Smart Pointers

This chapter is from the book

Modern C++ Design: Generic Programming and Design Patterns Applied

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This chapter is from the book

This chapter is from the book 

Modern C++ Design: Generic Programming and Design Patterns Applied

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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 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:

template
<
  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 1 apply.

Table 1 Storage Policy Constructs

Here is the default Storage policy implementation:

template <class T>
class DefaultSPStorage
{
protected:
  typedef T* StoredType;  //the type of the pointee_ object
  typedef T* PointerType;  //type returned by operator->
  typedef T& ReferenceType; //type returned by operator*
public:
  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_; }
protected:
  void Destroy()
  { delete pointee_; }
  static StoredType Default()
  { return 0; }
private:
  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

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. 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 Ownership- Policy. 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 2 apply.

Table 2 Ownership Policy Constructs

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

template <class P>
class RefCounted
{
  unsigned int* pCount_;
protected:
  RefCounted() : pCount_(new unsigned int(1)) {}
  P Clone(const P & val)
  {
   ++*pCount_;
   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
{
public:
  static P Clone(const P& val)
  {
   val->AddRef();
   return val;
  }
  static bool Release(const P& val)
  {
   val->Release();
   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 3 applies.
Table 3 Conversion Policy Construct




Expression


Semantics





bool allowConv =
 ConversionImpl<P>::allow;


If allow is true, SmartPtr allows implicit
conversion to its underlying pointer type.





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

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 4 apply.
Table 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 Smart-Ptr'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.