ASP.NET Web PDF Document Viewer/Editor Control Library

People often refer to reference types as being allocated on the heap and value types on the stack. C++ programmers will be familiar with these concepts, and C++ provided one syntax in the language to explicitly create items on the stack (a cheap form of storage local to a particular scope), and a different syntax for working on the heap (a slightly more expensive but sophisticated form of storage that could persist beyond the current scope). C# doesn t make that distinction in its syntax, because the .NET Framework itself makes no such distinction. These aspects of memory management are completely opaque to the developer, and it is actively wrong to think of value types as being always allocated on a stack. For people familiar with C++ this can take a while to get used to, especially as the myth is perpetuated on the Web, in the MSDN documentation and elsewhere. (For example, at the time of this writing, http://msdn.microsoft.com/library/aa288471 states that structs are created on the stack, and while that happens to be true of the ones in that example when running against the current version of .NET, it would have been helpful if the page had mentioned that it s not always true. For example, if a class has a field of value type, that field doesn t live on the stack it lives inside the object, and in all the versions of .NET released so far, objects live on the heap.)

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The important difference for the C# developer between these two kinds of types is the one of reference versus copy semantics.

As well as understanding the difference in behavior, you also need to be aware of some constraints. To be useful, a value type should be: Immutable Lightweight Something is immutable if it doesn t change over time. So, the integer 3 is immutable. It doesn t have any internal workings that can change its three-ness . You can replace the value of an int variable that currently contains a 3, by copying a 4 into it, but you can t change a 3 itself. (Unlike, say, a particular Plane object, which has a Direction property that you can change anytime you like without needing to replace the whole Plane.)

QPolygon class also provides methods for moving all the points around at once, as well as calculating

There s nothing in C# that stops you from creating a mutable value type. It is just a bad idea (in general). If your type is mutable, it is probably safer to make it a reference type, by declaring it as a class. Mutable value types cause problems because of the copy semantics if you modify a value, it s all too easy to end up modifying the wrong one, because there may be many copies.

It should be fairly apparent that a value type also needs to be pretty lightweight, because of all that copying going on Every time you pass it into a function, or assign it to a variable, a copy is made And copies are generally the enemy of good performance If your value type consists of more than two or three of the built-in types, it may be getting too big These constraints mean it is very rare that you will actually want to declare a value type yourself A lot of the obviously useful ones you might want are already defined in the NET Framework class libraries (things like 2D points, times, and dates) Custom value types are so rare that it was hard to come up with a useful example for this book that wasn t already provided in the class libraries.

(If you were wondering why our example application represents aircraft positions in such an idiosyncratic fashion, this is the reason) But that doesn t mean you should never, ever declare a value type Value types can have performance benefits when used in arrays (although as with most performance issues, this is not entirely clear-cut), and the immutability and copy semantics can make them safer when passing them in to functions you won t normally introduce side effects by working with a value type because you end up using a copy, rather than modifying shared data that other code might be relying on Our polar 3D point seems to comply with the requirements Any given point is just that: a specific point in 3D space a good candidate for immutability.

To draw actual lines, the drawLine, drawPolyline, and drawLines methods are called. Compare the differences between drawPolyline and drawLines. As you can see, drawPolyline joins all points, while drawLines joins each pair of points given. Listing 7-3. Drawing lines using drawLine, drawPolyline, and drawLines QPixmap pixmap( 200, 100 ); pixmap.fill( Qt::white ); QPainter painter( &pixmap ); painter.setPen( Qt::black ); QPolygon polyPoints; polyPoints << QPoint( << QPoint( << QPoint( << QPoint(