Bug 718938 - Add RAII helper to mfbt. r=waldo,luke,cjones

mfbt/Scoped.h

@@ -0,0 +1,224 @@
+/* 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_

mfbt/exported_headers.mk

@@ -52,6 +52,7 @@ EXPORTS_mozilla += \
   MSStdInt.h \
   RangedPtr.h \
   RefPtr.h \
+  Scoped.h \
   StandardInteger.h \
   Types.h \
   Util.h \