Why aren t we allowed to define a custom default constructor for a value type, given that we re allowed to do that for a reference type The short answer is that the specification for the relevant behavior in the .NET Framework doesn t let you. (The specification in question is called the Common Language Infrastructure [CLI], incidentally.) The slightly longer answer is: for efficiency reasons. By mandating that the default constructor for any value type always initializes everything to zero, large arrays of value types can be constructed very cheaply, just by allocating the required amount of memory and zeroing out the whole array in one step. And similarly, it simplifies the initialization of fields and variables everything can be initialized to zero.

excel 2010 free barcode font, barcode font for excel 2007 free, barcode font for excel mac, barcode mit excel erstellen kostenlos, barcode in excel 2003, download free barcode generator excel, barcode plugin excel 2007, barcode excel 2007, using barcode in excel 2007, generate barcode excel vba,

public PolarPoint3D(double distance, double angle, double altitude) : this() { Distance = distance; Angle = angle; Altitude = altitude; }

Figure 7-4. Lines drawn using different methods; from left to right: drawLine, drawPolylines, and drawLines (two lines)

You add the call just before the opening brace for the body of the constructor, and prefix it with a colon. We can also use this technique to avoid writing common initialization code multiple times. Say we wanted to provide another utility constructor that just took the polar coordinates, and initialized the altitude to zero by default. Instead of repeating all the code from the first constructor, we could just add this extra constructor to our definition for PolarPoint3D, as shown in Example 3-30.

public PolarPoint3D(double distance, double angle) : this(distance, angle, 0) { } public PolarPoint3D( double distance, double angle, double altitude) : this() { Distance = distance; Angle = angle; Altitude = altitude; }

Incidentally, this syntax for calling one constructor from another works equally well in classes, and is a great way of avoiding code duplication.

A line is drawn using the pen, so you can draw the line you need by altering the properties of the pen object. The two most commonly used properties of a QPen object are color and width, which control the color of the line drawn and the width. When drawing continuous lines using drawPolyline, it is useful to be able to control how the lines are joined together the joinStyle property can help. Figure 7-5 shows the available styles: bevel, miter, and rounded. The appropriate style is set by setting the joinStyle of your QPen object to Qt::BevelJoin, Qt::MiterJoin, or Qt::RoundJoin.

You should be careful of adding too many constructors to a class or struct. It is easy to lose track of which parameters are which, or to make arbitrary choices about which constructors you provide and which you don t. For example, let s say we wanted to add yet another constructor to PolarPoint3D that lets callers pass just the angle and altitude, initializing the distance to a default of zero, as Example 3-31 shows.

In 2 you saw how a service can be directly consumed in a client application through a script-based proxy to it. You can use the ScriptManager control to reference this using the <Services> child tag. This tag should contain one or more <atlas:ServiceReference> tags that specify the service you want to reference. This tag has two attributes: Path: This specifies the path to the service. You saw in 2 that JavaScript proxies to web services on Atlas web sites can be automatically generated by postfixing /js at the end of its URI. So, for example, the web service at test.asmx would return a JavaScript proxy that could be used to call it at test.asmx/js. When using the <atlas:ServiceReference> tag to specify the service, this would automatically be generated for you on the client side when the ScriptManager control is rendered. Here s an example: <atlas:ServiceReference Path="wstest.asmx"/> GenerateProxy: This is a Boolean value (true or false) that specifies whether the service reference should generate a proxy class or not. The default is true.

public PolarPoint3D( double altitude, double angle ) : this( 0, angle, altitude ) { }

Even before we compile, we can see that there s a problem we happen to have added the altitude parameter so that it is the first in the list, and angle stays second. In our main constructor, the altitude comes after the angle. Because they are both just doubles, there s nothing to stop you from accidentally passing the parameters the wrong way round. This is the exactly the kind of thing that surprises users of your class, and leads to hard-to-find bugs. But while inconsistent parameter ordering is bad design, it s not a showstopper. However, when we compile, things get even worse. We get another error:

   Copyright 2020.