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Property Declaration

The way to declare a property in a managed class has changed from Managed Extensions for C++ to Visual C++.

In the Managed Extensions design, each set or get property accessor is specified as an independent method. The declaration of each method is prefixed with the __property keyword. The method name begins with either set_ or get_ followed by the actual name of the property (as visible to the user). Thus, a Vector providing an x coordinate get property would name it get_x and the user would invoke it as x. This naming convention and separate specification of methods actually reflects the underlying runtime implementation of the property. For example, here is our Vector with a set of coordinate properties:

public __gc __sealed class Vector {
public:
   __property double get_x(){ return _x; }
   __property double get_y(){ return _y; }
   __property double get_z(){ return _z; }

   __property void set_x( double newx ){ _x = newx; }
   __property void set_y( double newy ){ _y = newy; }
   __property void set_z( double newz ){ _z = newz; }
};

This spreads out the functionality associated with a property and requires the user to lexically unify the associated sets and gets. Moreover, it is verbose. In the new syntax, which is more like that of C#, the property keyword is followed by the type of the property and its unadorned name. The set and get access methods are placed within a block following the property name. Note that unlike C#, the signature of the access method is specified. For example, here is the code example above translated into the new syntax.

public ref class Vector sealed { 
public:
   property double x {
      double get() {
         return _x;
      }

      void set( double newx ) {
         _x = newx;
      }
   } // Note: no semi-colon
};

If the access methods of the property reflect distinct access levels – such as a public get and a private or protected set, an explicit access label can be specified. By default, the access level of the property reflects that of the enclosing access level. For example, in the above definition of Vector, both the get and set methods are public. To make the set method protected or private, the definition would be revised as follows:

public ref class Vector sealed { 
public:
   property double x {
      double get() {
         return _x;
      }

   private:
      void set( double newx ) {
         _x = newx;
      }

   } // note: extent of private culminates here …

// note: dot is a public method of Vector
double dot( const Vector^ wv );

// etc.
};

The scope of an access keyword within a property extends until either the closing brace of the property or the specification of an additional access keyword. It does not extend beyond the definition of the property to the enclosing access level within which the property is defined. In the above declaration, for example, Vector::dot() is a public method.

Writing the set/get properties for the three Vector coordinates involves three steps:

  1. declare a private state member of the appropriate type.

  2. return it when the user wishes to get its value.

  3. assign it the new value.

In the new syntax, a shorthand property syntax is available which automates this usage pattern:

public ref class Vector sealed { 
public:
   // equivalent shorthand property syntax
   property double x; 
   property double y;
   property double z;
};

The interesting side effect of the shorthand property syntax is that although the backstage state member is generated by the compiler, it is not accessible within the class except through the set/get accessors.

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