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CHAPTER 5 DEFINING MANAGED TYPES
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The CTS allows you to define custom value types. The following code shows an example: public value class Point { int x; int y; public: Point(int x, int y); void Move(int x, int y); }; In most cases, it is preferable to define a reference type instead of a value type. As discussed in 2, your clients have to consider various boxing pitfalls when using your value type. Furthermore, there are several restrictions for value type definitions. Value types do not support inheritance. You can neither derive a value type from another type, nor can you use a value type as a base class. Special member functions for value types (e.g., the default constructor, copy constructors, operators for copy assignment, and the destructor) are not supported either. Copy constructors, copy-assignment operators, and destructors are not supported because there are scenarios in which the compiler cannot guarantee that these functions are called. For example, when a value is boxed, neither would the copy constructor be called to create the boxed value, nor could it be guaranteed that the destructor would be called. Defining managed value types can be useful when many instances are needed in the same time frame. For value types, multiple instances can be created in one chunk by creating a managed array. When V is a value type, the expression gcnew array<V>(100) allocates the memory for 100 instances of V in one chunk. For a reference type R, however, the expression gcnew array<R^>(100) would allocate memory for 100 tracking handles only. All instances would have to be created manually. Using value types in this case can also improve the GC performance. For value type arrays, only one managed object needs to be collected, whereas if you create a reference type array with n elements and initialize it so that each element refers to a new instance of the reference type, the GC would have to collect the array as well as each instance individually. Due to the lack of a default constructor for value types, initializing value type arrays is a fast operation, because it is sufficient to set the whole memory to 0 values. If the element type had a default constructor, this constructor would have to be called for each element of the array. C++ Managed Extensions (the .NET extensions for Visual C++ 2002 and 2003) supported the definition of custom default constructors for value types. If you compile code based on C++ Managed Extensions with the compiler switch /clr:oldSyntax of Visual C++ 2005, you
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CHAPTER 5 DEFINING MANAGED TYPES
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can still define value types with custom default constructors. However, this can be dangerous, since arrays of such a value type would not be initialized correctly by the C# compiler. C# simply leaves the value type array with zero-initializations for every element, even if the element type supports a default constructor. To perform the initialization correctly, the base class System::Array comes with the Initialize method, which a C# programmer must call explicitly. In contrast to C#, C++/CLI calls this method automatically as part of the array initialization, if the element type has a default constructor.
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Like the C++ type system, the CTS supports enums as a distinct kind of type. To define a managed enum, the keyword enum class is used. enum class Color { Red = 1, Blue = 2, Yellow = 4 }; There are a few differences between managed enums and native enums that you should know. First of all, values of managed enums must be written in a type-qualified way, as shown in the following line: Color c = Color::Red; To achieve a similar type-scoped behavior for native enums, they can be defined as nested types. In this case, the nesting type is the naming scope for the enum literals. The width of a managed enum value defaults to the width of int, but it can be customized with an inheritance-like syntax. The following code defines an 8-bit-wide managed enum: enum class Color : unsigned char { /*...*/ }; To define the width of the enum, the type bool, as well as the primitive signed and unsigned integer types, can be used. In contrast to native enums, managed enums do not have a standard conversion to and from int. Neither does such a conversion exist for the primitive type that specifies the width of the enum type. Instead of standard conversions, casts must be used. When the C++/CLI compiler generates a managed enum type, it extends a class from System::Enum, which itself derives from System::ValueType. Since a managed enum type is implicitly a value type, it can be instantiated as a value on the stack. This implies boxing when an enum value is casted or converted to a tracking handle type. As an example, when an enum value is passed to Console::WriteLine, the overload WriteLine(Object^) is called. To pass the enum value as an argument, a standard conversion to Object^ is applied, which implies boxing.
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