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 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<T>):
//   {
//     scoped_ptr<Foo> foo(new Foo("wee"));
//   }  // foo goes out of scope, releasing the pointer with it.
//
//   {
//     scoped_ptr<Foo> foo;          // No pointer managed.
//     foo.reset(new Foo("wee"));    // Now a pointer is managed.
//     foo.reset(new Foo("wee2"));   // Foo("wee") was destroyed.
//     foo.reset(new Foo("wee3"));   // Foo("wee2") was destroyed.
//     foo->Method();                // Foo::Method() called.
//     foo.get()->Method();          // Foo::Method() called.
//     SomeFunc(foo.release());      // SomeFunc takes ownership, foo no longer
//                                   // manages a pointer.
//     foo.reset(new Foo("wee4"));   // foo manages a pointer again.
//     foo.reset();                  // Foo("wee4") destroyed, foo no longer
//                                   // manages a pointer.
//   }  // foo wasn't managing a pointer, so nothing was destroyed.
//
// Example usage (scoped_ptr<T[]>):
//   {
//     scoped_ptr<Foo[]> foo(new Foo[100]);
//     foo.get()->Method();  // Foo::Method on the 0th element.
//     foo[10].Method();     // Foo::Method on the 10th element.
//   }
//
// These scopers also implement part of the functionality of C++11 unique_ptr
// in that they are "movable but not copyable."  You can use the scopers in
// the parameter and return types of functions to signify ownership transfer
// in to and out of a function.  When calling a function that has a scoper
// as the argument type, it must be called with an rvalue of a scoper, which
// can be created by using std::move(), or the result of another function that
// generates a temporary; passing by copy will NOT work.  Here is an example
// using scoped_ptr:
//
//   void TakesOwnership(scoped_ptr<Foo> arg) {
//     // Do something with arg.
//   }
//   scoped_ptr<Foo> CreateFoo() {
//     // No need for calling std::move() for returning a move-only value, or
//     // when you already have an rvalue as we do here.
//     return scoped_ptr<Foo>(new Foo("new"));
//   }
//   scoped_ptr<Foo> PassThru(scoped_ptr<Foo> arg) {
//     return arg;
//   }
//
//   {
//     scoped_ptr<Foo> ptr(new Foo("yay"));  // ptr manages Foo("yay").
//     TakesOwnership(std::move(ptr));       // ptr no longer owns Foo("yay").
//     scoped_ptr<Foo> ptr2 = CreateFoo();   // ptr2 owns the return Foo.
//     scoped_ptr<Foo> ptr3 =                // ptr3 now owns what was in ptr2.
//         PassThru(std::move(ptr2));        // ptr2 is correspondingly nullptr.
//   }
//
// Notice that if you do not call std::move() when returning from PassThru(), or
// when invoking TakesOwnership(), the code will not compile because scopers
// are not copyable; they only implement move semantics which require calling
// the std::move() function to signify a destructive transfer of state.
// CreateFoo() is different though because we are constructing a temporary on
// the return line and thus can avoid needing to call std::move().
//
// The conversion move-constructor properly handles upcast in initialization,
// i.e. you can use a scoped_ptr<Child> to initialize a scoped_ptr<Parent>:
//
//   scoped_ptr<Foo> foo(new Foo());
//   scoped_ptr<FooParent> parent(std::move(foo));

#ifndef BASE_MEMORY_SCOPED_PTR_H_
#define BASE_MEMORY_SCOPED_PTR_H_

// This is an implementation designed to match the anticipated future TR2
// implementation of the scoped_ptr class.

#include <assert.h>
#include <stddef.h>
#include <stdlib.h>

#include <iosfwd>
#include <memory>
#include <type_traits>
#include <utility>

#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 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 specializations.
template <class T, class D>
class scoped_ptr_impl {
 public:
  explicit scoped_ptr_impl(T* p) : data_(p) {}

  // Initializer for deleters that have data parameters.
  scoped_ptr_impl(T* p, const D& d) : data_(p, d) {}

  // Templated constructor that destructively takes the value from another
  // scoped_ptr_impl.
  template <typename U, typename V>
  scoped_ptr_impl(scoped_ptr_impl<U, V>* other)
      : data_(other->release(), other->get_deleter()) {
    // We do not support move-only deleters.  We could modify our move
    // emulation to have base::subtle::move() and base::subtle::forward()
    // functions that are imperfect emulations of their C++11 equivalents,
    // but until there's a requirement, just assume deleters are copyable.
  }

  template <typename U, typename V>
  void TakeState(scoped_ptr_impl<U, V>* other) {
    // See comment in templated constructor above regarding lack of support
    // for move-only deleters.
    reset(other->release());
    get_deleter() = other->get_deleter();
  }

