c# data matrix barcode Figure 4-1. A handle and a tracking reference designating the same object on the managed heap in Visual C#.NET

Painting Data Matrix in Visual C#.NET Figure 4-1. A handle and a tracking reference designating the same object on the managed heap

Figure 4-1. A handle and a tracking reference designating the same object on the managed heap
Data Matrix Printer In C#.NET
Using Barcode drawer for .NET Control to generate, create Data Matrix image in Visual Studio .NET applications.
www.OnBarcode.com
Scanning DataMatrix In C#.NET
Using Barcode recognizer for VS .NET Control to read, scan read, scan image in .NET applications.
www.OnBarcode.com
CHAPTER 4 OBJECT S EMAN TICS IN C+ +/CLI
Code 128A Maker In C#
Using Barcode printer for VS .NET Control to generate, create USS Code 128 image in .NET framework applications.
www.OnBarcode.com
Barcode Generation In C#
Using Barcode generation for Visual Studio .NET Control to generate, create Barcode image in Visual Studio .NET applications.
www.OnBarcode.com
Tracking references allow more options for returning information from a method or function, in addition to using the return value. Since a function can only have one return value, this is useful. In classic C++, you might have used a pointer to a pointer to return a pointer to allocated memory. Similarly, in C++/CLI, you can use a tracking reference to a handle to return a handle to an object that is allocated within a method or function. The code in Listing 4-12 shows the use of a tracking reference to a handle. This technique allows the handle to be modified in the function and retain its new value outside the function. Listing 4-12. Using a Tracking Reference void Query(String^ search, Recordset^% records) { // Query the database, generate recordset, // and set the records handle to point to it. records = gcnew Recordset(); } The function would be called as in Listing 4-13. Listing 4-13. Calling a Function with Tracking References static Recordset^ PlantQuery(String^ search) { DBConnection connection; Recordset^ records; connection.Query( search, records ); return records; } This example is a very typical use of tracking references. Without the tracking reference, you could change the object in the function and have those changes preserved, but you would not be able to make the handle reference a different object entirely or assign it to a newly created object.
Painting EAN128 In C#
Using Barcode printer for .NET framework Control to generate, create EAN / UCC - 13 image in .NET applications.
www.OnBarcode.com
USS Code 39 Creation In C#
Using Barcode generator for .NET Control to generate, create Code 39 Extended image in .NET applications.
www.OnBarcode.com
Dereferencing Handles
1D Creator In C#
Using Barcode maker for .NET framework Control to generate, create Linear image in VS .NET applications.
www.OnBarcode.com
Painting Code-27 In Visual C#.NET
Using Barcode generator for Visual Studio .NET Control to generate, create 2 of 7 Code image in VS .NET applications.
www.OnBarcode.com
In classic C++, you dereference a pointer using the * operator. The same is true of handles in C++/CLI. Subsequent assignment to a variable without a handle will result in a copy being made. Dereferenced handles may be assigned to tracking references without a copy being made. If a copy is made, there must be a copy constructor defined. Remember that copy constructors are not defined automatically for reference types as they are for native types. Listing 4-14 shows the basic syntax. Listing 4-14. Dereferencing Handles R^ r_handle = gcnew R(); R r_auto = *r_handle; // copy ctor used R% r_ref = *r_handle; // not copied
Data Matrix 2d Barcode Recognizer In .NET
Using Barcode decoder for VS .NET Control to read, scan read, scan image in VS .NET applications.
www.OnBarcode.com
Scanning ECC200 In Java
Using Barcode scanner for Java Control to read, scan read, scan image in Java applications.
www.OnBarcode.com
CHAPTER 4 OBJECT SE MAN TICS IN C++ /CLI
Make Code 39 Extended In None
Using Barcode encoder for Software Control to generate, create Code 3/9 image in Software applications.
www.OnBarcode.com
Scan Barcode In Visual Studio .NET
Using Barcode reader for VS .NET Control to read, scan read, scan image in VS .NET applications.
www.OnBarcode.com
Copy Constructors
Barcode Scanner In None
Using Barcode recognizer for Software Control to read, scan read, scan image in Software applications.
www.OnBarcode.com
Recognize PDF417 In Java
Using Barcode recognizer for Java Control to read, scan read, scan image in Java applications.
