Item 37. AUTO_PTR

Difficulty: 8

This Item covers basics about how you can use the standard auto_ptr safely and effectively.

Historical note: The original simpler form of this Item, appearing as a Special Edition of Guru of the Week, was first published in honor of the voting out of the Final Draft International Standard for Programming Language C++. It was known/suspected that auto_ptr would change one last time at the final meeting, where the standard was to be voted complete (Morristown, New Jersey, November 1997), so this problem was posted the day before the meeting began. The solution, freshly updated to reflect the prior day's changes to the standard, became the first published treatment of the standard auto_ptr.

Many thanks from all of us to Bill Gibbons, Greg Colvin, Steve Rumsby, and others who worked hard on the final refinement of auto_ptr. Greg, in particular, has labored over auto_ptr and related smart pointer classes for many years to satisfy various committee concerns and requirements, and deserves public recognition for that work.

This problem, now with a considerably more comprehensive and refined solution, illustrates the reasons for the eleventh-hour changes that were made, and it shows how you can make the best possible use of auto_ptr.

Comment on the following code: What's good, what's safe, what's legal, and what's not?

auto_ptr<T> source() 
{
  return auto_ptr<T>( new T(1) );
}
void sink( auto_ptr<T> pt ) { }
void f()
{
  auto_ptr<T> a( source() );
  sink( source() );
  sink( auto_ptr<T>( new T(1) ) );
  vector< auto_ptr<T> > v;
  v.push_back( auto_ptr<T>( new T(3) ) );
  v.push_back( auto_ptr<T>( new T(4) ) );
  v.push_back( auto_ptr<T>( new T(1) ) );
  v.push_back( a );
  v.push_back( auto_ptr<T>( new T(2) ) );
  sort( v.begin(), v.end() );
  cout << a->Value();
}
class C
{
public:    /*...*/
protected: /*...*/
private:   /*...*/
  auto_ptr<CImpl> pimpl_;
};