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Smart Pointers

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7.9 Ordering Comparisons

The ordering comparison operators are operator<, operator<=, operator>, and operator>=. You can implement them all in terms of operator<.

Whether to allow ordering of smart pointers is an interesting question in and of itself and relates to the dual nature of pointers, which consistently confuses programmers. Pointers are two concepts in one: iterators and monikers. The iterative nature of pointers allows you to walk through an array of objects using a pointer. Pointer arithmetic, including comparisons, supports this iterative nature of pointers. At the same time, pointers are monikers—inexpensive object representatives that can travel quickly and access the objects in a snap. The dereferencing operators * and -> support the moniker concept.

The two natures of pointers can be confusing at times, especially when you need only one of them. For operating with a vector, you might use both iteration and dereferencing, whereas for walking through a linked list or for manipulating individual objects, you use only dereferencing.

Ordering comparisons for pointers is defined only when the pointers belong to the same contiguous memory. In other words, you can use ordering comparisons only for pointers that point to elements in the same array.

Defining ordering comparisons for smart pointers boils down to this question: Do smart pointers to the objects in the same array make sense? On the face of it, the answer is no. Smart pointers' main feature is to manage object ownership, and objects with separate ownership do not usually belong to the same array. Therefore, it would be dangerous to allow users to make nonsensical comparisons.

If you really need ordering comparisons, you can always use explicit access to the raw pointer. The issue here is, again, to find the safest and most expressive behavior for most situations.

The previous section concludes that an implicit conversion to a raw pointer type is optional. If SmartPtr's client chooses to allow implicit conversion, the following code compiles:

SmartPtr<Something> sp1, sp2;
if (sp1 < sp2) // Converts sp1 and sp2 to raw pointer type,
        // then performs the comparison
...

This means that if we want to disable ordering comparisons, we must be proactive, disabling them explicitly. A way of doing this is to declare them and never define them, which means that any use will trigger a link-time error.

template <class T>
class SmartPtr
{ ... };

template <class T, class U>
bool operator<(const SmartPtr<T>&, const U&); // Not defined
template <class T, class U>
bool operator<(const T&, const SmartPtr<U>&); // Not defined

However, it is wiser to define all other operators in terms of operator<, as opposed to leaving them undefined. This way, if SmartPtr's users think it's best to introduce smart pointer ordering, they need only define operator<.

// Ambiguity buster
template <class T, class U>
bool operator<(const SmartPtr<T>& lhs, const SmartPtr<U>& rhs)
{
  return lhs < GetImpl(rhs);
}
// All other operators
template <class T, class U>
bool operator>(SmartPtr<T>& lhs, const U& rhs)
{
  return rhs < lhs;
}
... similarly for the other operators ...

Note the presence, again, of an ambiguity buster. Now if some library user thinks that SmartPtr<Widget> should be ordered, the following code is the ticket:

inline bool operator<(const SmartPtr<Widget>& lhs,
  const Widget* rhs)
{
  return GetImpl(lhs) < rhs;
}

inline bool operator<(const Widget* lhs,
  const SmartPtr<Widget>& rhs)
{
  return lhs < GetImpl(rhs);
}

It's a pity that the user must define two operators instead of one, but it's so much better than defining eight.

This would conclude the issue of ordering, were it not for an interesting detail. Sometimes it is very useful to have an ordering of arbitrarily located objects, not just objects belonging to the same array. For example, you might need to store supplementary per-object information, and you need to access that information quickly. A map ordered by the address of objects is very effective for such a task.

Standard C++ helps in implementing such designs. Although pointer comparison for arbitrarily located objects is undefined, the standard guarantees that std::less yields meaningful results for any two pointers of the same type. Because the standard associative containers use std::less as the default ordering relationship, you can safely use maps that have pointers as keys.

SmartPtr should support this idiom, too; therefore, SmartPtr specializes std::less. The specialization simply forwards the call to std::less for regular pointers:

namespace std
{
  template <class T>
  struct less<SmartPtr<T> >
   : public binary_function<SmartPtr<T>, SmartPtr<T>, bool>
  {
   bool operator()(const SmartPtr<T>& lhs,
     const SmartPtr<T>& rhs) const
   {
     return less<T*>()(GetImpl(lhs), GetImpl(rhs));
   }
  };
}

In summary, SmartPtr does not define ordering operators by default. It declares—without implementing—two generic operator<s and implements all other ordering operators in terms of operator<. The user can define either specialized or generic versions of operator<.

SmartPtr specializes std::less to provide an ordering of arbitrary smart pointer objects.

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