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Smart Pointers in C++

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7.14 Putting It All Together

Not much to go! Here comes the fun part. So far we have treated each issue in isolation. It's now time to collect all the decisions into a unique SmartPtr implementation.

The strategy we'll use is the one described in Chapter 1: policy-based class design. Each design aspect that doesn't have a unique solution migrates to a policy. The SmartPtr class template accepts each policy as a separate template parameter. SmartPtr inherits all these template parameters, allowing the corresponding policies to store state.

Let's recap the previous sections by enumerating the variation points of SmartPtr. Each variation point translates into a policy.

  • Storage policy (Section 7.3). By default, the stored type is T* (T is the first template parameter of SmartPtr), the pointer type is again T*, and the reference type is T&. The means of destroying the pointee object is the delete operator.

  • Ownership policy (Section 7.5). Popular implementations are deep copy, reference counting, reference linking, and destructive copy. Note that Ownership is not concerned with the mechanics of destruction itself; this is Storage's task. Ownership controls the moment of destruction.

  • Conversion policy (Section 7.7). Some applications need automatic conversion to the underlying raw pointer type; others do not.

  • Checking policy (Section 7.10). This policy controls whether an initializer for SmartPtr is valid and whether a SmartPtr is valid for dereferencing.

Other issues are not worth dedicating separate policies to them or have an optimal solution:

  • The address-of operator (Section 7.6) is best not overloaded.

  • Equality and inequality tests are handled with the tricks shown in Section 7.8.

  • Ordering comparisons (Section 7.9) are left unimplemented; however, Loki specializes std::less for SmartPtr objects. The user may define an operator<, and Loki helps by defining all other ordering comparisons in terms of operator<.

  • Loki defines const-correct implementations for the SmartPtr object, the pointee object, or both.

  • There is no special support for arrays, but one of the canned Storage implementations can dispose of arrays by using operator delete[].

The presentation of the design issues surrounding smart pointers made these issues easier to understand and more manageable because each issue was discussed in isolation. It would be very helpful, then, if the implementation could decompose and treat issues in isolation instead of fighting with all the complexity at once.

Divide et Impera— this old principle coined by Julius Caesar can be of help even today with smart pointers. (I'd bet money he didn't predict that.) We break the problem into small component classes, called policies. Each policy class deals with exactly one issue. SmartPtr inherits all these classes, thus inheriting all their features. It's that simple—yet incredibly flexible, as you will soon see. Each policy is also a template parameter, which means you can mix and match existing stock policy classes or build your own.

The pointed-to type comes first, followed by each of the policies. Here is the resulting declaration of SmartPtr:

   typename T,
   template <class> class OwnershipPolicy = RefCounted,
   class ConversionPolicy = DisallowConversion,
   template <class> class CheckingPolicy = AssertCheck,
   template <class> class StoragePolicy = DefaultSPStorage
class SmartPtr;

The order in which the policies appear in SmartPtr's declaration puts the ones that you customize most often at the top.

The following four subsections discuss the requirements of the four policies we have defined. A rule for all policies is that they must have value semantics; that is, they must define a proper copy constructor and assignment operator.

7.14.1 The Storage Policy

The Storage policy abstracts the structure of the smart pointer. It provides type definitions and stores the actual pointee_ object.

If StorageImpl is an implementation of the Storage policy and storageImpl is an object of type StorageImpl<T>, then the constructs in Table 7.1 apply.

Here is the default Storage policy implementation:

template <class T>
class DefaultSPStorage
   typedef T* StoredType;    //the type of the pointee_ object
   typedef T* PointerType;   //type returned by operator->
   typedef T& ReferenceType; //type returned by operator*
   DefaultSPStorage() : pointee_(Default())
   DefaultSPStorage(const StoredType& p): pointee_(p) {}
   PointerType operator->() const { return pointee_; }
   ReferenceType operator*() const { return *pointee_; }
   friend inline PointerType GetImpl(const DefaultSPStorage& sp)
   { return sp.pointee_; }
   friend inline const StoredType& GetImplRef(
      const DefaultSPStorage& sp)
   { return sp.pointee_; }
   friend inline StoredType& GetImplRef(DefaultSPStorage& sp)
   { return sp.pointee_; }
   void Destroy()
   { delete pointee_; }
   static StoredType Default()
   { return 0; }
   StoredType pointee_;

In addition to DefaultSPStorage, Loki also defines the following:

  • ArrayStorage, which uses operator delete[] inside Release

  • LockedStorage, which uses layering to provide a smart pointer that locks data while dereferenced (see Section 7.13.1)

  • HeapStorage, which uses an explicit destructor call followed by std::free to release the data

Table 7.1. Storage Policy Constructs

Expression Semantics
StorageImpl<T>::StoredType The type actually stored by the implementation. Default: T*.
StorageImpl<T>::PointerType The pointer type defined by the implementation.

This is the type returned by SmartPtr's operator->. Default: T*. Can be different from StorageImpl<T>::StoredType when you're using smart pointer layering (see Sections 7.3 and 7.13.1).

StorageImpl<T>::ReferenceType The reference type. This is the type returned by SmartPtr's operator*. Default: T&.
GetImpl(storageImpl) Returns an object of type StorageImpl<T> ::StoredType.
GetImplRef(storageImpl) Returns an object of type StorageImpl<T> ::StoredType&, qualified with const if storageImpl is const.
storageImpl.operator->() Returns an object of type StorageImpl<T> ::PointerType. Used by SmartPtr's own operator->.
storageImpl.operator*() Returns an object of type StorageImpl<T> ::ReferenceType. Used by SmartPtr's own operator*.

