Add Slice: a (pointer, size) array view that acts like a container

src/Makefile.am

@@ -312,6 +312,7 @@ libbitcoin_consensus_a_SOURCES = \
   script/script_error.cpp \
   script/script_error.h \
   serialize.h \
+  span.h \
   tinyformat.h \
   uint256.cpp \
   uint256.h \

src/span.h

@@ -0,0 +1,40 @@
+// Copyright (c) 2018 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_SPAN_H
+#define BITCOIN_SPAN_H
+
+#include <type_traits>
+#include <cstddef>
+
+/** A Span is an object that can refer to a contiguous sequence of objects.
+ *
+ * It implements a subset of C++20's std::span.
+ */
+template<typename C>
+class Span
+{
+    C* m_data;
+    std::ptrdiff_t m_size;
+
+public:
+    constexpr Span() noexcept : m_data(nullptr), m_size(0) {}
+    constexpr Span(C* data, std::ptrdiff_t size) noexcept : m_data(data), m_size(size) {}
+
+    constexpr C* data() const noexcept { return m_data; }
+    constexpr std::ptrdiff_t size() const noexcept { return m_size; }
+};
+
+/** Create a span to a container exposing data() and size().
+ *
+ * This correctly deals with constness: the returned Span's element type will be
+ * whatever data() returns a pointer to. If either the passed container is const,
+ * or its element type is const, the resulting span will have a const element type.
+ *
+ * std::span will have a constructor that implements this functionality directly.
+ */
+template<typename V>
+constexpr Span<typename std::remove_pointer<decltype(std::declval<V>().data())>::type> MakeSpan(V& v) { return Span<typename std::remove_pointer<decltype(std::declval<V>().data())>::type>(v.data(), v.size()); }
+
+#endif