[ fromfile: qtfirstapp.xml id: qtfirstapp ]

Example 1.15 shows a rewrite of the first C++ example, using standard Qt dialogs instead of standard input/output and QString instead of Standard Library strings. We include this example here, even though the code contains a few things that have not been discussed yet so that you can see another way to ask for input and present output, using Qt GUI convenience functions.

#include <QtGui>

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

    // Declarations of variables
    int answer = 0;                                 

    do {
        // local variables to the loop:
        int factArg = 0;
        int fact(1);
        factArg = QInputDialog::getInt(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("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 Qt types (classes) listed next.

The code in Example 1.15 contains items discussed in the next few chapters:

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

Figure 1.2.  QInputDialog – Getting an int

The input "widget" inside the dialog box is called a spin box and is implemented as QSpinBox. It displays the current value and, if the user clicks on the up or down button located at the right end of the display space, it displays other acceptable choices. The user could also press the up or down arrow key. The implementation is flexible and can easily be customized (e.g., by specifying the minimum and maximum acceptable values). After the user enters a number and clicks OK, the dialog box is replaced with a QMessageBox that pops up with the calculated result.

Figure 1.3.  QMessageBox Question

Any application that uses Qt classes needs a project file. Recall that a project file describes the project by listing all of the files and all of the options and file locations that are needed to build the project. Because this is a simple application, the project file is also quite simple, as shown in Example 1.16.

TEMPLATE = app
include (../../common.pri)
SOURCES += fac1.cpp

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 you might have your source code distributed among several subdirectories, each having its own project file. In such a case you 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 second line includes the optional common project settings from Example 1.6. Finally, the source file is listed in SOURCES.