Deleting a shared pointer (C++)

Question

I have a pointer to a QScriptEngine that I'm passing through the overloaded class constructor of class Evaluator and assigns it to QScriptEngine *engine_ (class Property subclasses Evaluator, and calls this constructor of Evaluator, passing it an already allocated QScriptEngine). The constructor with no arguments creates the new QScriptEngine pointer (class Generic subclasses Evaluator in this way). In the destructor I test if engine_ is not NULL, delete the pointer, then assign it NULL. Should the pointer (engine_) in the derived Property now also be NULL? Something tells me this is not the case. If not, how do you deal with this situation? I need the QScriptEngine to be the same instance throughout. QScriptEngine's = operator is private, or I would be avoiding the pointer all together.

I saw some info on shared pointers (boost::shared_ptr and std:tr1::shared_ptr) in another SO question. I'm already using boost for the regex library, so boost is not out of the question if that's the best way to deal with this. Hopefully there's a non-boost way, for general C++ knowledge and future projects.

Answer 1

You can solve this by giving one of the classes (class A) lifetime control of that pointer, along with a 'getter' method. The other class (class B) would always call A's getter whenever it needed the pointer. That way, A remains in control of the pointer at all times. Downside is the getter function (it will probably inline, but it's still a second indirection). Also, B is going to have to check that pointer for NULL on pretty much every use.

Your other choice is to wrap the pointer in something like boost::shared_ptr which takes care of the problem (if used properly) by holding the underlying pointer, and only deleting it when all objects that share that pointer are deleted. You could write this yourself, but since you already have boost in play, I'd just use their implementation.

A third choice is to re-factor the whole thing so that you don't need a shared pointer. I'd personally never design a C++ program that needed shared pointers, just because it's a spot where memory management bugs could easily creep in over the years, but that's just me.