How to define instantiation functions

An object definition's instantiation function does not have the usual new(ELeave)..PushL()...ConstructL()...Pop() structure. Instead, it delegates construction to a static function REComSession::CreateImplementationL().

There are a number of overloads of the CreateImplementationL() function that offer different parameter options:

• a UID that uniquely identifies the interface is always passed. Every implementation uses the same UID in its registration resource file to declares which interface it implements.

• the instance identifier key can be passed by reference, or by passing the offset to the data member (calculated using the _FOFF macro).

• optionally, resolution data, usually ultimately supplied by the client, can be passed, which a resolver will use to select the implemenation to use. This data must be wrapped in a TEComResolverParams object.

• optionally, a UID can be passed to specify a particular resolver to use, if the default resolver is not appropriate.

• optionally, a data block can be given for passing to the instantiated object.

An interface definition can have more than one such instantiation function, offering different options to the client.

Example

The following example declares a NewL() function that takes a string to use for resolution from the client, wraps this in a TEComResolverParams, and calls REComSession::CreateImplementationL() to get an instance.

Typically, the NewL() function is static, and means that you need to pass the offset to the data member that is to contain the instance identifier key. This offset value is passed as the second parameter in the call to REComSession::CreateImplementationL(), which then puts the instance identifier key value into that position in the returned object (which is cast as a CExampleInterfaceDefinition type).

inline CExampleInterfaceDefinition* CExampleInterfaceDefinition::NewL(const TDesC& aMatchString)
    {
    const TUid KExampleInterfaceDefinitionUid = {0x10009DC0};

    TEComResolverParams resolverParams; 
    resolverParams.SetDataType(aMatchString);
    TAny* ptr = REComSession::CreateImplementationL(
       KExampleInterfaceDefinitionUid,
        _FOFF(CExampleInterfaceDefinition,iDtor_ID_Key),
        resolverParams);

     return REINTERPRET_CAST(CExampleInterfaceDefinition*, ptr);
     }

Note

CreateImplementationL() returns a TAny* (i.e. a void*), so this must be cast to the actual pointer type.