Example Project: Using QApplication and QLabel

Example Project Using QApplication and QLabel

Example 3.1 shows a simple main program that creates a QApplication and a QLabel, manipulates some QStrings, and then pops up a graphical window.

This example makes use of Qt's QString class, which is a dynamic string implementation that supports the Unicode standard.[1] QString contains many helpful member functions for converting and formatting strings in different ways. This example also makes use of QTextStream, a very flexible Qt class that can provide a well-developed stream interface for reading or writing text to various objects (e.g., text files, strings, and byte arrays).

[1] http://www.unicode.org/standard/standard.html

Example 3.1. src/qapp/main.cpp

#include 
#include 
#include 
#include 
#include 
#include 

int main(int argc, char * argv[]) {
 QApplication myapp(argc, argv); <-- 1
 QWidget wid; 	 <-- 2
 qDebug() << "sizeof widget: " << sizeof(wid)
 << " sizeof qapplication: " << sizeof(myapp) ;
 QString message;
 QTextStream buf(&message); <-- 3
 buf << "A QWdget is " << sizeof(wid)
 << " bytes. 
A QObject is " << sizeof(QObject)
 << " bytes. 
A QApplication is " << sizeof(myapp)
 << " bytes.";
 qDebug() << message;
 QLabel label(message); 	 <-- 4
 label.show(); 	 <-- 5
 return myapp.exec(); 	 <-- 6
};

 

(1)All Qt GUI applications need to create one of these at the start of main().

(2)We are only creating this to see how big it is.

(3)This is a stream that allows us to "write" to the string, similar in usage to std::stringstream.

(4)Create a GUI widget with the message.

(5)Make the label visible.

(6)Enter the event loop, and wait for the user to do something. When the user exits, so does myapp.exec().

To build this app, we need a project file. 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 simple, as shown in Example 3.2.

Example 3.2. src/qapp/qapp.pro

TEMPLATE = app
SOURCES += main.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 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.