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Smart Pointers

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This chapter is from the book
In this chapter from Modern C++ Design, Andrei Alexandrescu navigates through the sometimes treacherous waters of using smart pointers, which imitate built-in pointers in syntax and semantics but perform a host of additional tasks that built-in pointers can't.
Andrei Alexandrescu is the author of Modern C++ Design: Generic Programming and Design Patterns Applied (Addison-Wesley, 2001, ISBN 0-201-70431-5).

Smart pointers have been the subject of hecatombs of code written and rivers of ink consumed by programmers and writers around the world. Perhaps the most popular, intricate, and powerful C++ idiom, smart pointers are interesting in that they combine many syntactic and semantic issues. This chapter discusses smart pointers, from their simplest aspects to their most complex ones and from the most obvious errors in implementing them to the subtlest ones—some of which also happen to be the most gruesome.

In brief, smart pointers are C++ objects that simulate simple pointers by implementing operator-> and the unary operator*. In addition to sporting pointer syntax and semantics, smart pointers often perform useful tasks—such as memory management or locking—under the covers, thus freeing the application from carefully managing the lifetime of pointed-to objects.

This chapter not only discusses smart pointers but also implements a SmartPtr class template. SmartPtr is designed around policies, and the result is a smart pointer that has the exact levels of safety, efficiency, and ease of use that you want.

After reading this chapter, you will be an expert in smart pointer issues such as the following:

  • The advantages and disadvantages of smart pointers
  • Ownership management strategies
  • Implicit conversions
  • Tests and comparisons
  • Multithreading issues

This chapter implements a generic SmartPtr class template. Each section presents one implementation issue in isolation. At the end, the implementation puts all the pieces together. In addition to understanding the design rationale of SmartPtr, you will know how to use, tweak, and extend it.

7.1 Smart Pointers 101

So what's a smart pointer? A smart pointer is a C++ class that mimics a regular pointer in syntax and some semantics, but it does more. Because smart pointers to different types of objects tend to have a lot of code in common, almost all good-quality smart pointers in existence are templated by the pointee type, as you can see in the following code:

template <class T>
class SmartPtr
{
public:
  explicit SmartPtr(T* pointee) : pointee_(pointee);
  SmartPtr& operator=(const SmartPtr& other);
  ~SmartPtr();
  T& operator*() const
  {
   ...
   return *pointee_;
  }
  T* operator->() const
  {
   ...
   return pointee_;
  }
private:
  T* pointee_;
  ...
};

SmartPtr<T> aggregates a pointer to T in its member variable pointee_. That's what most smart pointers do. In some cases, a smart pointer might aggregate some handles to data and compute the pointer on the fly.

The two operators give SmartPtr pointer-like syntax and semantics. That is, you can write

class Widget
{
public:
  void Fun();
};

SmartPtr<Widget> sp(new Widget);
sp->Fun();
(*sp).Fun();

Aside from the definition of sp, nothing reveals it as not being a pointer. This is the mantra of smart pointers: You can replace pointer definitions with smart pointer definitions without incurring major changes to your application's code. You thus get extra goodies with ease. Minimizing code changes is very appealing and vital for getting large applications to use smart pointers. As you will soon see, however, smart pointers are not a free lunch.

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