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CHAPTER 1 INTRODUCING C++/CLI
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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 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. 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; void PrintGreeting(String^ name) { Console::WriteLine(greeting + name + punctuator); } };
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CHAPTER 1 INTRODUCING C++/CLI
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As you can see, the code uses value struct in place of value class. Throughout this text, whenever I use the term class, I mean class or structure. As in classic C++, the difference between a structure and a class is that structure members are public by default, and class members are private by default. As you know, the main function, also known as the entry point, may take additional arguments that are passed in by the operating system: the number of arguments (traditionally called argc) and an array of the arguments as character arrays (traditionally called argv). This information is also available to C++/CLI programmers, but instead of using the traditional arguments, you use a managed array type. In this case, the array parameter is an array of handles to String, each string representing one of the supplied arguments. The managed array type is one of the many fundamental types defined by the CLR that has special language support in C++/CLI. These CLR analogs of C++ types provide bounds checking, but also are objects in and of themselves, and so provide features called properties (discussed in the next chapter), such as the Length property used in Listing 1-6, and useful methods. The old int parameter of classic C++ s main function, argc, isn t necessary since the Length property can be used to get the count of command-line arguments. With this array of arguments, you can supply a person s name on the command line and print a greeting customized to that person, as demonstrated in Listing 1-6. Listing 1-6. Using Command-Line Arguments // greeting.cpp using namespace System; value struct Greeting { String^ greeting; Char punctuator; void PrintGreeting(String^ name) { Console::WriteLine(greeting + name + punctuator); } }; int main(array<String^>^ args) { Greeting greet; greet.greeting = "Hi "; greet.punctuator = '!'; if (args->Length < 1) { Console::WriteLine("Enter names on the command line, like this:" " greeting <name1> <name2> ..."); Console::WriteLine("Use quotes around names with spaces."); return 1; }
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