www.OnBarcode.com
Copy constructors are a special type of constructor that takes an object of the same type as a parameter creates a new copy of it. Copy constructors take a tracking reference (%) as a parameter, as shown in Listing 4-15. Listing 4-15. Using Tracking References with Copy Constructors // passing_with_copy_ctor.cpp using namespace System; ref class R { int val; String^ str; public: property int Value { int get() { return val; } void set(int i) { val = i; } } property String^ Label { String^ get() { return str; } void set(String^ s) { str = s; } } R(int val_in, String^ label) : val(val), str(label) { } R(const R% r) { // Copy the elements of R. // Value is a value type, so it gets copied. val = r.val; // Strings in C++/CLI are immutable, so even though this does not copy // the object (it only copies a reference to the object), this is OK here. str = r.str; } }; Unlike classic C++, there is no default copy constructor generated for a managed class, so you need to define one explicitly in order to use this method of parameter passing. The parameter to a copy constructor is normally declared const, although the language does not enforce this. If you do use a const reference as a parameter, you cannot use properties in the copy constructor, since properties take nonconstant parameters. The solution is to use the fields directly in the body of the copy constructor, as in Listing 4-15.
Barcode Maker In .NET Framework
Using Barcode drawer for .NET framework Control to generate, create Barcode image in Visual Studio .NET applications.
www.OnBarcode.com
Encode PDF-417 2d Barcode In None
Using Barcode printer for Font Control to generate, create PDF417 image in Font applications.
www.OnBarcode.com
CHAPTER 4 OBJECT S EMAN TICS IN C+ +/CLI
GS1-128 Creation In Java
Using Barcode drawer for Eclipse BIRT Control to generate, create USS-128 image in Eclipse BIRT applications.
www.OnBarcode.com
UPC A Generator In Java
Using Barcode drawer for Android Control to generate, create GTIN - 12 image in Android applications.
www.OnBarcode.com
Lvalues, GC-Lvalues, Rvalues, and GC-Rvalues
Barcode Maker In Java
Using Barcode maker for Eclipse BIRT Control to generate, create Barcode image in Eclipse BIRT applications.
www.OnBarcode.com
Barcode Creator In None
Using Barcode maker for Font Control to generate, create Barcode image in Font applications.
www.OnBarcode.com
When working with tracking references, handle dereferencing, and so on, you ll find it much easier to understand how it all works if you know how the compiler understands whether a given expression may be on the left or the right of an assignment. In classic C++, expressions that are addressable may appear on the left side of an assignment. An addressable entity is one that has a memory address associated with it, into which a value may be placed. Such an entity is called an lvalue (see Listing 4-16). Other expressions, which may not be assigned to, are referred to as rvalues. They may be used on the right side of an assignment, but not the left. Listing 4-16. Using Lvalues // lvalues.cpp int main() { int i; int j = 10; i = 15; j = i; 15 = 10; }
// // // //
OK: "int j" is an lvalue. OK: "i" is an lvalue. OK: "j" is an lvalue Error: 15 is NOT an lvalue!
C++/CLI introduces the concept of a gc-lvalue, an lvalue that has a managed heap address. All gc-lvalues are lvalues, but not all lvalues are gc-lvalues. In practical terms, this means that you can always use a gc-lvalue to represent either managed or native data, but that you cannot use an ordinary lvalue to represent managed data. gc-lvalues are different from lvalues in general because the address of the value may be changed, since it could be moved around by the garbage collection process; the ordinary lvalue is incapable of tracking this. Thus, to ensure type safety, the compiler does not allow lvalues to represent addresses on the managed heap. A gc-rvalue is an entity that can be the addressee of a managed heap address these can appear on the right side of assignment expressions where the left side is a gc-lvalue. A native reference or native pointer cannot be used to refer to a gc-lvalue; instead, a handle or tracking reference must be used to represent a gc-lvalue. Because value types may live on the stack or may live inside a managed heap object, whether they are considered gc-rvalues depends on where they are stored. Listing 4-17 illustrates the various cases. Listing 4-17. GC-Lvalues and GC-Rvalues // gc_lvalues.cpp value struct V { int i; };
Copyright © OnBarcode.com . All rights reserved.