Added thirdparty: boost library

thirdparty/include/boost/histogram/detail/relaxed_equal.hpp

@@ -0,0 +1,46 @@
+// Copyright 2015-2019 Hans Dembinski
+//
+// Distributed under the Boost Software License, Version 1.0.
+// (See accompanying file LICENSE_1_0.txt
+// or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#ifndef BOOST_HISTOGRAM_DETAIL_RELAXED_EQUAL_HPP
+#define BOOST_HISTOGRAM_DETAIL_RELAXED_EQUAL_HPP
+
+#include <boost/histogram/detail/priority.hpp>
+#include <type_traits>
+
+namespace boost {
+namespace histogram {
+namespace detail {
+
+struct relaxed_equal {
+  template <class T, class U>
+  constexpr auto impl(const T& t, const U& u, priority<1>) const noexcept
+      -> decltype(t == u) const {
+    return t == u;
+  }
+
+  // consider T and U not equal, if there is no operator== defined for them
+  template <class T, class U>
+  constexpr bool impl(const T&, const U&, priority<0>) const noexcept {
+    return false;
+  }
+
+  // consider two T equal if they are stateless
+  template <class T>
+  constexpr bool impl(const T&, const T&, priority<0>) const noexcept {
+    return std::is_empty<T>::value;
+  }
+
+  template <class T, class U>
+  constexpr bool operator()(const T& t, const U& u) const noexcept {
+    return impl(t, u, priority<1>{});
+  }
+};
+
+} // namespace detail
+} // namespace histogram
+} // namespace boost
+
+#endif