ASP.NET PDF Viewer using C#, VB/NET

This chapter starts by taking an ordinary C++ class and integrating it with Qt to make it more reusable and easier to use. In the process, you have a look at the build system used to compile and link Qt applications as well as installing and setting up Qt on your platform. The chapter then discusses how Qt can enable you to build components that can be interconnected in very flexible ways. This is what makes Qt such a powerful tool it makes it easy to build components that can be reused, exchanged, and interconnected. Finally, you learn about the collection and helper classes offered by Qt.

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// Frobnicator v2.41 has a bug where it crashes occasionally if // we try to set the target to "Norfolk". Setting it to an empty // string first seems to work around the problem. Frobnicator.SetTarget(""); Frobnicator.SetTarget("Norfolk");

This is now less mysterious. Someone coming across this code knows why the apparently redundant line was added. It s clear what problem it solves and the conditions under which that problem occurs, which makes it possible to find out whether the problem has been fixed in the most recent version of the offending component, making it possible to remove the fix. This makes it much easier to maintain code in the long run. As far as C# is concerned, this example is identical to the one without comments. The // character sequence tells it to ignore any further text up to the end of the line. So you can either put comments on their own line as shown earlier, or tack them onto the end of an existing line:

Like most of the C-family languages, C# supports two forms of comment syntax. As well as the single-line // form, you can write a comment that spans multiple lines, denoting the start with /* and the end with */, for example:

/* This is part of a comment. This continues to be part of the same comment. Here endeth the comment. */

Before you can start developing Qt applications, you need to download and set up Qt. You will use the open source edition of Qt because it is freely available for all. If you have a commercial license for Qt, you have received installations instructions with it. The installation procedure differs slightly depending on the platform that you are planning to use for development. Because Mac OS X and Linux are both based on Unix, the installation process is identical for the two (and all Unix platforms). Windows, on the other hand, is different and is covered separately. You can start all three platforms by downloading the edition suitable for your platform from www.trolltech.com/products/qt/downloads.

With the advent of the personal computer, much of the old server functionality wasn t necessary anymore. This was because the main use of servers at that time was for functions that could easily take place on a personal computer, such as calculations or analyses (see Figure 1-3). Functionality that required connectivity, such as email and network newsreaders, could still be achieved on a personal computer through terminal emulation. Then someone had the bright idea of using the power of the personal computer to enhance the online experience, taking it away from the green-and-black terminal and allowing features such as email, news, and graphics to appear in full four-color glory (see Figure 1-4). The personal computer flourished in power, and the early personal computers gave way to much more powerful machines with better graphics, faster processing chips, more memory, and persistent storage through hard drives.

new Dictionary<long, double>(); public double this[long index] { get { double result; nonEmptyValues.TryGetValue(index, out result); return result; } set { nonEmptyValues[index] = value; } }

}

Notice that this example doesn t bother to check the return value from TryGetValue. That s because when it fails to find the entry, it sets the result to the default value, and in the case of a double, that means 0. And 0 is what we want to return for an entry whose value has not been set yet. The following code uses the SparseArray class:

All platforms except Windows can be said to be Unix platforms. However, Mac OS X differs from the rest because it does not use the X Window System, more commonly known as X11,

SparseArray big = new SparseArray(); big[0] = 123; big[10000000000] = 456; Console.WriteLine(big[0]); Console.WriteLine(big[2]); Console.WriteLine(big[10000000000]);

This sets the value of the first element, and also the element with an index of 10 billion this simply isn t possible with an ordinary array. And yet it works fine here, with minimal memory usage. The code prints out values for three indexes, including one that hasn t been set. Here are the results:

123 0 456

for handling graphics. So Mac OS X needs a different Qt edition; the necessary file (qt-macopensource-src-version.tar.gz) can be downloaded from Trolltech. The X11-based Unix platforms use the qt-x11-opensource-src-version.tar.gz file from Trolltech.