  ~scoped_ptr_impl() {
    // Match libc++, which calls reset() in its destructor.
    // Use nullptr as the new value for three reasons:
    // 1. libc++ does it.
    // 2. Avoids infinitely recursing into destructors if two classes are owned
    //    in a reference cycle (see ScopedPtrTest.ReferenceCycle).
    // 3. If |this| is accessed in the future, in a use-after-free bug, attempts
    //    to dereference |this|'s pointer should cause either a failure or a
    //    segfault closer to the problem. If |this| wasn't reset to nullptr,
    //    the access would cause the deleted memory to be read or written
    //    leading to other more subtle issues.
    reset(nullptr);
  }

  void reset(T* p) {
    // Match C++11's definition of unique_ptr::reset(), which requires changing
    // the pointer before invoking the deleter on the old pointer. This prevents
    // |this| from being accessed after the deleter is run, which may destroy
    // |this|.
    T* old = data_.ptr;
    data_.ptr = p;
    if (old != nullptr)
      static_cast<D&>(data_)(old);
  }

  T* get() const { return data_.ptr; }

  D& get_deleter() { return data_; }
  const D& 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<D&>(data_), static_cast<D&>(p2.data_));
    swap(data_.ptr, p2.data_.ptr);
  }

  T* release() {
    T* old_ptr = data_.ptr;
    data_.ptr = nullptr;
    return old_ptr;
  }

 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 D
  // member, while avoiding any space overhead for it when D is an
  // empty class.  See e.g. http://www.cantrip.org/emptyopt.html for a good
  // discussion of this technique.
  struct Data : public D {
    explicit Data(T* ptr_in) : ptr(ptr_in) {}
    Data(T* ptr_in, const D& other) : D(other), ptr(ptr_in) {}
    T* ptr;
  };

  Data data_;

  DISALLOW_COPY_AND_ASSIGN(scoped_ptr_impl);
};

}  // 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 nullptr 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 scoped_ptr is small. On most compilers, when using the
// std::default_delete, sizeof(scoped_ptr<T>) == sizeof(T*). Custom deleters
// will increase the size proportional to whatever state they need to have. See
// comments inside scoped_ptr_impl<> for details.
//
// Current implementation targets having a strict subset of  C++11's
// unique_ptr<> features. Known deficiencies include not supporting move-only
// deleteres, function pointers as deleters, and deleters with reference
// types.
template <class T, class D = std::default_delete<T>>
class scoped_ptr {
  DISALLOW_COPY_AND_ASSIGN_WITH_MOVE_FOR_BIND(scoped_ptr)

  static_assert(!std::is_array<T>::value,
                "scoped_ptr doesn't support array with size");
  static_assert(base::internal::IsNotRefCounted<T>::value,
                "T is a refcounted type and needs a scoped_refptr");

 public:
  // The element and deleter types.
  using element_type = T;
  using deleter_type = D;

  // Constructor.  Defaults to initializing with nullptr.
  scoped_ptr() : impl_(nullptr) {}

  // Constructor.  Takes ownership of p.
  explicit scoped_ptr(element_type* p) : impl_(p) {}

  // Constructor.  Allows initialization of a stateful deleter.
  scoped_ptr(element_type* p, const D& d) : impl_(p, d) {}

  // Constructor.  Allows construction from a nullptr.
  scoped_ptr(std::nullptr_t) : impl_(nullptr) {}

  // Move constructor.
  //
  // IMPLEMENTATION NOTE: Clang requires a move constructor to be defined (and
  // not just the conversion constructor) in order to warn on pessimizing moves.
  // The requirements for the move constructor are specified in C++11
  // 20.7.1.2.1.15-17, which has some subtleties around reference deleters. As
  // we don't support reference (or move-only) deleters, the post conditions are
  // trivially true: we always copy construct the deleter from other's deleter.
  scoped_ptr(scoped_ptr&& other) : impl_(&other.impl_) {}

  // Conversion constructor.  Allows construction from a scoped_ptr rvalue for a
  // convertible type and deleter.
  //
  // IMPLEMENTATION NOTE: C++ 20.7.1.2.1.19 requires this constructor to only
  // participate in overload resolution if all the following are true:
  // - U is implicitly convertible to T: this is important for 2 reasons:
  //     1. So type traits don't incorrectly return true, e.g.
  //          std::is_convertible<scoped_ptr<Base>, scoped_ptr<Derived>>::value
  //        should be false.
  //     2. To make sure code like this compiles:
  //        void F(scoped_ptr<int>);
  //        void F(scoped_ptr<Base>);
  //        // Ambiguous since both conversion constructors match.
  //        F(scoped_ptr<Derived>());
  // - U is not an array type: to prevent conversions from scoped_ptr<T[]> to
  //   scoped_ptr<T>.
  // - D is a reference type and E is the same type, or D is not a reference
  //   type and E is implicitly convertible to D: again, we don't support
  //   reference deleters, so we only worry about the latter requirement.
  template <typename U,
            typename E,
            typename std::enable_if<!std::is_array<U>::value &&
                                    std::is_convertible<U*, T*>::value &&
                                    std::is_convertible<E, D>::value>::type* =
                nullptr>
  scoped_ptr(scoped_ptr<U, E>&& other)
      : impl_(&other.impl_) {}

