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Property Index Declaration

The syntax for declaring an indexed property has changed from Managed Extensions for C++ to Visual C++.

The two primary shortcoming of the Managed Extensions language support of indexed properties is the inability to provide class-level subscripting; that is, all indexed properties are required to be given a name, and thus there is no way, for example, to provide a managed subscript operator that can be directly applied to a Vector or Matrix class object. A second less significant shortcoming is that it is visually difficult to distinguish a property from an indexed property – the number of parameters is the only indication. Finally, indexed properties suffer from the same problems as those of non-indexed properties – the accessors are not treated as an atomic unit, but separated into individual methods. For example:

public __gc class Vector;
public __gc class Matrix {
   float mat[,];

public: 
   __property void set_Item( int r, int c, float value);
   __property float get_Item( int r, int c );

   __property void set_Row( int r, Vector* value );
   __property Vector* get_Row( int r );
};

As you can see here, the indexers are distinguished only by the additional parameters to specify a two or single dimension index. In the new syntax, the indexers are distinguished by the bracket ([,]) following the name of the indexer and indicating the number and type of each index:

public ref class Vector {};
public ref class Matrix {
private:
   array<float, 2>^ mat;

public:
   property float Item [int,int] {
      float get( int r, int c );
      void set( int r, int c, float value );
   }

   property Vector^ Row [int] {
      Vector^ get( int r );
      void set( int r, Vector^ value );
   }
};

To indicate a class level indexer that can be applied directly to objects of the class in the new syntax, the default keyword is reused to substitute for an explicit name. For example:

public ref class Matrix {
private:
   array<float, 2>^ mat;

public:
   // ok: class level indexer now
   //
   //     Matrix mat …
   //     mat[ 0, 0 ] = 1; 
   //
   // invokes the set accessor of the default indexer …

   property float default [int,int] {
      float get( int r, int c );
      void set( int r, int c, float value );
   }

   property Vector^ Row [int] {
      Vector^ get( int r );
      void set( int r, Vector^ value );
   }
};

In the new syntax, when the default indexed property is specified, the two following names are reserved: get_Item and set_Item. This is because these are the underlying names generated for the default indexed property.

Note that there is no simple index syntax analogous to the simple property syntax.

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