О да! Про supports.

Множество задач использования Supports (QueryInterface) сводятся примерно к следующему:

IMemoryPool = interface
end;//IMemoryPool

IString = interface(SomeAncestor1)
end;//IString

IMemoryStream = interface(SomeAncestor2)
end;//IMemoryStream

DoString(aString : IString);
begin
 if Supports(aString, IMemoryPool, l_Pool) then
 ... do something with l_Pool
end;

DoStream(aStream : IMemoryStream );
begin
 if Supports(aString, IMemoryPool, l_Pool) then
 ... do something with l_Pool
end;

хотя можно написать так:

IMemoryPool = interface
end;//IMemoryPool

IString = interface(SomeAncestor1{, implements IMemoryPool })
 function AsPool : IMemoryPool;
end;//IString

IMemoryStream = interface(SomeAncestor2{, implements IMemoryPool })
 function AsPool : IMemoryPool;
end;//IMemoryStream

DoString(aString : IString);
begin
 aString.AsPool
 ... do something with AsPool
end;

DoStream(aStream : IMemoryStream );
begin
 aStream.AsPool
 ... do something with AsPool
end;

-- второе - понятное дело - эффективнее.