Athena Developer Guide

Athena
Chapter 18 Framework packages, interfaces and libraries Version/Issue: 2.0.0
• Derived from IInterface. We follow the convention that all interfaces should be derived
from a basic interface IInterface. This interface defined 3 methods: addRef(), release()
and queryInterface(). This methods allow the framework to manage the reference counting of
the framework components and the possibility to obtain a different interface of a component
using any interface (see later).
• Pure Abstract Methods. All the methods should be pure abstract (virtual ReturnType
method(...) = 0;) With the exception of the static method interfaceID() (see
later) and some inline templated methods to facilitate the use of the interface by the end-user.
• Interface ID. Each interface should have a unique identification (see later) used by the query
interface mechanism.
18.3.1 Interface ID
We needed to introduce an interface ID for identifying interfaces for the queryInterface functionality.
The interface ID is made of a numerical identifier that needs to be allocated off-line, and a major and
minor version numbers. The version number is used to decide if the interface the service provider is
returning is compatible with the interface the client is expecting. The rules for deciding if the interface
request is compatible are:
• The interface identifier is the same
• The major version is the same
• The minor version of the client is less than or equal to the one of the service provider. This
allows the service provider to add functionality (incrementing minor version number) keeping
old clients still compatible.
The interface ID is defined in the same header file as the rest of the interface. Care should be taken of
globally allocating the interface identifier, and of modifying the version whenever a change of the
interface is required, according to the rules. Of course changes to interfaces should be minimized.
static const InterfaceID IID_Ixxx(2 /*id*/, 1 /*major*/, 0 /*minor*/);
class Ixxx : public IInterface {
. . .
static const InterfaceID& interfaceID() { return IID_Ixxx; }
};
The static method Ixxx::interfaceID() is useful for the implementation of templated methods
and classes using an interface as template parameter. The construct T::interfaceID() returns the
interface ID of interface T.
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