Extend scoped_ptr to be closer to unique_ptr. Support custom deleters, and deleting arrays.

base/memory/scoped_ptr.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Scopers help you manage ownership of a pointer, helping you easily manage the
 // a pointer within a scope, and automatically destroying the pointer at the
 // end of a scope.  There are two main classes you will use, which correspond
 // to the operators new/delete and new[]/delete[].
 //
 // Example usage (scoped_ptr):
@@ skipping 77 matching lines @@
 #define BASE_MEMORY_SCOPED_PTR_H_
 
 // This is an implementation designed to match the anticipated future TR2
 // implementation of the scoped_ptr class, and its closely-related brethren,
 // scoped_array, scoped_ptr_malloc.
 
 #include <assert.h>
 #include <stddef.h>
 #include <stdlib.h>
 
+#include <alogrithm>  // For std::swap().
+
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/move.h"
 #include "base/template_util.h"
 
 namespace base {
 
 namespace subtle {
 class RefCountedBase;
 class RefCountedThreadSafeBase;
 }  // namespace subtle
 
+// Function object which deletes its parameter, which must be a pointer.
+// If C is an array type, invokes 'delete[]' on the parameter; otherwise,
+// invokes 'delete'. The default deleter for scoped_ptr<T>.
+template <class C>
+struct DefaultDeleter {

[gromer, 2012/10/18 20:45:20]

I named this in camel-case because it's intended to be an implementation detail,
so there was no need to be consistent with std::default_deleter; I figured
nobody's likely to need to specialize the default deleter before we deprecate
scoped_ptr.

However, as noted in my other comment I gather Chromium's scoped_ptr is here for
the long haul. That being the case, you might consider either moving this to
namespace internal (which come to think of it is what I should've done from the
beginning) or naming it default_deleter, depending on whether you want to expose
it as a customization point.

+  DefaultDeleter() {}
+  template <typename U> DefaultDeleter(const DefaultDeleter<U>& other) {
+    // All default single-object deleters can trivially convert to one another.
+  }
+  inline void operator()(C* ptr) const {
+    enum { type_must_be_complete = sizeof(C) };
+    delete ptr;
+  }
+};
+
+// Specialization of DefaultDeleter for array types.
+template <class C>
+struct DefaultDeleter<C[]> {
+  inline void operator()(C* ptr) const {
+    enum { type_must_be_complete = sizeof(C) };
+    delete[] ptr;
+  }
+
+ private:
+  // Disable this operator for any U != C because it is unsafe to execute
+  // an array delete when the static type of the array mismatches the dynamic
+  // type.
+  template <typename U> void operator()(U* array) const;
+};
+
+// Function object which invokes 'free' on its parameter, which must be
+// a pointer. Can be used to store malloc-allocated pointers in scoped_ptr:
+//
+// scoped_ptr<int, base::FreeDeleter> foo_ptr(
+//     static_cast<int>(malloc(sizeof(int))));
+struct FreeDeleter {
+  inline void operator()(void* ptr) const {
+    free(ptr);
+  }
+};
+
 namespace internal {
 
 template <typename T> struct IsNotRefCounted {
   enum {
     value = !base::is_convertible<T*, base::subtle::RefCountedBase*>::value &&
         !base::is_convertible<T*, base::subtle::RefCountedThreadSafeBase*>::
             value
   };
 };
 
