1 files changed, 34 insertions, 1 deletions
diff --git a/src/span.h b/src/span.h
index 77de059fa6..73b37e35f3 100644
--- a/src/span.h
+++ b/src/span.h
@@ -1,4 +1,4 @@
-// Copyright (c) 2018 The Bitcoin Core developers
+// Copyright (c) 2018-2020 The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -8,6 +8,7 @@
 #include <type_traits>
 #include <cstddef>
 #include <algorithm>
+#include <assert.h>
 
 /** A Span is an object that can refer to a contiguous sequence of objects.
  *
@@ -24,9 +25,28 @@ public:
     constexpr Span(C* data, std::ptrdiff_t size) noexcept : m_data(data), m_size(size) {}
     constexpr Span(C* data, C* end) noexcept : m_data(data), m_size(end - data) {}
 
+    /** Implicit conversion of spans between compatible types.
+     *
+     *  Specifically, if a pointer to an array of type O can be implicitly converted to a pointer to an array of type
+     *  C, then permit implicit conversion of Span<O> to Span<C>. This matches the behavior of the corresponding
+     *  C++20 std::span constructor.
+     *
+     *  For example this means that a Span<T> can be converted into a Span<const T>.
+     */
+    template <typename O, typename std::enable_if<std::is_convertible<O (*)[], C (*)[]>::value, int>::type = 0>
+    constexpr Span(const Span<O>& other) noexcept : m_data(other.m_data), m_size(other.m_size) {}
+
+    /** Default copy constructor. */
+    constexpr Span(const Span&) noexcept = default;
+
+    /** Default assignment operator. */
+    Span& operator=(const Span& other) noexcept = default;
+
     constexpr C* data() const noexcept { return m_data; }
     constexpr C* begin() const noexcept { return m_data; }
     constexpr C* end() const noexcept { return m_data + m_size; }
+    constexpr C& front() const noexcept { return m_data[0]; }
+    constexpr C& back() const noexcept { return m_data[m_size - 1]; }
     constexpr std::ptrdiff_t size() const noexcept { return m_size; }
     constexpr C& operator[](std::ptrdiff_t pos) const noexcept { return m_data[pos]; }
 
@@ -41,6 +61,8 @@ public:
     friend constexpr bool operator<=(const Span& a, const Span& b) noexcept { return !(b < a); }
     friend constexpr bool operator>(const Span& a, const Span& b) noexcept { return (b < a); }
     friend constexpr bool operator>=(const Span& a, const Span& b) noexcept { return !(a < b); }
+
+    template <typename O> friend class Span;
 };
 
 /** Create a span to a container exposing data() and size().
@@ -57,4 +79,15 @@ constexpr Span<A> MakeSpan(A (&a)[N]) { return Span<A>(a, N); }
 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()); }
 
+/** Pop the last element off a span, and return a reference to that element. */
+template <typename T>
+T& SpanPopBack(Span<T>& span)
+{
+    size_t size = span.size();
+    assert(size > 0);
+    T& back = span[size - 1];
+    span = Span<T>(span.data(), size - 1);
+    return back;
+}
+
 #endif