[ fromfile: qtfirstapp.xml id: qtfirstapp ]

Example 1.12 shows a re-write of our first C++ example, using standard Qt dialogs instead of standard input/output and QString instead of Standard Library strings.

#include <QtGui>

int main (int argc, char* argv[]) { 
    QApplication app(argc, argv); 
    QTextStream cout(stdout, QIODevice::WriteOnly);     

    // Declarations of variables
    int answer = 0; 

    do {
        // local variables to the loop:
        int factArg = 0;
        int fact(1);
        factArg = QInputDialog::getInteger(0, "Factorial Calculator",
            "Factorial of:", 1); 
        cout << "User entered: " << factArg << endl;
        int i=2;
        while (i <= factArg) {
            fact = fact * i;
            ++i;
        }
        QString response = QString("The factorial of %1 is %2.\n%3")
            .arg(factArg).arg(fact)  
            .arg("Do you want to compute another factorial?");    
        answer = QMessageBox::question(0, "Play again?", response,
            QMessageBox::Yes | QMessageBox::No); 
    } while (answer == QMessageBox::Yes);
    return EXIT_SUCCESS;
}

This program makes use of the following classes listed below.

When we run this application, we will first see an input dialog like Figure 1.2.

Figure 1.2.  Factorial Input Dialog

After the user enters a number and clicks OK, we will see a QMessageBox with the calculated result.

As with any app that uses Qt classes, we need a project file. Recall that a project file describes the project by listing all of the other files, as well as all of the options and file locations that are needed to build the project. Because this is a very simple application, the project file is also quite simple, as shown in Example 1.13.

TEMPLATE = app
SOURCES += fac1.cpp
win {
        CONFIG += console
}

The first line, TEMPLATE = app, indicates that qmake should start with a templated Makefile suited for building applications. If this project file were for a library, you would see TEMPLATE = lib to indicate that a Makefile library template should be used instead. A third possibility is that we might have our source code distributed among several subdirectories, each having its own project file. In such a case we might see TEMPLATE = subdirs in the project file located in the parent directory, which would cause a Makefile to be produced in the parent directory and also in each subdirectory.

The last line, CONFIG += console, only necessary on Windows platforms, tells the compiler to build a “console” application, which can interact with the user via standard input/output streams. If you are using MS Dev studio, this is equivalent to selecting Project Properties - Configuration Properties - Linker - System - Subsystem - Console Without this option, you will not see messages to qDebug(), stdout, or stderr.

Qt provides classes to replace almost every class in the C++ standard library. We saw examples of this in the last few sections. Generally, we prefer to use Qt classes instead of the corresponding C++ Standard Library classes. Sometimes, however, it sometimes makes sense to (carefully!) use classes from both libraries in the same program; for example, in case we need to reuse library code that uses iostream or standard library classes in a Qt application.