gtest-linked_ptr.h

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// Copyright 2003 Google Inc.
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//     * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
//     * Neither the name of Google Inc. nor the names of its
// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//
// Authors: Dan Egnor (egnor@google.com)
//
// A "smart" pointer type with reference tracking.  Every pointer to a
// particular object is kept on a circular linked list.  When the last pointer
// to an object is destroyed or reassigned, the object is deleted.
//
// Used properly, this deletes the object when the last reference goes away.
// There are several caveats:
// - Like all reference counting schemes, cycles lead to leaks.
// - Each smart pointer is actually two pointers (8 bytes instead of 4).
// - Every time a pointer is assigned, the entire list of pointers to that
//   object is traversed.  This class is therefore NOT SUITABLE when there
//   will often be more than two or three pointers to a particular object.
// - References are only tracked as long as linked_ptr<> objects are copied.
//   If a linked_ptr<> is converted to a raw pointer and back, BAD THINGS
//   will happen (double deletion).
//
// A good use of this class is storing object references in STL containers.
// You can safely put linked_ptr<> in a vector<>.
// Other uses may not be as good.
//
// Note: If you use an incomplete type with linked_ptr<>, the class
// *containing* linked_ptr<> must have a constructor and destructor (even
// if they do nothing!).
//
// Bill Gibbons suggested we use something like this.
//
// Thread Safety:
//   Unlike other linked_ptr implementations, in this implementation
//   a linked_ptr object is thread-safe in the sense that:
//     - it's safe to copy linked_ptr objects concurrently,
//     - it's safe to copy *from* a linked_ptr and read its underlying
//       raw pointer (e.g. via get()) concurrently, and
//     - it's safe to write to two linked_ptrs that point to the same
//       shared object concurrently.
// TODO(wan@google.com): rename this to safe_linked_ptr to avoid
// confusion with normal linked_ptr.

#ifndef GTEST_INCLUDE_GTEST_INTERNAL_GTEST_LINKED_PTR_H_
#define GTEST_INCLUDE_GTEST_INTERNAL_GTEST_LINKED_PTR_H_

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

#include "gtest/internal/gtest-port.h"

namespace testing {
namespace internal {

// Protects copying of all linked_ptr objects.
GTEST_API_ GTEST_DECLARE_STATIC_MUTEX_(g_linked_ptr_mutex);

// This is used internally by all instances of linked_ptr<>.  It needs to be
// a non-template class because different types of linked_ptr<> can refer to
// the same object (linked_ptr<Superclass>(obj) vs linked_ptr<Subclass>(obj)).
// So, it needs to be possible for different types of linked_ptr to participate
// in the same circular linked list, so we need a single class type here.
//
// DO NOT USE THIS CLASS DIRECTLY YOURSELF.  Use linked_ptr<T>.
class linked_ptr_internal {
 public:
  // Create a new circle that includes only this instance.
  void join_new() {
    next_ = this;
  }

  // Many linked_ptr operations may change p.link_ for some linked_ptr
  // variable p in the same circle as this object.  Therefore we need
  // to prevent two such operations from occurring concurrently.
  //
  // Note that different types of linked_ptr objects can coexist in a
  // circle (e.g. linked_ptr<Base>, linked_ptr<Derived1>, and
  // linked_ptr<Derived2>).  Therefore we must use a single mutex to
  // protect all linked_ptr objects.  This can create serious
  // contention in production code, but is acceptable in a testing
  // framework.

  // Join an existing circle.
  void join(linked_ptr_internal const* ptr)
      GTEST_LOCK_EXCLUDED_(g_linked_ptr_mutex) {
    MutexLock lock(&g_linked_ptr_mutex);

    linked_ptr_internal const* p = ptr;
    while (p->next_ != ptr) p = p->next_;
    p->next_ = this;
    next_ = ptr;
  }

  // Leave whatever circle we're part of.  Returns true if we were the
  // last member of the circle.  Once this is done, you can join() another.
  bool depart()
      GTEST_LOCK_EXCLUDED_(g_linked_ptr_mutex) {
    MutexLock lock(&g_linked_ptr_mutex);

    if (next_ == this) return true;
    linked_ptr_internal const* p = next_;
    while (p->next_ != this) p = p->next_;
    p->next_ = next_;
    return false;
  }

 private:
  mutable linked_ptr_internal const* next_;
};

template <typename T>
class linked_ptr {
 public:
  typedef T element_type;

  // Take over ownership of a raw pointer.  This should happen as soon as
  // possible after the object is created.
  explicit linked_ptr(T* ptr = NULL) { capture(ptr); }
  ~linked_ptr() { depart(); }

  // Copy an existing linked_ptr<>, adding ourselves to the list of references.
  template <typename U> linked_ptr(linked_ptr<U> const& ptr) { copy(&ptr); }
  linked_ptr(linked_ptr const& ptr) {  // NOLINT
    assert(&ptr != this);
    copy(&ptr);
  }

  // Assignment releases the old value and acquires the new.
  template <typename U> linked_ptr& operator=(linked_ptr<U> const& ptr) {
    depart();
    copy(&ptr);
    return *this;
  }

  linked_ptr& operator=(linked_ptr const& ptr) {
    if (&ptr != this) {
      depart();
      copy(&ptr);
    }
    return *this;
  }

  // Smart pointer members.
  void reset(T* ptr = NULL) {
    depart();
    capture(ptr);
  }
  T* get() const { return value_; }
  T* operator->() const { return value_; }
  T& operator*() const { return *value_; }

  bool operator==(T* p) const { return value_ == p; }
  bool operator!=(T* p) const { return value_ != p; }
  template <typename U>
  bool operator==(linked_ptr<U> const& ptr) const {
    return value_ == ptr.get();
  }
  template <typename U>
  bool operator!=(linked_ptr<U> const& ptr) const {
    return value_ != ptr.get();
  }

 private:
  template <typename U>
  friend class linked_ptr;

  T* value_;
  linked_ptr_internal link_;

  void depart() {
    if (link_.depart()) delete value_;
  }

  void capture(T* ptr) {
    value_ = ptr;
    link_.join_new();
  }

  template <typename U> void copy(linked_ptr<U> const* ptr) {
    value_ = ptr->get();
    if (value_)
      link_.join(&ptr->link_);
    else
      link_.join_new();
  }
};

template<typename T> inline
bool operator==(T* ptr, const linked_ptr<T>& x) {
  return ptr == x.get();
}

template<typename T> inline
bool operator!=(T* ptr, const linked_ptr<T>& x) {
  return ptr != x.get();
}

// A function to convert T* into linked_ptr<T>
// Doing e.g. make_linked_ptr(new FooBarBaz<type>(arg)) is a shorter notation
// for linked_ptr<FooBarBaz<type> >(new FooBarBaz<type>(arg))
template <typename T>
linked_ptr<T> make_linked_ptr(T* ptr) {
  return linked_ptr<T>(ptr);
}

}  // namespace internal
}  // namespace testing

#endif  // GTEST_INCLUDE_GTEST_INTERNAL_GTEST_LINKED_PTR_H_