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Class Implementing Two Interfaces with Duplicate Method Names

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Delphi only supports single inheritance. A Delphi class can only descend from a single parent class, but a Delphi class can implement multiple interfaces.

The TAthlete descends from the THuman parent class (which presumably descends from TInterfacedObject) and it implements both the IWalker and IJumper interfaces. What if both IWalker and IJumper contain a run method.

Data Model

Right now TAthlete doesn’t implement the members of IWalker or IJumper.

  • [dcc32 Error] E2291 Missing implementation of interface method IJumper.run
  • [dcc32 Error] E2291 Missing implementation of interface method IWalker.run

When we implement these interfaces in TAthlete, what if we want to have a different run method for IWalker vs IJumper? Enter the Method Resolution Clause.

Interface Method Resolution Clause

When a class implements two or more interfaces that have identically named methods, use method resolution clauses to resolve the naming conflicts. You can override the default name-based mappings by including method resolution clauses in a class declaration. We might implement those interfaces like this:

But what happens if I call Run on a class reference to an TAthlete object? It doesn’t exist. There is no Run method on TAthlete, and both PowerWalk and RealRun are private, so they aren’t accessible via a class reference either.

If we wanted to call Run on TAthlete we could do that with a little change.

Now IJumper uses the default name-based mapping, which IWalker uses the manually mapped method

It seems like it would usually be a good idea to be explicit in all the methods implemented by interfaces when you have a conflict like this, but there could be a reason to be less explicit in certain use cases. It is great that Delphi gives you the flexibility to implement this either way necessary.

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Chief Developer Advocate for Embarcadero Technologies.

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