Question

What is the best approach to encapsulate objects and manage their lifetime? Example: I have a class A, that contains an object of type B and is solely responsible for it.

Solution 1, clone b object to ensure that only A is able to clean it up.

class A {     B *b; public:     A(B &b)     {         this->b = b.clone();     }      ~A()     {         delete b; // safe     } }; 

Solution 2, directly use the passed object, we risk a potential double free here.

class A {     B *b; public:     A(B *b)     {         this->b = b;     }      ~A()     {         delete b; // unsafe     } }; 

In my actual case, solution #2 would fit best. However I wonder if this is considered bad code because someone might not know about the behavior of A, even if it’s documented. I can think of these scenarios:

B *myB = new B(); A *myA = new A(myB); delete myB; // myA contains a wild pointer now 

Or,

B *myB = new B(); A *firstA = new A(myB); A *secondA = new A(myB); // bug! double assignment delete firstA; // deletes myB, secondA contains a wild pointer now delete secondA; // deletes myB again, double free 

Can I just ignore these issues if I properly document the behavior of A? Is it enough to declare the responsibility and leave it up to the others to read the docs? How is this managed in your codebase?

Answer 1

You should define your object so that the ownership semantics are, as much as possible, defined by the interface. As David Thornley pointed out, std::auto_ptr is the smart pointer of choice to indicate transfer of ownership. Define your class like so:

class A {         std::auto_ptr<B> b; public:         A(std::auto_ptr<B> b)         {         this->b = b;     }     // Don't need to define this for this scenario     //~A()     //{      //   delete b; // safe     //} }; 

Since the contract of std::auto_ptr is that assignment = transfer of ownership, your constructor now states implicitly that an A object has ownership of the pointer to B it’s passed. In fact, if a client tries to do something with a std::auto_ptr<B> that they used to construct an A after the construction, the operation will fail, as the pointer they hold will be invalid.