How to do it…

Let's learn how to convert various data types in Qt by following these steps:

1. Open Qt Creator and create a new Qt Console Application project by going to File| New File or Project:

2. Open main.cpp and add the following headers to it:

#include <QCoreApplication>
#include <QDebug>
#include <QtMath>
#include <QDateTime>
#include <QTextCodec>
#include <iostream>

3. In the main() function, add the following code to convert a string into a number:

int numberA = 2;
QString numberB = "5";
qDebug() << "1) " << "2 + 5 =" << numberA + numberB.toInt();

4. Convert a number back into a string:

float numberC = 10.25;
float numberD = 2;
QString result = QString::number(numberC * numberD);
qDebug() << "2) " << "10.25 * 2 =" << result;

5. Let's see how to round down a value using qFloor():

float numberE = 10.3;
float numberF = qFloor(numberE);
qDebug() << "3) " << "Floor of 10.3 is" << numberF;

6. Using qCeil(), we are able to round a number to the smallest integral value not smaller than its initial value:

float numberG = 10.3;
float numberH = qCeil(numberG);
qDebug() << "4) " << "Ceil of 10.3 is" << numberH;

7. Create a date-time variable by converting from a piece of date-time data written in string format:

QString dateTimeAString = "2016-05-04 12:24:00";
QDateTime dateTimeA = QDateTime::fromString(dateTimeAString, "yyyy-MM-dd hh:mm:ss");
qDebug() << "5) " << dateTimeA;

8. Convert the date-time variable back into a string with our custom format:

QDateTime dateTimeB = QDateTime::currentDateTime();
QString dateTimeBString = dateTimeB.toString("dd/MM/yy hh:mm");
qDebug() << "6) " << dateTimeBString;

9. Call the QString::toUpper() function to convert a string variable into all capital letters:

QString hello1 = "hello world!";
qDebug() << "7) " << hello1.toUpper();

10. Calling QString::toLower() will convert the string completely into lowercase:

QString hello2 = "HELLO WORLD!";
qDebug() << "8) " << hello2.toLower();

11. The QVariant class provided by Qt is a very powerful data type that can be easily converted to other types without any effort from the programmer:

QVariant aNumber = QVariant(3.14159);
double aResult = 12.5 * aNumber.toDouble();
qDebug() << "9) 12.5 * 3.14159 =" << aResult;

12. This demonstrates how a single QVariant variable can be simultaneously converted to multiple data types without any effort from the programmer:

qDebug() << "10) ";
QVariant myData = QVariant(10);
qDebug() << myData;
myData = myData.toFloat() / 2.135;
qDebug() << myData;
myData = true;
qDebug() << myData;
myData = QDateTime::currentDateTime();
qDebug() << myData;
myData = "Good bye!";
qDebug() << myData;

13. The full source code in main.cpp will now look like this:

#include <QCoreApplication>
#include <QDebug>
#include <QtMath>
#include <QDateTime>
#include <QTextCodec>
#include <iostream>

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

14. After that, let's add in the code to convert a string into a number, and vice versa:

    // String to number
    int numberA = 2;
    QString numberB = "5";
    qDebug() << "1) " << "2 + 5 =" << numberA + numberB.toInt();

    // Number to string
    float numberC = 10.25;
    float numberD = 2;
    QString result = QString::number(numberC * numberD);
    qDebug() << "2) " << "10.25 * 2 =" << result;

15. Write the code to convert floating-point numbers to their nearest succeeding or preceding integer number, respectively:

    // Floor
    float numberE = 10.3;
    float numberF = qFloor(numberE);
    qDebug() << "3) " << "Floor of 10.3 is" << numberF;

    // Ceil
    float numberG = 10.3;
    float numberH = qCeil(numberG);
    qDebug() << "4) " << "Ceil of 10.3 is" << numberH;

16. Convert a string into date-time format, and vice versa:

    // Date time from string
    QString dateTimeAString = "2016-05-04 12:24:00";
    QDateTime dateTimeA = QDateTime::fromString(dateTimeAString, "yyyy-MM-dd hh:mm:ss");
    qDebug() << "5) " << dateTimeA;

    // Date time to string
    QDateTime dateTimeB = QDateTime::currentDateTime();
    QString dateTimeBString = dateTimeB.toString("dd/MM/yy hh:mm");
    qDebug() << "6) " << dateTimeBString;

17. Continue to add code to convert strings into uppercase or lowercase characters:

    // String to all uppercase
    QString hello1 = "hello world!";
    qDebug() << "7) " << hello1.toUpper();

    // String to all lowercase
    QString hello2 = "HELLO WORLD!";
    qDebug() << "8) " << hello2.toLower();

18. Convert the QVariant data type into other types:

    // QVariant to double
    QVariant aNumber = QVariant(3.14159);
    double aResult = 12.5 * aNumber.toDouble();
    qDebug() << "9) 12.5 * 3.14159 =" << aResult;

    // QVariant different types
    qDebug() << "10) ";
    QVariant myData = QVariant(10);
    qDebug() << myData;
    myData = myData.toFloat() / 2.135;
    qDebug() << myData;
    myData = true;
    qDebug() << myData;

19. Convert the QVariant data type into QDateTime and QString:

    myData = QDateTime::currentDateTime();
    qDebug() << myData;
    myData = "Good bye!";
    qDebug() << myData;

    return a.exec();
}

20. Compile and run the project and you should see something like this: