Aggregation explained

// ComplexCalculator.idl : IDL source for ComplexCalculator.dll
//

// This file will be processed by the MIDL tool to
// produce the type library (ComplexCalculator.tlb) and marshalling code.

import "oaidl.idl";
import "ocidl.idl";

//Aggregation: copy the object and interface module from calculator.idl
	[
		object,
		uuid(1E48B0F0-010D-4658-A0B6-70300FDA56F1),
		dual,
		helpstring("ISimpleMath Interface"),
		pointer_default(unique)
	]
	interface ISimpleMath : IDispatch
	{
		[id(1), helpstring("method Add")] HRESULT Add([in] int x, [in] int y, [out,retval] int* z );
	};
//End

	[
		object,
		uuid(F4A87CF0-EB3C-447A-8B00-5062ECCBA164),
		dual,
		helpstring("IComplexMath Interface"),
		pointer_default(unique)
	]
	interface IComplexMath : IDispatch
	{
		[id(1), helpstring("method Divide")] HRESULT Divide([in] int x, [in] int y, [out,retval] int* z );
	};

[
	uuid(B6B78EBB-87E3-4738-819B-DDF1903DF46A),
	version(1.0),
	helpstring("ComplexCalculator 1.0 Type Library")
]
library COMPLEXCALCULATORLib
{
	importlib("stdole32.tlb");
	importlib("stdole2.tlb");

	[
		uuid(D90899E4-3043-491E-B606-52FE3B39B104),
		helpstring("ComplexMath Class")
	]
	coclass ComplexMath
	{
		[default] interface IComplexMath;
		interface ISimpleMath;
	};
};