Returning pointers from functions

Question

I got caught out recently by accidentally calling a RemoveItem function that returned a pointer that I should have taken ownership of (but didn't), instead of calling a DeleteItem method which returned void.

To avoid this kind of leak, what's the right kind of pointer to return from a function where the caller is expected to take ownership of the returned object?

Base * factory()
{
   if (condition)
     return new DerivedA;
   else
     return new DerivedB;
 }

 ...

 boost::scoped_ptr<Base> b(factory());  // no leak here
 factory();  // but this leaks, obviously

Should factory() return a shared pointer to prevent the leak?

The factory example should be familiar, but here's the kind of thing that caused me a problem:-

void DeleteItem(ItemName);  // delete named item from structure.

Item* RemoveItem(ItemName); // removes named item from the structure, and returns it.       
                           //Caller can then re-insert it elsewhere. 

RemoveItem("Fred"); // whoops! Should have called DeleteItem.
                    // Apart from the leak, everything appears OK...

An std::unique_ptr<Base> would work. A shared_ptr doesn't seem to have the semantics you require. — juanchopanza
– juanchopanza, Commented Oct 23, 2013 at 8:52
the problem is why would you call factory() without getting hold of the return value from first place? I won't totally rely on tech to avoid nonsense things. — billz
– billz, Commented Oct 23, 2013 at 8:56
Actually, std::auto_ptr<Base> might be suitable in this case. — juanchopanza
– juanchopanza, Commented Oct 23, 2013 at 9:00
@Roddy Then use std::auto_ptr. A factory function should never return a shared_ptr (Boost or standard), because this forces the client to use shared_ptr, even if he doesn't want to share. — James Kanze
– James Kanze, Commented Oct 23, 2013 at 11:26

James Kanze · Accepted Answer · 2013-10-23 11:30:08Z

1

The obvious solution is to have only one function, which returns and std::auto_ptr (since you say you don't have C++11). This will cause the object to be deleted unless the caller does something with it.

answered Oct 23, 2013 at 11:30

James Kanze

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