Can i pass auto_ptr by reference to functions?

Question

is the following function OK:

void DoSomething(auto_ptr< … >& a)....

Klansman

Answer 1

You can do it but I'm not sure why you would want to do it.

If you're using the auto_ptr to indicate ownership of the ptr (as people generally do), then you only need to pass the auto_ptr to a function if you want to transfer ownership of the ptr to the function, in which case you would pass the auto_ptr by value:

void DoSomething(auto_ptr<int> a)

So any code calling DoSomething relinquishes ownership of the ptr:

auto_ptr<int> p (new int (7));
DoSomething (p);
// p is now empty.

Otherwise just pass the ptr by value:

void DoSomething(int* a)
{...}

...

auto_ptr<int> p (new int (7));
DoSomething (p.get ());
// p still holds the ptr.

or pass a ref to the pointed to object:

void DoSomething(int& a)
{...}

...

auto_ptr<int> p (new int (7));
DoSomething (*p);
// p still holds the ptr.

The second is usually preferable as it makes it more explicit that DoSomething is unlikely to attempt to delete the object.

Muddler

passing the pointer by value is not safe - memory leaks could occur. creating the object without 'new' means that the object will be on the stack - not sure it is the best approach either. – Klansman

@eriks: Memory leaks can occur in many situations; even if you pass an auto_ptr by reference, someone could call delete ptr.get() which would be worse. Passing pointers by value passes no information about transfer of ownership or the correct form of deletion so - in the absence of documentation to the contrary - I wouldn't expect the callee to delete the object pointed to. – Inerney

@eriks: You won't get memory leak, you'll just get dangling pointers. – Larisa

@eriks: I've expanded my examples to hopefully clarify. – Muddler