From 2a458005d99882e1e7487bcb7782dee3b1a1de72 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ion=20Gazta=C3=B1aga?= <igaztanaga@gmail.com>
Date: Tue, 10 Nov 2020 10:08:03 +0100
Subject: [PATCH] Fixes #171 ("deque::clear() uses undefined behaviour")

---
 doc/container.qbk                 |  2 ++
 include/boost/container/deque.hpp | 31 +++++++++++++++++--------------
 2 files changed, 19 insertions(+), 14 deletions(-)

diff --git a/doc/container.qbk b/doc/container.qbk
index f654c3b..9baf77a 100644
--- a/doc/container.qbk
+++ b/doc/container.qbk
@@ -1341,6 +1341,8 @@ use [*Boost.Container]? There are several reasons for that:
 [section:release_notes_boost_1_76_00 Boost 1.76 Release]
 
 * Added [[no-discard]] attribute in all containers to catch bugs related to unused return values.
+* Fixed bugs/issues:
+   * [@https://github.com/boostorg/container/issues/171    GitHub #171: ['"deque::clear() uses undefined behaviour"]].
 
 [endsect]
 
diff --git a/include/boost/container/deque.hpp b/include/boost/container/deque.hpp
index 9ebb2ea..e94ea7b 100644
--- a/include/boost/container/deque.hpp
+++ b/include/boost/container/deque.hpp
@@ -1843,22 +1843,25 @@ class deque : protected deque_base<typename real_allocator<T, Allocator>::type,
    //! <b>Complexity</b>: Linear to the number of elements in the deque.
    void clear() BOOST_NOEXCEPT_OR_NOTHROW
    {
-      for (index_pointer node = this->members_.m_start.m_node + 1;
-            node < this->members_.m_finish.m_node;
-            ++node) {
-         this->priv_destroy_range(*node, *node + get_block_size());
-         this->priv_deallocate_node(*node);
-      }
+      if (this->members_.m_finish != this->members_.m_start) {
+  
+         for (index_pointer node = this->members_.m_start.m_node + 1;
+               node < this->members_.m_finish.m_node;
+               ++node) {
+            this->priv_destroy_range(*node, *node + get_block_size());
+            this->priv_deallocate_node(*node);
+         }
 
-      if (this->members_.m_start.m_node != this->members_.m_finish.m_node) {
-         this->priv_destroy_range(this->members_.m_start.m_cur, this->members_.m_start.m_last);
-         this->priv_destroy_range(this->members_.m_finish.m_first, this->members_.m_finish.m_cur);
-         this->priv_deallocate_node(this->members_.m_finish.m_first);
-      }
-      else
-         this->priv_destroy_range(this->members_.m_start.m_cur, this->members_.m_finish.m_cur);
+         if (this->members_.m_start.m_node != this->members_.m_finish.m_node) {
+            this->priv_destroy_range(this->members_.m_start.m_cur, this->members_.m_start.m_last);
+            this->priv_destroy_range(this->members_.m_finish.m_first, this->members_.m_finish.m_cur);
+            this->priv_deallocate_node(this->members_.m_finish.m_first);
+         }
+         else
+            this->priv_destroy_range(this->members_.m_start.m_cur, this->members_.m_finish.m_cur);
 
-      this->members_.m_finish = this->members_.m_start;
+         this->members_.m_finish = this->members_.m_start;
+      }
    }
 
    //! <b>Effects</b>: Returns true if x and y are equal