StorageImpl<T>::StoredType p;

p = storageImpl.Default();

Returns the default value (usually zero).
storageImpl.Destroy() Destroys the pointee object.

7.14.2 The Ownership Policy

The Ownership policy must support intrusive as well as nonintrusive reference counting. Therefore, it uses explicit function calls rather than constructor/destructor techniques, as Koenig (1996) does. The reason is that you can call member functions at any time, whereas constructors and destructors are called automatically and only at specific times.

The Ownership policy implementation takes one template parameter, which is the corresponding pointer type. SmartPtr passes StoragePolicy<T>::PointerType to OwnershipPolicy. Note that OwnershipPolicy's template parameter is a pointer type, not an object type.

If OwnershipImpl is an implementation of Ownership and ownershipImpl is an object of type OwnershipImpl<P>, then the constructs in Table 7.2 apply.

Table 7.2. Ownership Policy Constructs

Expression Semantics
P val1; P val2 = OwnershipImplImpl. Clone(val1); Clones an object. It can modify the source value if OwnershipImpl uses destructive copy.
const P val1; P val2 = ownershipImpl. Clone(val1); Clones an object.
P val;bool unique = ownershipImpl. Release(val); Releases ownership of an object. Returns true if the last reference to the object was released.
bool dc = OwnershipImpl<P>::destructiveCopy; States whether OwnershipImpl uses destructive copy. If that's the case, SmartPtr uses the Colvin/Gibbons trick (Meyers 1999) used in std::auto_ptr.

An implementation of Ownership that supports reference counting is shown in the following:

template <class P>
class RefCounted
   unsigned int* pCount_;
   RefCounted() : pCount_(new unsigned int(1)) {}
   P Clone(const P & val)
      return val;
   bool Release(const P&)
      if (!--*pCount_)
         delete pCount_;
         return true;
      return false;
   enum { destructiveCopy = false }; // see below

Implementing a policy for other schemes of reference counting is very easy. Let's write an Ownership policy implementation for COM objects. COM objects have two functions: AddRef and Release. Upon the last Release call, the object destroys itself. You need only direct Clone to AddRef and Release to COM's Release:

template <class P>
class COMRefCounted
   static P Clone(const P& val)
      return val;
   static bool Release(const P& val)
      return false;
   enum { destructiveCopy = false }; // see below

Loki defines the following Ownership implementations:

  • DeepCopy, described in Section 7.5.1. DeepCopy assumes that pointee class implements a member function Clone.

  • RefCounted, described in Section 7.5.3 and in this section.

  • RefCountedMT, a multithreaded version of RefCounted.

  • COMRefCounted, a variant of intrusive reference counting described in this section.

  • RefLinked, described in Section 7.5.4.

  • DestructiveCopy, described in Section 7.5.5.

  • NoCopy, which does not define Clone, thus disabling any form of copying.

7.14.3 The Conversion Policy

Conversion is a simple policy: It defines a Boolean compile-time constant that says whether or not SmartPtr allows implicit conversion to the underlying pointer type.

If ConversionImpl is an implementation of Conversion, then the construct in Table 7.3 applies.

The underlying pointer type of SmartPtr is dictated by its Storage policy and is StorageImpl<T>::PointerType.

As you would expect, Loki defines precisely two Conversion implementations:

  • AllowConversion

  • DisallowConversion

Table 7.3. Conversion Policy Construct

Expression Semantics
bool allowConv = ConversionImpl<P>::allow; If allow is true, SmartPtr allows implicit conversion to its underlying pointer type.

7.14.4 The Checking Policy

As discussed in Section 7.10, there are two main places to check a SmartPtr object for consistency: during initialization and before dereference. The checks themselves might use assert, exceptions, or lazy initialization or not do anything at all.

The Checking policy operates on the StoredType of the Storage policy, not on the PointerType. (See Section 7.14.1 for the definition of Storage.)

If S is the stored type as defined by the Storage policy implementation, and if CheckingImpl is an implementation of Checking, and if checkingImpl is an object of type CheckingImpl<S>, then the constructs in Table 7.4 apply.

Table 7.4. Checking Policy Constructs

Expression Semantics
S value; checkingImpl.OnDefault(value); SmartPtr calls OnDefault in the default constructor call. If CheckingImpl does not define this function, it disables the default constructor at compile time.
S value; checkingImpl.OnInit(value); SmartPtr calls OnInit upon a constructor call.
S value; checkingImpl.OnDereference (value); SmartPtr calls OnDereference before returning from operator-> and operator*.
const S value;checkingImpl.OnDereference(value); SmartPtr calls OnDereference before returning from the const versions of operator-> and operator*.

Loki defines the following implementations of Checking:

  • AssertCheck, which uses assert for checking the value before dereferencing.

  • AssertCheckStrict, which uses assert for checking the value upon initialization.

  • RejectNullStatic, which does not define OnDefault. Consequently, any use of SmartPtr's default constructor yields a compile-time error.

  • RejectNull, which throws an exception if you try to dereference a null pointer.

  • RejectNullStrict, which does not accept null pointers as initializers (again, by throwing an exception).

  • NoCheck, which handles errors in the grand C and C++ tradition—that is, it does no checking at all.

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