+// Minimal implementation of the core logic of scoped_ptr, suitable for
+// reuse in both scoped_ptr and its specialization.
+template <class Element, class Deleter>
+class scoped_ptr_impl {
+  MOVE_ONLY_TYPE_FOR_CPP_03(scoped_ptr_impl, RValue)
+
+ public:
+  explicit scoped_ptr_impl(Element* p) : data_(p) { }
+
+  template <typename U, typename V>
+  scoped_ptr_impl(scoped_ptr_impl<U, V> other) : data_(NULL) {
+    // We do not support move-only deleters.  We could modify our move
+    // emulation to have a base::subtle::move() function that is an imperfect
+    // emulation of C++11 std::move().  But until there's a requirement, this
+    // is simpler.
+    reset(other.release());
+    get_deleter() = other.get_deleter();
+  }
+
+  template <typename U, typename V>
+  const scoped_ptr_impl& operator=(scoped_ptr_impl<U, V> rhs) {
+    // See comment in move type-coverting constructor above regarding lack of
+    // support for move-only deleters.
+    reset(rhs.release());
+    get_deleter() = rhs.get_deleter();
+    return *this;
+  }
+
+  scoped_ptr_impl(RValue rvalue) : data_(NULL) {
+    swap(*rvalue.object);
+  }
+
+  ~scoped_ptr_impl() {
+    if (data_.ptr != NULL) {
+      get_deleter()(data_.ptr);
+    }
+  }
+
+  void reset(Element* p) {
+    // This self-reset check is deprecated.
+    // this->reset(this->get()) currently works, but it is DEPRECATED, and
+    // will be removed once we verify that no one depends on it.
+    //
+    // TODO(ajwong): File bug for the deprecation and ordering issue below.
+    if (p != data_.ptr) {
+      if (data_.ptr != NULL) {
+        // Note that this can lead to undefined behavior and memory leaks
+        // in the unlikely but possible case that get_deleter()(get())
+        // indirectly deletes this. The fix is to reset ptr_ before deleting
+        // its old value, but first we need to clean up the code that relies
+        // on the current sequencing.
+        get_deleter()(data_.ptr);
+      }
+      data_.ptr = p;
+    }
+  }
+
+  Element* get() const { return data_.ptr; }
+
+  Deleter& get_deleter() { return data_; }
+  const Deleter& get_deleter() const { return data_; }
+
+  void swap(scoped_ptr_impl& p2) {
+    // Standard swap idiom: 'using std::swap' ensures that std::swap is
+    // present in the overload set, but we call swap unqualified so that
+    // any more-specific overloads can be used, if available.
+    using std::swap;
+    swap(static_cast<Deleter&>(data_), static_cast<Deleter&>(p2.data_));
+    swap(data_.ptr, p2.data_.ptr);
+  }
+
+  Element* release() {
+    Element* retVal = data_.ptr;
+    data_.ptr = NULL;
+    return retVal;
+  }
+
+ private:
+  // Needed to allow type-converting constructor.
+  template <typename U, typename V> friend class scoped_ptr_impl;
+
+  // Use the empty base class optimization to allow us to have a Deleter
+  // member, while avoiding any space overhead for it when Deleter is an
+  // empty class.  See e.g. http://www.cantrip.org/emptyopt.html for a good
+  // discussion of this technique.
+  struct Data : public Deleter {
+    explicit Data(Element* ptr_in) : ptr(ptr_in) {}
+    Element* ptr;
+  };
+
+  Data data_;
+};
+
 }  // namespace internal
+
 }  // namespace base
 
 // A scoped_ptr<T> is like a T*, except that the destructor of scoped_ptr<T>
 // automatically deletes the pointer it holds (if any).
 // That is, scoped_ptr<T> owns the T object that it points to.
 // Like a T*, a scoped_ptr<T> may hold either NULL or a pointer to a T object.
 // Also like T*, scoped_ptr<T> is thread-compatible, and once you
 // dereference it, you get the thread safety guarantees of T.
 //
 // The size of a scoped_ptr is small:
 // sizeof(scoped_ptr<C>) == sizeof(C*)
-template <class C>
+template <class Element, class Deleter = base::DefaultDeleter<Element> >
 class scoped_ptr {
   MOVE_ONLY_TYPE_FOR_CPP_03(scoped_ptr, RValue)
 
-  COMPILE_ASSERT(base::internal::IsNotRefCounted<C>::value,
-                 C_is_refcounted_type_and_needs_scoped_refptr);
+  COMPILE_ASSERT(base::internal::IsNotRefCounted<Element>::value,
+                 Element_is_refcounted_type_and_needs_scoped_refptr);
 
  public:
-
-  // The element type
-  typedef C element_type;
+  // The element and deleter types.
+  typedef Element element_type;
+  typedef Deleter deleter_type;
 
   // Constructor.  Defaults to initializing with NULL.
-  // There is no way to create an uninitialized scoped_ptr.
-  // The input parameter must be allocated with new.
-  explicit scoped_ptr(C* p = NULL) : ptr_(p) { }
+  scoped_ptr() : impl_(NULL) { }
+
+  // Constructor.  Takes ownership of p.
+  explicit scoped_ptr(element_type* p) : impl_(p) { }
 
