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__property implements Support in C++Builder

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C++Builder introduces the implements attribute for the __property keyword. The implements attribute enables you to implement interfaces efficiently, without involving multiple inheritance. In the past, classes based on TObject were allowed to implement interfaces. However, implementing interfaces in C++ was tedious compared to Delphi, primarily because of the way multiple inheritance works in C++ (see Implementing Interfaces: Delphi and C++).

The implements Attribute Is Helpful for Using DAX: The __property implements has the following advantages for ActiveX:

  • Enables a C++ class to easily use the ActiveX helpers provided by DAX (Delphi ActiveX, the new framework for ActiveX in C++Builder XE).
  • Makes it easier for C++Builder XE ActiveX projects to make the transition from ATL to DAX.

The implements attribute of the C++ __property keyword allows you to implement an interface by specifying it as an attribute or field of a class. This implementation is analagous to how the Delphi implements directive allows a class to implement an interface by delegating to a property.

In a __property statement, the implements attribute is placed last, similar to the placement of the __nodefault attribute.

Syntax of __property implements

Here is the syntax of the implements attribute of the __property keyword:

 class TMyPersist: public TInterfacedObject
 {
   IPersist* FPersist;
 public:
   __property IPersist* Persist = {read=FPersist, write=FPersist, implements};
 };

The syntax omits the initialization of the FPersist field. The class constructor might initialize FPersist to some DAX helper for IPersist.

In response to the implements attribute, the C++ compiler records the interface (IPersist, in this case) in the InterfaceTable of the RTTI for TMyPersist (as well as the offset of the FFooBar field).

Note: The __property implements attribute works only with fields, not with getters or setters.

For a full code example, see Implementing Interfaces: Delphi and C++.


See Also

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