QUOTE]That's not required for a Composition. It's true, though, that this is a quite strong indicator for a Composition relationship.
I disagree, here's why:
If composition implies lifecycle management by the composer (it does), then any outside object that can obtain a handle to the composed object (the Engine, in the example) can prevent garbage collection of an Engine that has been made available for garbage collection by virtue of its composer (the Car) being made available for garbage collection. In other words, by gaining access to the Engine explicitly, you've violated the contract of composition by subverting the lifecycle management defined by composition.
So, in this case, by destructing the car, you may not be able to destruct the engine because you've allowed outside access to the engine.