format and precision.
Let’s see the following example.
There are 3 double values x, y, z.
We want to return the result of the following equation:
(x / z) * (5.961 / y)
We also want to ask the user to set the desired precision of the result.
Here is how I do with Qt.
You can also Fork the complete qt project from my GitHub.
* note = there are many ways to solve this issue, much simpler, but this time I want to demonstrate how we do in Qt.
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 |
// (c) Roland Ihasz - https://github.com/lnrsoft #include <QCoreApplication> #include <iostream> #include <QDebug> int main(int argc, char* argv[]) { QCoreApplication a(argc, argv); double x = 1.190; double z = 6.634; double y = 3.398; int precision; double result = (x / z) * (5.961 / y); std::cout << "\nEnter the required precision of the result: "; std::cin >> precision; QString resultstring = QString::number(result, 'g', precision); std::cout << "\n1.In this case the output is a double value." << std::endl; std::cout << "The result by the compiler default setting is: " << result << "\n" << std::endl; std::cout << "2. Using the QString::number member function." << std::endl; std::cout << "The result when we set the precision to " << precision << " is: " << qPrintable(resultstring) << "\n" << std::endl; /* * using qPrintable to remove qoutes from the output */ return 0; } |
Argument Formats
In member functions where an argument format can be specified (e.g., arg(), number()), the argument format can be one of the following:
Format Meaning
e format as [-]9.9e[+|-]999
E format as [-]9.9E[+|-]999
f format as [-]9.9
g use e or f format, whichever is the most concise
G use E or f format, whichever is the most concise
A precision is also specified with the argument format. For the ‘e’, ‘E’, and ‘f’ formats, the precision represents the number of digits after the decimal point. For the ‘g’ and ‘G’ formats, the precision represents the maximum number of significant digits (trailing zeroes are omitted).
How to return a string equivalent of the number n using Qt was written by Roland Ihasz>, you can modify or distribute. You can do anything you want with this software, just don’t say you wrote it.
More info on doc.qt.io
There is a simple method to insert images such as a png file using QPixmap. I personally prefer to use a QLabel to show simple images on QWidget, particularry if there is no any other fancy requirements.
Basically you can locate the QLabel anywhere on your QWidget. You will find the pixelmap option at the text option section in your Qt Creator, right below the textFormat. When you click on the Value section you’ll find a small arrow pointing down.
Simple click on it, then choose your png file.
If you just simple want to display an image to the QWidget, I found the following option works well. You just need to insert the path of the image file right after the
|
1 |
ui->setupUi(this); |
member function in your main parent widget, then insert the following source code.
|
1 2 |
QPixmap png_img("/Users/lnrsoft/software_development/repository/test_app/queq.png"); ui->label->setPixmap(png_img); |
If you know other method to display an image on a QtWidget Please, let me know by dropping a line, I can add it to this post if it is relevant and you agree to do.
Just reIf you just started to familiar yourself with Qt’s IDE, this is my advice. After a long time of daily usage became my favorite developing IDE, as it is covering almost every part of the work I do on a daily basis. However there are three (3rd party) plugins am always adding to my Qt Creator IDE wherever I work on-site or in the office they are very useful. Especially CppCheck but gtest and the DB viewing tool can coming handy.
Qt Creator CppCheck integration plugin
Qt Creator Google C++ Testing Framework integration plugin
Qt Creator SQL database viewer plugin
All of the plugins mentioned above developed by Gres.
Bottom line: a good and comfortable development enviromet setup if playing a key role of utilizing your almost always limited time and result an efficient way of your daily developing work.
Valgrind (adding memory and function profiler feature to Qt Creator), sometimes preferring GDB instead of LLDB especially project on OSX.
I also use Doxygen for generating documentation from annotated C++ sources.
You can enable/disable plugins, I strongly recommend to enable this very convenient plugging, it is much more helpful compare to my previously used tool. It recognizes notes: TODO, BUG, FIXME, NOTE, HACK by default.
Very simply to use. Here is one of my projects TODO list just started.
Click image to zoom in
Qt 5.9 Beta has been Released. You can read more about the new features in Tuukka Turunen’s recent post on Qt Blog.