From 7c48447ffab2266462ca084e1e16afd57104fcc1 Mon Sep 17 00:00:00 2001
From: "Peter D. Barnes, Jr" <barnes26@llnl.gov>
Date: Wed, 15 Sep 2021 22:01:48 +0200
Subject: [PATCH] core: Add hashing functor for creating hash of a Ptr

---
 CHANGES.html         |  1 +
 RELEASE_NOTES        |  1 +
 src/core/model/ptr.h | 35 +++++++++++++++++++++++++++++++++++
 3 files changed, 37 insertions(+)

diff --git a/CHANGES.html b/CHANGES.html
index 6e6535832..782e1c071 100644
--- a/CHANGES.html
+++ b/CHANGES.html
@@ -60,6 +60,7 @@ us a note on ns-developers mailing list.</p>
 <li>In class <b>Ipv4InterfaceAddress</b>, a new function IsInSameSubnet () is added to check if both the IPv4 addresses are in the same subnet. Also, it is consistent with <b>Ipv6InterfaceAddress::IsInSameSubnet ()</b>.</li>
 <li>In class <b>ConfigStore</b>, a new Attribue <b>SaveDeprecated</b> allows to not save DEPRECATED Attributes. The default value is <b>false</b> (save DEPRECATED Attributes).</li>
 <li>In class <b>TracedCallback</b>, a new function <b>IsEmpty</b> allows to know if the TracedCallback will call any callback.</li>
+<li>A new specialization of std::hash for Ptr allows to use seamlessly Ptrs as keys in unordered_map and unordered_set.</li>
 </ul>
 <h2>Changes to existing API:</h2>
 <ul>
diff --git a/RELEASE_NOTES b/RELEASE_NOTES
index b24fd480f..3c3f25199 100644
--- a/RELEASE_NOTES
+++ b/RELEASE_NOTES
@@ -36,6 +36,7 @@ New user-visible features
 - (config-store) OBSOLETE Attributes are not anymore saved.
 - (config-store) New ConfigStore::SaveDeprecated Attribute (default false) to avoid to save DEPRECATED attributes.
 - (core) Add TracedCallback::IsEmpty to know if a TracedCallback has any callback associated.
+- (core) Add std::hash<ns3::Ptr<T>> to allow seamless use of Ptr as key in unordered_maps.
 
 Bugs fixed
 ----------
diff --git a/src/core/model/ptr.h b/src/core/model/ptr.h
index 91e342d6c..36ed7a5cc 100644
--- a/src/core/model/ptr.h
+++ b/src/core/model/ptr.h
@@ -695,4 +695,39 @@ Ptr<T>::operator Tester * () const
 
 } // namespace ns3
 
+
+/****************************************************
+ *      Global Functions (outside namespace ns3)
+ ***************************************************/
+
+/**
+ * \ingroup ptr
+ * Hashing functor taking a `Ptr` and returning a @c std::size_t.
+ * For use with `unordered_map` and `unordered_set`.
+ *
+ * \note When a `Ptr` is used in a container the lifetime of the underlying
+ * object is at least as long as the container.  In other words,
+ * you need to remove the object from the container when you are done with
+ * it, otherwise the object will persist until the container itself is
+ * deleted.
+ *
+ * \tparam T \deduced The type held by the `Ptr`
+ */
+template<class T>
+struct
+std::hash<ns3::Ptr<T>>
+{
+  /**
+   * The functor.
+   * \param p The `Ptr` value to hash.
+   * \return the hash
+   */
+  std::size_t
+  operator () (ns3::Ptr<T> p) const
+  {
+    return std::hash<const T *> () (ns3::PeekPointer (p));
+  }
+};
+
+
 #endif /* PTR_H */