When we define more than one member in a type with the same name (be it a constructor or, as we ll see later, a method) we call this overloading. Initially, we created two constructors (two overloads of the constructor) for Polar Point3D, and they compiled just fine. This is because they took different sets of parameters. One took three doubles, the other two. In fact, there was also the third, hidden constructor that took no parameters at all. All three constructors took different numbers of parameters, meaning there s no ambiguity about which constructor we want when we initialize a new PolarPoint3D. The constructor in Example 3-31 seems different: the two doubles have different names. Unfortunately, this doesn t matter to the C# compiler it only looks at the types of the parameters, and the order in which they are declared. It does not use names for

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Figure 7-5. Line segments can be joined in three ways: bevel, miter, and rounded. The QPen can be set up to draw dotted and dashed lines as well as completely freely dashed lines. The different variations of this are shown in Figure 7-6.

disambiguation. This should hardly be surprising, because we re not required to provide argument names when we call methods or constructors. If we add the overload in Example 3-31, it s not clear what new PolarPoint3D(0, 0) would mean, and that s why we get an error we ve got two members with the same name (PolarPoint3D the constructor), and exactly the same parameter types, in the same order. Looking at overloaded functions will emphasize that it really is only the method name and the parameters that matter a function s return type is not considered to be a disambiguating aspect of the member for overload purposes. That means there s nothing we can do about it: we re going to have to get rid of this third constructor (just delete it); and while we re in the code, we ll finish up the declaration of the data portion of our Plane by adding a property for its position, shown in Example 3-32.

public PolarPoint3D Position { get; set; }

Just as with constructors, we can provide more than one method with the same name, but a different list of parameter types. It is, in general, a bad idea to provide two overloads with the same name if they perform a semantically different operation (again that s the kind of thing that surprises developers using your class), so the most common reason for overloading is to provide several different ways to do something. We can provide users of our code with flexible methods that take lots of arguments to control different aspects of the code, and we can also provide developers that don t need this flexibility with simpler options by providing overloads that don t need as many arguments. Suppose we added a method to our Plane class enabling messages to be sent to aircraft. Perhaps in our first attempt we define a method whose signature looks like this:

Figure 7-6. Lines can be drawn solid or dashed in different patterns there are predefined patterns as well as capabilities to do custom patterns. The pattern is picked by setting the style property of the QPen object to Qt::SolidLine, Qt::DotLine, Qt::DashLine, Qt::DotDashLine, Qt::DotDotDashLine, or Qt::CustomDashLine. If you use a custom line, you must also set a custom dash pattern through the dashPattern property (Listing 7-4 shows how it s done). The output from the listing is shown in Figure 7-7. The dashPattern consists of a vector list of qreal values. The values determine the width of the dashes and gaps, where the first value is the first dash, then a gap, then a dash, then another gap, and so on.

public void SendMessage(string messageText)

When running a page that contains this within a ScriptManager control, you will get the following code on the client: <script type="text/xml-script"> <page xmlns:script="http://schemas.microsoft.com/xml-script/2005"> <references> <add src="wstest.asmx/js" /> </references> <components /> </page> </script> As you can see, a <reference> tag has been added to the Atlas Script, and this references the JavaScript proxy for the wstest.asmx web service. You ll see some examples on how to use web services and their proxies in 7.

But suppose that as the project progresses, we find that it would be useful to be able to delay transmission of certain messages. We could modify the SendMessage method so that it accepts an extra argument. There s a handy type in the framework called TimeSpan which lets us specify duration. We could modify the method to make use of it:

public void SendMessage(string messageText, TimeSpan delay)

Listing 7-4. Drawing lines using predefined or custom patterns QPixmap pixmap( 200, 100 ); pixmap.fill( Qt::white ); QPainter painter( &pixmap ); QPen pen( Qt::black ); pen.setStyle( Qt::SolidLine ); painter.setPen( pen ); painter.drawLine( QPoint( 10, 10 ), QPoint( 190, 10 ) ); pen.setStyle( Qt::DashDotLine ); painter.setPen( pen ); painter.drawLine( QPoint( 10, 50 ), QPoint( 190, 50 ) ); pen.setDashPattern( QVector<qreal>() << 1 << 1 << 1 << 2 << 2 << 4 << 8 << 8 << 8 pen.setStyle( Qt::CustomDashLine ); painter.setPen( pen ); painter.drawLine( QPoint( 10, 90 ), QPoint( 190, 90 << 1 << 2 << 2 << 4 << 4 << 4 << 8 );

Alas! If we already had code in our project depending on the original signature, we d start to see this compiler error:

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