c# data matrix barcode INTRODUCING C ++/C LI in C#

Draw Data Matrix in C# INTRODUCING C ++/C LI

CHAPTER 1 INTRODUCING C ++/C LI
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Hello, World
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Now let s look at our first program (Listing 1-1) and see how the language looks in actual code. Listing 1-1. Hello, World // hello_world1.cpp int main() { System::Console::WriteLine("Hello, World!"); } The program in Listing 1-1 illustrates the classic Hello, World application. It shows several features from classic C++ a method call with a string argument, the qualification of a method name by the class and the namespace to which it belongs (with the usual double-colon scope operator), and the main method. It shows a few features new to the .NET Framework, such as the System namespace, the Console class, and the Console class s WriteLine method. You ll notice that there is no #include directive. Instead, managed type libraries in C++/CLI are referenced from their compiled form with #using. You could also write this program as shown in Listing 1-2. Listing 1-2. Hello, World with #using Directive // hello_world2.cpp #using "mscorlib.dll" using namespace System; int main() { Console::WriteLine("Hello World!"); } The #using directive references the DLL file mscorlib.dll. The program also employs the using declaration in the classic C++ sense, which as you know is simply used to avoid having to use fully qualified names for program elements in the System namespace. The #using directive is a new C++/CLI concept used to reference the types contained in a DLL. This is very different from #include, which references types declared before compilation. The first example you saw works because the compiler automatically inserts #using "mscorlib.dll". This is convenient since nearly all CLI programs require the types that it defines. The DLL is a CLI assembly, which contains not just executable code, but also metadata that exposes information about the types and program elements in the assembly. No header file is needed. Listing 1-3 illustrates a few more features of the language. Listing 1-3. More C++/CLI Features // hello_world3.cpp using namespace System;
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C HA PTER 1 INTRODUC IN G C++ /CLI
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ref class Hello { String^ greeting; public: Hello(String^ str) : greeting(str) { } void Greet() { Console::WriteLine(greeting + "!"); } void SetGreeting(String^ newGreeting) { greeting = newGreeting; } }; int main() { Hello^ hello = gcnew Hello("Hi there!"); hello->SetGreeting("Hello World"); hello->Greet(); hello->SetGreeting("Howdy"); hello->Greet(); } This code creates a reference class, as indicated by the ref keyword. It s called Hello, with a constructor, a method called Greet, and another method called SetGreeting. The SetGreeting method takes a System::String parameter. The caret indicates that the parameter type is handle to String. The String class is the CLI version of a (Unicode) character string. Unlike a native string, the String object is invariant, which means it cannot be changed without creating a brand new string. In 5, you ll see how to create a string that can be manipulated and changed.
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Note Actually, ref is not a keyword in exactly the same sense as a C++ keyword. For one thing, it is
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sensitive to the context in which it is used. Unlike keywords, context-sensitive keywords introduced in C++/CLI can be used as variable names without causing program errors. Also, keywords like ref class are considered whitespace keywords, which obey certain special rules. See the appendix for information about contextsensitive keywords and whitespace keywords.
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Also notice the Greet method uses a new C++/CLI method of concatenating strings using the + operator. Also, the constructor and the SetGreeting method take a String, but the code
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CHAPTER 1 INTRODUCING C ++/C LI
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passes a string literal. The compiler creates a String object from the string literal passed in. You ll learn the details of how this works in 5, but for now just notice that you can use string literals in a natural way with the String type, without concerning yourself with the subtleties of whether it s a narrow or wide character string literal. Just as in classic C++, the main method does not need to explicitly return a value, even though its return value is properly int, not void. This is because the compiler inserts return 0; automatically. In the main method in Listing 1-3, you saw a very important pattern that is used throughout all C++/CLI code. The Hello class is a reference type, lives on the managed heap, is created with gcnew instead of new, and is referred to using a handle, a named object that refers to the unnamed object on the managed heap. The indirection operator is used, just as if the handle were a pointer to the object. Notice that there is no call to any form of delete either. I ve demonstrated a simple reference type, but you may be wondering whether the Hello class could also be a value type. Indeed, it can be, because it has no explicit inheritance relationship with any other class (although, because it is a managed type, it implicitly inherits from Object); it has no special initialization that would require you to define a special default constructor; it has no other special member functions; and it contains no data. Listing 1-4 shows how the code would look with Hello as a value type. Listing 1-4. Using a Value Type // hello_world4.cpp using namespace System; value class Hello { // This code is unchanged. }; int main() { Hello hello; hello.Greet("Hello World"); } In the second version, hello is created as a local stack variable in the main function, rather than on the managed heap, which might result in some performance gain, although with only one object, this hardly matters. Also, a real value type would probably have member variables, perhaps as in Listing 1-5. Listing 1-5. A Value Type with Members value struct Greeting { String^ greeting; Char punctuator;
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