From a31e5cffa85f58b64a39fa7c4a1bd3bd9228b069 Mon Sep 17 00:00:00 2001
From: Conrad Poelman <cpgithub@stellarscience.com>
Date: Tue, 4 Aug 2020 17:20:40 -0400
Subject: [PATCH] Remove deprecated inheritance from std::iterator (#97)

std::iterator was deprecated in C++17 and removed in C++20. I replaced the inheritance with the 5 equivalent typedefs, even though they're not all used by ublas, for compatibility in case clients depend on them.
---
 .../boost/numeric/ublas/detail/iterator.hpp   | 24 ++++++++++++++-----
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/include/boost/numeric/ublas/detail/iterator.hpp b/include/boost/numeric/ublas/detail/iterator.hpp
index 1723a301c..7aebf2f9f 100644
--- a/include/boost/numeric/ublas/detail/iterator.hpp
+++ b/include/boost/numeric/ublas/detail/iterator.hpp
@@ -107,8 +107,12 @@ namespace boost { namespace numeric { namespace ublas {
    * via the post increment operator.
    */
     template<class IC, class I, class T>
-    struct forward_iterator_base:
-        public std::iterator<IC, T> {
+    struct forward_iterator_base {
+        typedef IC iterator_category;
+        typedef T value_type;
+        typedef std::ptrdiff_t difference_type;
+        typedef T* pointer;
+        typedef T& reference;
         typedef I derived_iterator_type;
         typedef T derived_value_type;
 
@@ -145,8 +149,12 @@ namespace boost { namespace numeric { namespace ublas {
    * via the post increment and post decrement operator.
    */
     template<class IC, class I, class T>
-    struct bidirectional_iterator_base:
-        public std::iterator<IC, T> {
+    struct bidirectional_iterator_base {
+        typedef IC iterator_category;
+        typedef T value_type;
+        typedef std::ptrdiff_t difference_type;
+        typedef T* pointer;
+        typedef T& reference;
         typedef I derived_iterator_type;
         typedef T derived_value_type;
 
@@ -200,8 +208,12 @@ namespace boost { namespace numeric { namespace ublas {
    */
     template<class IC, class I, class T, class D = std::ptrdiff_t>
     // ISSUE the default for D seems rather dangerous as it can easily be (silently) incorrect
-    struct random_access_iterator_base:
-        public std::iterator<IC, T> {
+    struct random_access_iterator_base {
+        typedef IC iterator_category;
+        typedef T value_type;
+        typedef D difference_type;
+        typedef T* pointer;
+        typedef T& reference;
         typedef I derived_iterator_type;
         typedef T derived_value_type;
         typedef D derived_difference_type;