You can use mp.dynamicCast<MessagePart>() ;-)

Is it possible that messageParts() ever returns an empty list? Also, you can use messageParts().last().staticCast<MessagePart>() as well ;-)

This operation does not look very const to me

no massageParts can't be empty because it is filled in the if block above, and the else block has a continue, so only if something was added it gets to this line.

well - but it is const in terms of the object itself. But yes the contrain why I made it const is gone in meanwhile - so it is better to mak it as const.

delete newline