  // operator=.
  //
  // IMPLEMENTATION NOTE: Unlike the move constructor, Clang does not appear to
  // require a move assignment operator to trigger the pessimizing move warning:
  // in this case, the warning triggers when moving a temporary. For consistency
  // with the move constructor, we define it anyway. C++11 20.7.1.2.3.1-3
  // defines several requirements around this: like the move constructor, the
  // requirements are simplified by the fact that we don't support move-only or
  // reference deleters.
  scoped_ptr& operator=(scoped_ptr&& rhs) {
    impl_.TakeState(&rhs.impl_);
    return *this;
  }

  // operator=.  Allows assignment from a scoped_ptr rvalue for a convertible
  // type and deleter.
  //
  // IMPLEMENTATION NOTE: C++11 unique_ptr<> keeps this operator= distinct from
  // the normal move assignment operator. C++11 20.7.1.2.3.4-7 contains the
  // requirement for this operator, but like the conversion constructor, the
  // requirements are greatly simplified by not supporting move-only or
  // reference deleters.
  template <typename U,
            typename E,
            typename std::enable_if<!std::is_array<U>::value &&
                                    std::is_convertible<U*, T*>::value &&
                                    // Note that this really should be
                                    // std::is_assignable, but <type_traits>
                                    // appears to be missing this on some
                                    // platforms. This is close enough (though
                                    // it's not the same).
                                    std::is_convertible<D, E>::value>::type* =
                nullptr>
  scoped_ptr& operator=(scoped_ptr<U, E>&& rhs) {
    impl_.TakeState(&rhs.impl_);
    return *this;
  }

  // operator=.  Allows assignment from a nullptr. Deletes the currently owned
  // object, if any.
  scoped_ptr& operator=(std::nullptr_t) {
    reset();
    return *this;
  }

  // Reset.  Deletes the currently owned object, if any.
  // Then takes ownership of a new object, if given.
  void reset(element_type* p = nullptr) { impl_.reset(p); }

  // Accessors to get the owned object.
  // operator* and operator-> will assert() if there is no current object.
  element_type& operator*() const {
    assert(impl_.get() != nullptr);
    return *impl_.get();
  }
  element_type* operator->() const  {
    assert(impl_.get() != nullptr);
    return impl_.get();
  }
  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).
  //
  // Note that this trick is only safe when the == and != operators
  // are declared explicitly, as otherwise "scoped_ptr1 ==
  // scoped_ptr2" will compile but do the wrong thing (i.e., convert
  // to Testable and then do the comparison).
 private:
  typedef base::internal::scoped_ptr_impl<element_type, deleter_type>
      scoped_ptr::*Testable;

 public:
  operator Testable() const {
    return impl_.get() ? &scoped_ptr::impl_ : nullptr;
  }

  // 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 nullptr, the return value is nullptr. After this operation, this
  // object will hold a nullptr, and will not own the object any more.
  element_type* release() WARN_UNUSED_RESULT {
    return impl_.release();
  }

 private:
  // 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_;

  // Forbidden for API compatibility with std::unique_ptr.
  explicit scoped_ptr(int disallow_construction_from_null);
};

template <class T, class D>
class scoped_ptr<T[], D> {
  DISALLOW_COPY_AND_ASSIGN_WITH_MOVE_FOR_BIND(scoped_ptr)

 public:
  // The element and deleter types.
  using element_type = T;
  using deleter_type = D;

  // Constructor.  Defaults to initializing with nullptr.
  scoped_ptr() : impl_(nullptr) {}

  // Constructor. Stores the given array. Note that the argument's type
  // must exactly match T*. In particular:
  // - it cannot be a pointer to a type derived from T, because it is
  //   inherently unsafe in the general case to access an array through a
  //   pointer whose dynamic type does not match its static type (eg., if
  //   T and the derived types had different sizes access would be
  //   incorrectly calculated). Deletion is also always undefined
  //   (C++98 [expr.delete]p3). If you're doing this, fix your code.
  // - it cannot be const-qualified differently from T per unique_ptr spec
  //   (http://cplusplus.github.com/LWG/lwg-active.html#2118). Users wanting
  //   to work around this may use const_cast<const T*>().
  explicit scoped_ptr(element_type* array) : impl_(array) {}

