No, it isn't. Whoever owns this object will likely try to destroy it later on, and then you'll have a double deletion. The first version isn't safe either, because almost anything you do in the "..." section will be undefined behavior. The safe thing to do would be to schedule destruction of the object for the end of the frame, or something like that. entityManager would have a container of IDs to destroy and once per frame it would go through it and destroy everything in it.

I am 100% sure that this object will get deleted only once. And  will it be fine if I don't try to access the object after "delete this"?
[source lang="cpp"]Entity::handleMessage(Message* msg){...//do all kind of stuff beforehand  if(msg->type==DIE)    entityManager.destroy(this->ID);//(delete this;)   //nothing will happen after this}[/source]

No, it isn't. Whoever owns this object will likely try to destroy it later on, and then you'll have a double deletion. The first version isn't safe either, because almost anything you do in the "..." section will be undefined behavior. The safe thing to do would be to schedule destruction of the object for the end of the frame, or something like that. entityManager would have a container of IDs to destroy and once per frame it would go through it and destroy everything in it.

I am 100% sure that this object will get deleted only once. And  will it be fine if I don't try to access the object after "delete this"
[source lang="cpp"]Entity::handleMessage(Message* msg){...//do all kind of stuff beforehand  if(msg->type==DIE)    delete this;   //nothing will happen after this}[/source]