Memory Management

Introduction

The Zorba processor allows you to customize various functionality via its C++ API. As expected, this is done by providing C++ classes that you derive from and provide your own implementations of virtual functions. You then provide Zorba with instances of your derived classes.However, there are two problems: one general and one that's Windows-specific.

The General Problem

The general problem is instance ownership, i.e., after you provide Zorba with an instance of one of your derived classes, should it be destroyed by Zorba?For example, suppose the API for the full-text Stemmer were this:

// NOTE: THIS IS NOT THE ACTUAL API -- IT'S A "WHAT IF" API

class Stemmer {
public:
  virtual ~Stemmer();
  // ...
};

class StemmerProvider {
public:
  // ...
  Stemmer* getStemmer( locale::iso639_1::type lang ) const = 0;
};

and that you implemented it like this:

class MyStemmerProvider : public StemmerProvider {
public:
  Stemmer* getStemmer( locale::iso639_1::type lang ) const;
};

Stemmer* MyStemmerProvider::getStemmer( locale::iso639_1::type lang ) const {
  MyStemmer stemmer;
  return &stemmer;
}

Assume that your stemmer is used for all languages and that it maintains no state. For efficiency, you could therefore use a singleton instance of it. The problem is that Zorba can't tell the difference between a non-singleton (dynamically allocated) instance and a singleton (statically allocated) instance, i.e., whether to call delete on it or not. (In C++, there is no built-in way to determine whether a pointer points to an object that is statically allocated or was dynamically allocated.)

The Windows-Specific Problem

A further complication is that, on the Windows platform, there is an issue with dynamically allocating instances in your executable and deallocating them in a library (or vice versa). This has to do with the Windows C Runtime Library that's used by the executable and library: if they don't match and you dynamically allocate an instance in your executable and it's deallocated in the library (or vice versa), your program will crash.The only way to avoid this problem is to ensure that all instances that are dynamically allocated in your executable are also deallocated in your executable; and the same goes for the library.

The Solution to Both Problems

A solution to both problems is to replace what would have been a call to delete with a call to a virtual member function destroy(). The API for the full-text Stemmer would now be (and actually is):

class Stemmer {
public:
  typedef /* implementation-defined */ ptr;
  virtual void destroy() const = 0;
  // ...
protected:
  virtual ~Stemmer();
};

class StemmerProvider {
public:
  // ...
  Stemmer::ptr getStemmer( locale::iso639_1::type lang ) const = 0;
};

The changes are:

• A new ptr type that is to be used in the place of a native C++ pointer. It uses the relatively new C++ class and specifies a custom deleter that calls destroy() rather than delete. (If your C++ implementation doesn't have unique_ptr, Zorba provides its own.)
• A new pure virtual destroy() member function.
• The destructor has been made protected to prevent destruction by calling delete from outside the class.

If you now implemented your stemmer like this:

class MyStemmer {
public:
  void destroy();
  // ...
private:
  MyStemmer();
  friend class MyStemmerProvider; // only it can create instances
};

void MyStemmer::destroy() const {
  // Do nothing since our stemmer is statically allocated.
}

class MyStemmerProvider : public StemmerProvider {
public:
  Stemmer::ptr getStemmer( locale::iso639_1::type lang ) const;
};

Stemmer::ptr MyStemmerProvider::getStemmer( locale::iso639_1::type lang ) const {
  static MyStemmer stemmer;
  return Stemmer::ptr( &stemmer );
}

then it would work for a statically allocated instance of your stemmer. On the other hand, if your StemmerProvider dynamically allocates instances, then your implementation should be like this:

void MyStemmer::destroy() const {
  delete this;
}

Stemmer::ptr MyStemmerProvider::getStemmer( locale::iso639_1::type lang ) const {
  return Stemmer::ptr( new MyStemmer );
}