  // Constructor.  Allows construction from a nullptr.
  scoped_ptr(std::nullptr_t) : impl_(nullptr) {}

  // Constructor.  Allows construction from a scoped_ptr rvalue.
  scoped_ptr(scoped_ptr&& other) : impl_(&other.impl_) {}

  // operator=.  Allows assignment from a scoped_ptr rvalue.
  scoped_ptr& operator=(scoped_ptr&& rhs) {
    impl_.TakeState(&rhs.impl_);
    return *this;
  }

  // operator=.  Allows assignment from a nullptr. Deletes the currently owned
  // array, if any.
  scoped_ptr& operator=(std::nullptr_t) {
    reset();
    return *this;
  }

  // Reset.  Deletes the currently owned array, if any.
  // Then takes ownership of a new object, if given.
  void reset(element_type* array = nullptr) { impl_.reset(array); }

  // Accessors to get the owned array.
  element_type& operator[](size_t i) const {
    assert(impl_.get() != nullptr);
    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_ : nullptr;
  }

  // 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 nullptr, the return value is nullptr. After this operation, this
  // object will hold a nullptr, 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 U> explicit scoped_ptr(U* array);
  explicit scoped_ptr(int disallow_construction_from_null);

  // Disable reset() from any type other than element_type*, for the same
  // reasons as the constructor above.
  template <typename U> void reset(U* array);
  void reset(int disallow_reset_from_null);
};

// Free functions
template <class T, class D>
void swap(scoped_ptr<T, D>& p1, scoped_ptr<T, D>& p2) {
  p1.swap(p2);
}

template <class T1, class D1, class T2, class D2>
bool operator==(const scoped_ptr<T1, D1>& p1, const scoped_ptr<T2, D2>& p2) {
  return p1.get() == p2.get();
}
template <class T, class D>
bool operator==(const scoped_ptr<T, D>& p, std::nullptr_t) {
  return p.get() == nullptr;
}
template <class T, class D>
bool operator==(std::nullptr_t, const scoped_ptr<T, D>& p) {
  return p.get() == nullptr;
}

template <class T1, class D1, class T2, class D2>
bool operator!=(const scoped_ptr<T1, D1>& p1, const scoped_ptr<T2, D2>& p2) {
  return !(p1 == p2);
}
template <class T, class D>
bool operator!=(const scoped_ptr<T, D>& p, std::nullptr_t) {
  return !(p == nullptr);
}
template <class T, class D>
bool operator!=(std::nullptr_t, const scoped_ptr<T, D>& p) {
  return !(p == nullptr);
}

template <class T1, class D1, class T2, class D2>
bool operator<(const scoped_ptr<T1, D1>& p1, const scoped_ptr<T2, D2>& p2) {
  return p1.get() < p2.get();
}
template <class T, class D>
bool operator<(const scoped_ptr<T, D>& p, std::nullptr_t) {
  return p.get() < nullptr;
}
template <class T, class D>
bool operator<(std::nullptr_t, const scoped_ptr<T, D>& p) {
  return nullptr < p.get();
}

template <class T1, class D1, class T2, class D2>
bool operator>(const scoped_ptr<T1, D1>& p1, const scoped_ptr<T2, D2>& p2) {
  return p2 < p1;
}
template <class T, class D>
bool operator>(const scoped_ptr<T, D>& p, std::nullptr_t) {
  return nullptr < p;
}
template <class T, class D>
bool operator>(std::nullptr_t, const scoped_ptr<T, D>& p) {
  return p < nullptr;
}

template <class T1, class D1, class T2, class D2>
bool operator<=(const scoped_ptr<T1, D1>& p1, const scoped_ptr<T2, D2>& p2) {
  return !(p1 > p2);
}
template <class T, class D>
bool operator<=(const scoped_ptr<T, D>& p, std::nullptr_t) {
  return !(p > nullptr);
}
template <class T, class D>
bool operator<=(std::nullptr_t, const scoped_ptr<T, D>& p) {
  return !(nullptr > p);
}

template <class T1, class D1, class T2, class D2>
bool operator>=(const scoped_ptr<T1, D1>& p1, const scoped_ptr<T2, D2>& p2) {
  return !(p1 < p2);
}
template <class T, class D>
bool operator>=(const scoped_ptr<T, D>& p, std::nullptr_t) {
  return !(p < nullptr);
}
template <class T, class D>
bool operator>=(std::nullptr_t, const scoped_ptr<T, D>& p) {
  return !(nullptr < p);
}

// 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);
}

template <typename T>
std::ostream& operator<<(std::ostream& out, const scoped_ptr<T>& p) {
  return out << p.get();
}

#endif  // BASE_MEMORY_SCOPED_PTR_H_