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CHAPTER 7 FEATURES OF A .NET CLASS
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int main() { ElementType oxygen; oxygen.AtomicWeight = 15.9994; ValueAccessor^ get_method = gcnew ValueAccessor(oxygen, &ElementType::AtomicWeight::get); Console::WriteLine("{0}", get_method->Invoke()); } Say we d like to also have some static properties in our Element class. In fact, we d like to make a periodic table class with a static array property. There is nothing special about a static property; all the rules for static methods and fields apply. Static properties are intended to be used for properties of a type, not properties of a particular instance. Listing 7-5 is a first attempt at this. Listing 7-5. Trying to Define a Static Property // property_static.cpp value class ElementType { public: // Periodic Table of the Elements static property array<ElementType>^ PeriodicTable; static ElementType() { PeriodicTable = gcnew array<ElementType>(120); // Initialize each element and its properties. } }; That s great, but if we later want to change the implementation from an array to a List or Hashtable, we might need to rewrite the code that uses the property. A better way to implement collection-like properties is to use vector properties, also called indexed properties.
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A special type of property is allowed in C++/CLI that enables properties to act like arrays. You can also use indexed properties to provide array indexing on objects, the equivalent of defining the array indirection operator (operator[]) for your type. To make a property support the indexing syntax, use the square brackets in the property declaration. Inside the square brackets, put the type you will use as the index. You can index on any type. Listing 7-6 shows a simple indexed property named ordinal. Note the type of the index appears inside square brackets, and the index is used as the first parameter of both the get and set methods.
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CHAPTER 7 FEATURES OF A .NET CLASS
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Listing 7-6. Using an Indexed Property // properties_indexed1.cpp using namespace System; ref class Numbers { array<String^>^ ordinals; public: Numbers() { ordinals = gcnew array<String^> { "zero", "one", "two", "three" }; } property String^ ordinal[unsigned int] { String^ get(unsigned int index) { return ordinals[index]; } void set(unsigned int index, String^ value) { ordinals[index] = value; } } }; int main() { Numbers^ nums = gcnew Numbers(); // Access the property values using the indexer // with an unsigned int as the index. Console::WriteLine( nums->ordinal[0] ); } Here is the output of Listing 7-6:
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You can also define a default indexed property by naming the property default, which enables the index to be used directly on the instance of the object (see Listing 7-7). Whether you are accessing a default indexed property using a handle or a variable declared with stack semantics, you can use the array indirection operator directly.
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CHAPTER 7 FEATURES OF A .NET CLASS
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Listing 7-7. Using a Default Property // properties_indexed2.cpp using namespace System; ref class Numbers { array<String^>^ ordinals; public: Numbers() { ordinals = gcnew array<String^> { "zero", "one", "two", "three" }; } property String^ default[unsigned int] { String^ get(unsigned int index) { return ordinals[index]; } void set(unsigned int index, String^ value) { ordinals[index] = value; } } }; int main() { Numbers nums; // Access property using array indexing operators on the // instance directly. Console::WriteLine( nums[0] ); // If using a handle, you can still use array syntax. Numbers^ nums2 = gcnew Numbers(); Console::WriteLine( nums2[1] ); // You can also use the name "default" and access like a // named property. Console::WriteLine( nums.default[2] ); Console::WriteLine( nums2->default[3] ); }
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CHAPTER 7 FEATURES OF A .NET CLASS
The output of Listing 7-7 is as follows: zero one two three Listing 7-8 shows a class with an indexed property whose backing store is a collection class. The indexed property on the class PeriodicTable invokes the default indexed property on a .NET Framework collection class, Hashtable (here accessed through the interface IDictionary). The ElementType class now overrides the ToString method on Object to allow custom output. 8 discusses the override keyword. Listing 7-8. Backing a Property with a Collection // periodic_table.cpp using namespace System; using namespace System::Collections; value class ElementType { public: property unsigned int AtomicNumber; property double AtomicWeight; property String^ Name; property String^ Symbol; // You cannot use initializer list syntax to initialize properties. ElementType(String^ name, String^ symbol, double a, double n) { AtomicNumber = n; AtomicWeight = a; Name = name; Symbol = symbol; } // Override the ToString method (you'll learn more about the override // keyword in the next chapter). virtual String^ ToString() override { return String::Format( "Element {0} Symbol {1} Atomic Number {2} Atomic Weight {3}", Name, Symbol, AtomicNumber, AtomicWeight); } };
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