   // Constructor.  Allows construction from a scoped_ptr rvalue for a
-  // convertible type.
-  template <typename U>
-  scoped_ptr(scoped_ptr<U> other) : ptr_(other.release()) { }
+  // convertible type and deleter.
+  template <typename U, typename V>
+  scoped_ptr(scoped_ptr<U, V> other) : impl_(other.impl_.Pass()) { }
 
   // Constructor.  Move constructor for C++03 move emulation of this type.
-  scoped_ptr(RValue rvalue)
-      : ptr_(rvalue.object->release()) {
-  }
-
-  // Destructor.  If there is a C object, delete it.
-  // We don't need to test ptr_ == NULL because C++ does that for us.
-  ~scoped_ptr() {
-    enum { type_must_be_complete = sizeof(C) };
-    delete ptr_;
-  }
+  scoped_ptr(RValue rvalue) : impl_(rvalue.object->release()) { }
 
   // operator=.  Allows assignment from a scoped_ptr rvalue for a convertible
-  // type.
-  template <typename U>
-  scoped_ptr& operator=(scoped_ptr<U> rhs) {
-    reset(rhs.release());
+  // type and deleter.
+  template <typename U, typename V>
+  scoped_ptr& operator=(scoped_ptr<U, V> rhs) {
+    impl_ = rhs.impl_.Pass();
     return *this;
   }
 
   // operator=.  Move operator= for C++03 move emulation of this type.
   scoped_ptr& operator=(RValue rhs) {
     swap(*rhs->object);
     return *this;
   }
 
   // Reset.  Deletes the current owned object, if any.
+  void reset() { impl_.reset(NULL); }
+
+  // Reset.  Deletes the currently owned object, if any.
   // Then takes ownership of a new object, if given.
-  // this->reset(this->get()) works.
-  void reset(C* p = NULL) {
-    if (p != ptr_) {
-      enum { type_must_be_complete = sizeof(C) };
-      delete ptr_;
-      ptr_ = p;
-    }
-  }
+  void reset(element_type* p) { impl_.reset(p); }
 
   // Accessors to get the owned object.
   // operator* and operator-> will assert() if there is no current object.
-  C& operator*() const {
-    assert(ptr_ != NULL);
-    return *ptr_;
+  element_type& operator*() const {
+    assert(impl_.get() != NULL);
+    return *impl_.get();
   }
-  C* operator->() const  {
-    assert(ptr_ != NULL);
-    return ptr_;
+  element_type* operator->() const  {
+    assert(impl_.get() != NULL);
+    return impl_.get();
   }
-  C* get() const { return ptr_; }
+  element_type* get() const { return impl_.get(); }
 
-  // Allow scoped_ptr<C> to be used in boolean expressions, but not
+  // Access to the deleter.
+  deleter_type& get_deleter() { return impl_.get_deleter(); }
+  const deleter_type& get_deleter() const { return impl_.get_deleter(); }
+
+  // Allow scoped_ptr<element_type> to be used in boolean expressions, but not
   // implicitly convertible to a real bool (which is dangerous).
-  typedef C* scoped_ptr::*Testable;
-  operator Testable() const { return ptr_ ? &scoped_ptr::ptr_ : NULL; }
+ private:
+  typedef base::internal::scoped_ptr_impl<element_type, deleter_type>
+      scoped_ptr::*Testable;
+
+ public:
+  operator Testable() const { return impl_.get() ? &scoped_ptr::impl_ : NULL; }
 
   // Comparison operators.
   // These return whether two scoped_ptr refer to the same object, not just to
   // two different but equal objects.
-  bool operator==(C* p) const { return ptr_ == p; }
-  bool operator!=(C* p) const { return ptr_ != p; }
+  bool operator==(element_type* p) const { return impl_.get() == p; }
+  bool operator!=(element_type* p) const { return impl_.get() != p; }
 
   // Swap two scoped pointers.
   void swap(scoped_ptr& p2) {
-    C* tmp = ptr_;
-    ptr_ = p2.ptr_;
-    p2.ptr_ = tmp;
+    impl_.swap(p2.impl_);
   }
 
