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
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 or lnrrepo.ddns.net/git/lnrsoft/sqlite3_cpp_basics_updated
Use this link local_db.sqlite.zip (59 downloads) if you would like to download the local_db.sqlite file that I used in this tutorial.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 | #include #include #include 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; } } |
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. |
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 |
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 #include #include 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; } |
1 2 3 4 | $ clang++ 02_sqlite3_create_table.cpp -Wall -Wextra -lsqlite3 -std=c++17 $ ./a.out Database successfully opened Table successfully created |
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 #include #include 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; } |
1 2 3 4 | $ clang++ 03_sqlite3_insert.cpp -Wall -Wextra -lsqlite3 -std=c++17 $ ./a.out Database successfully opened Records created successfully |
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 | #include #include #include #include #include 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; } |
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 |
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 #include #include #include #include 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; } |
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 |
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 #include #include #include #include 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; } |
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 |
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
mysql_db_connection_test.pro1 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 */; |
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 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 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.
Download libmysqlclient.18.dylib
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