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//
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//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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// 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>


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) {
      assert(p->next_ != this &&
             "Trying to join() a linked ring we are already in. "
             "Is GMock thread safety enabled?");
      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) {