pjs/mfbt/Scoped.h

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this file,
 * You can obtain one at http://mozilla.org/MPL/2.0/. */

/* A number of structures to simplify scope-based RAII management. */

#ifndef mozilla_Scoped_h_
#define mozilla_Scoped_h_

/*
 * Resource Acquisition Is Initialization is a programming idiom used
 * to write robust code that is able to deallocate resources properly,
 * even in presence of execution errors or exceptions that need to be
 * propagated.  The Scoped* classes defined in this header perform the
 * deallocation of the resource they hold once program execution
 * reaches the end of the scope for which they have been defined.
 *
 * This header provides the following RAII classes:
 *
 * - |ScopedFreePtr| - a container for a pointer, that automatically calls
 *   |free()| at the end of the scope;
 * - |ScopedDeletePtr| - a container for a pointer, that automatically calls
 *   |delete| at the end of the scope;
 * - |ScopedDeleteArray| - a container for a pointer to an array, that
 *   automatically calls |delete[]| at the end of the scope.
 *
 * The general scenario for each of the RAII classes is the following:
 *
 * ScopedClass foo(create_value());
 * // ... In this scope, |foo| is defined. Use |foo.get()| or |foo.rwget()|
 *        to access the value.
 * // ... In case of |return| or |throw|, |foo| is deallocated automatically.
 * // ... If |foo| needs to be returned or stored, use |foo.forget()|
 *
 * Note that the RAII classes defined in this header do _not_ perform any form
 * of reference-counting or garbage-collection. These classes have exactly two
 * behaviors:
 * - if |forget()| has not been called, the resource is always deallocated at
 *   the end of the scope;
 * - if |forget()| has been called, any control on the resource is unbound
 *   and the resource is not deallocated by the class.
 *
 *
 * Extension:
 *
 * In addition, this header provides class |Scoped| and macro |SCOPED_TEMPLATE|
 * to simplify the definition of RAII classes for other scenarios. These macros
 * have been used to automatically close file descriptors/file handles when
 * reaching the end of the scope, graphics contexts, etc.
 */

#include "mozilla/Attributes.h"
#include "mozilla/GuardObjects.h"

/*
 * Scoped is a helper to create RAII wrappers
 * Type argument |Traits| is expected to have the following structure:
 *
 *   struct Traits {
 *     // Define the type of the value stored in the wrapper
 *     typedef value_type type;
 *     // Returns the value corresponding to the uninitialized or freed state
 *     const static type empty();
 *     // Release resources corresponding to the wrapped value
 *     // This function is responsible for not releasing an |empty| value
 *     const static void release(type);
 *   }
 */
template <typename Traits>
class Scoped
{
public:
  typedef typename Traits::type Resource;

  explicit Scoped(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM)
    : value(Traits::empty())
  {
    MOZ_GUARD_OBJECT_NOTIFIER_INIT;
  }
  explicit Scoped(const Resource& value
                  MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
    : value(value)
  {
    MOZ_GUARD_OBJECT_NOTIFIER_INIT;
  }
  ~Scoped() {
    Traits::release(value);
  }

  // Constant getter
  operator const Resource&() const { return value; }
  const Resource& operator->() const { return value; }
  const Resource& get() const { return value; }
  // Non-constant getter.
  Resource& rwget() { return value; }

  /*
   * Forget the resource.
   *
   * Once |forget| has been called, the |Scoped| is neutralized, i.e. it will
   * have no effect at destruction (unless it is reset to another resource by
   * |operator=|).
   *
   * @return The original resource.
   */
  Resource forget() {
    Resource tmp = value;
    value = Traits::empty();
    return tmp;
  }

  /*
   * Perform immediate clean-up of this |Scoped|.
   *
   * If this |Scoped| is currently empty, this method has no effect.
   */
  void dispose() {
    Traits::release(value);
    value = Traits::empty();
  }

  bool operator==(const Resource& other) const {
    return value == other;
  }

  /*
   * Replace the resource with another resource.
   *
   * Calling |operator=| has the side-effect of triggering clean-up. If you do
   * not want to trigger clean-up, you should first invoke |forget|.
   *
   * @return this
   */
  Scoped<Traits>& operator=(const Resource& other) {
    return reset(other);
  }
  Scoped<Traits>& reset(const Resource& other) {
    Traits::release(value);
    value = other;
    return *this;
  }

private:
  explicit Scoped(const Scoped<Traits>& value) MOZ_DELETE;
  Scoped<Traits>& operator=(const Scoped<Traits>& value) MOZ_DELETE;

private:
  Resource value;
  MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
};

/*
 * SCOPED_TEMPLATE defines a templated class derived from Scoped
 * This allows to implement templates such as ScopedFreePtr.
 *
 * @param name The name of the class to define.
 * @param Traits A struct implementing clean-up. See the implementations
 * for more details.
 */
#define SCOPED_TEMPLATE(name, Traits)                          \
template <typename Type>                                       \
struct name : public Scoped<Traits<Type> >                     \
{                                                              \
  typedef Scoped<Traits<Type> > Super;                         \
  typedef typename Super::Resource Resource;                   \
  name& operator=(Resource ptr) {                              \
    Super::operator=(ptr);                                     \
    return *this;                                              \
  }                                                            \
  explicit name(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM)          \
    : Super(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_TO_PARENT)    \
  {}                                                           \
  explicit name(Resource ptr                                   \
                MOZ_GUARD_OBJECT_NOTIFIER_PARAM)               \
    : Super(ptr MOZ_GUARD_OBJECT_NOTIFIER_PARAM_TO_PARENT)     \
  {}                                                           \
private:                                                       \
  explicit name(name& source) MOZ_DELETE;                      \
  name& operator=(name& source) MOZ_DELETE;                    \
};

/*
 * ScopedFreePtr is a RAII wrapper for pointers that need to be free()d.
 *
 *   struct S { ... };
 *   ScopedFreePtr<S> foo = malloc(sizeof(S));
 *   ScopedFreePtr<char> bar = strdup(str);
 */
template <typename T>
struct ScopedFreePtrTraits
{
  typedef T* type;
  static T* empty() { return NULL; }
  static void release(T* ptr) { free(ptr); }
};
SCOPED_TEMPLATE(ScopedFreePtr, ScopedFreePtrTraits)

/*
 * ScopedDeletePtr is a RAII wrapper for pointers that need to be deleted.
 *
 *   struct S { ... };
 *   ScopedDeletePtr<S> foo = new S();
 */
template <typename T>
struct ScopedDeletePtrTraits : public ScopedFreePtrTraits<T> {
  static void release(T* ptr) { delete ptr; }
};
SCOPED_TEMPLATE(ScopedDeletePtr, ScopedDeletePtrTraits)

/*
 * ScopedDeleteArray is a RAII wrapper for pointers that need to be delete[]ed.
 *
 *   struct S { ... };
 *   ScopedDeleteArray<S> foo = new S[42];
 */
template <typename T>
struct ScopedDeleteArrayTraits : public ScopedFreePtrTraits<T>
{
  static void release(T* ptr) { delete [] ptr; }
};
SCOPED_TEMPLATE(ScopedDeleteArray, ScopedDeleteArrayTraits)


#endif // mozilla_Scoped_h_