   // Release a pointer.
   // The return value is the current pointer held by this object.
   // If this object holds a NULL pointer, the return value is NULL.
   // After this operation, this object will hold a NULL pointer,
   // and will not own the object any more.
-  C* release() WARN_UNUSED_RESULT {
-    C* retVal = ptr_;
-    ptr_ = NULL;
-    return retVal;
+  element_type* release() WARN_UNUSED_RESULT {
+    return impl_.release();
   }
 
+  // C++98 doesn't support functions templates with default parameters which
+  // makes it hard to write a PassAs() that understands converting the deleter
+  // while preserving simple calling semantics.
+  //
+  // Since there isn't a use case yet for PassAs() with custom deleters, we
+  // just ignore the custom deleter for now.
   template <typename PassAsType>
   scoped_ptr<PassAsType> PassAs() {
-    return scoped_ptr<PassAsType>(release());
+    return scoped_ptr<PassAsType>(Pass());
   }
 
  private:
-  C* ptr_;
+  // Needed to reach into |impl_| in the constructor.
+  template <typename U, typename V> friend class scoped_ptr;
+  base::internal::scoped_ptr_impl<element_type, deleter_type> impl_;
 
-  // Forbid comparison of scoped_ptr types.  If C2 != C, it totally doesn't
-  // make sense, and if C2 == C, it still doesn't make sense because you should
-  // never have the same object owned by two different scoped_ptrs.
-  template <class C2> bool operator==(scoped_ptr<C2> const& p2) const;
-  template <class C2> bool operator!=(scoped_ptr<C2> const& p2) const;
+  // Forbid comparison of scoped_ptr types.  If U != Element, it totally
+  // doesn't make sense, and if U == Element, it still doesn't make sense
+  // because you should never have the same object owned by two different
+  // scoped_ptrs.
+  template <class U> bool operator==(scoped_ptr<U> const& p2) const;
+  template <class U> bool operator!=(scoped_ptr<U> const& p2) const;
+};
 
+template <class Element, class Deleter>
+class scoped_ptr<Element[], Deleter> {
+  MOVE_ONLY_TYPE_FOR_CPP_03(scoped_ptr, RValue)
+
+  COMPILE_ASSERT(base::internal::IsNotRefCounted<Element>::value,
+                 Element_is_refcounted_type_and_needs_scoped_refptr);
+
+ public:
+  // The element and deleter types.
+  typedef Element element_type;
+  typedef Deleter deleter_type;
+
+  // Constructor.  Defaults to initializing with NULL.
+  // There is no way to create an uninitialized scoped_ptr.
+  scoped_ptr() : impl_(NULL) { }
+
+  // Constructor. Stores the given array. Note that the argument's type
+  // must exactly match Element*. In particular:
+  // - it cannot be a pointer to a type derived from Element, because it is
+  //   inherently unsafe to access an array through a pointer whose
+  //   dynamic type does not match its static type. If you're doing this,
+  //   fix your code.
+  // - it cannot be NULL, because NULL is an integral expression, not a
+  //   pointer to Element. Use the no-argument version instead of explicitly
+  //   passing NULL.
+  // - it cannot be const-qualified differently from Element. You can work
+  //   around this using implicit_cast (from base/casts.h):
+  //
+  //   int* i;
+  //   scoped_ptr<const int[]> arr(implicit_cast<const int[]>(i));
+  //
+  // TODO(ajwong): Find citations for the above.  Also see if we want to keep
+  // the implicit_cast<> comment.
+  explicit scoped_ptr(element_type* array) : impl_(array) { }
+
+  // Constructor.  Move constructor for C++03 move emulation of this type.
+  scoped_ptr(RValue rvalue) : impl_(rvalue.object->release()) { }
+
+  // operator=.  Move operator= for C++03 move emulation of this type.
+  scoped_ptr& operator=(RValue rhs) {
+    swap(*rhs->object);
+    return *this;
+  }
+
+  // Reset.  Deletes the currently owned array, if any.
+  void reset() { impl_.reset(NULL); }
+
+  // Reset.  Deletes the currently owned array, if any.
+  // Then takes ownership of a new object, if given.
+  void reset(element_type* array) { impl_.reset(array); }
+
+  // Accessors to get the owned array.
+  // operator* and operator-> will assert() if there is no current array.
+  element_type& operator[](size_t i) const {
+    assert(impl_.get() != NULL);
+    return impl_.get()[i];
+  }
+  element_type* get() const { return impl_.get(); }
+
+  // Access to the deleter.
+  deleter_type& get_deleter() { return impl_.get_deleter(); }
+  const deleter_type& get_deleter() const { return impl_.get_deleter(); }
+
+  // Allow scoped_ptr<element_type> to be used in boolean expressions, but not
+  // implicitly convertible to a real bool (which is dangerous).
+ private:
+  typedef base::internal::scoped_ptr_impl<element_type, deleter_type>
+      scoped_ptr::*Testable;
+
+ public:
+  operator Testable() const { return impl_.get() ? &scoped_ptr::impl_ : NULL; }
+
+  // Comparison operators.
+  // These return whether two scoped_ptr refer to the same object, not just to
+  // two different but equal objects.
+  bool operator==(element_type* array) const { return impl_.get() == array; }
+  bool operator!=(element_type* array) const { return impl_.get() != array; }
+
+  // Swap two scoped pointers.
+  void swap(scoped_ptr& p2) {
+    impl_.swap(p2.impl_);
+  }
+
+  // Release a pointer.
+  // The return value is the current pointer held by this object.
+  // If this object holds a NULL pointer, the return value is NULL.
+  // After this operation, this object will hold a NULL pointer,
+  // and will not own the object any more.
+  element_type* release() WARN_UNUSED_RESULT {
+    return impl_.release();
+  }
+
+ private:
+  // Force element_type to be a complete type.
+  enum { type_must_be_complete = sizeof(element_type) };
+
+  // Actually hold the data.
+  base::internal::scoped_ptr_impl<element_type, deleter_type> impl_;
+
+  // Disable initialization from any type other than element_type*, by
+  // providing a constructor that matches such an initialization, but is
+  // private and has no definition. This is disabled because it is not safe to
+  // call delete[] on an array whose static type does not match its dynamic
+  // type.
+  template <typename T>
+  explicit scoped_ptr(T* array);
+
+  // Disable reset() from any type other than element_type*, for the same
+  // reasons as the constructor above.
+  template <typename T>
+  void reset(T* array);
+
+  // Forbid comparison of scoped_ptr types.  If U != Element, it totally
+  // doesn't make sense, and if U == Element, it still doesn't make sense
+  // because you should never have the same object owned by two different
+  // scoped_ptrs.
+  template <class U> bool operator==(scoped_ptr<U> const& p2) const;
+  template <class U> bool operator!=(scoped_ptr<U> const& p2) const;
 };
 
 // Free functions
-template <class C>
-void swap(scoped_ptr<C>& p1, scoped_ptr<C>& p2) {
+template <class C, class D>
+void swap(scoped_ptr<C, D>& p1, scoped_ptr<C, D>& p2) {
   p1.swap(p2);
 }
 
-template <class C>
-bool operator==(C* p1, const scoped_ptr<C>& p2) {
+template <class C, class D>
+bool operator==(C* p1, const scoped_ptr<C, D>& p2) {
   return p1 == p2.get();
 }
 
-template <class C>
-bool operator!=(C* p1, const scoped_ptr<C>& p2) {
+template <class C, class D>
+bool operator!=(C* p1, const scoped_ptr<C, D>& p2) {
   return p1 != p2.get();
 }
 
 // scoped_array<C> is like scoped_ptr<C>, except that the caller must allocate
 // with new [] and the destructor deletes objects with delete [].
 //
 // As with scoped_ptr<C>, a scoped_array<C> either points to an object
 // or is NULL.  A scoped_array<C> owns the object that it points to.
 // scoped_array<T> is thread-compatible, and once you index into it,
 // the returned objects have only the thread safety guarantees of T.
@@ skipping 244 matching lines @@
 
 // A function to convert T* into scoped_ptr<T>
 // Doing e.g. make_scoped_ptr(new FooBarBaz<type>(arg)) is a shorter notation
 // for scoped_ptr<FooBarBaz<type> >(new FooBarBaz<type>(arg))
 template <typename T>
 scoped_ptr<T> make_scoped_ptr(T* ptr) {
   return scoped_ptr<T>(ptr);
 }
 
 #endif  // BASE_MEMORY_SCOPED_PTR_H_