Partial bitcoin#18468: Span improvements

bitcoin@1f790a1
bitcoin@bb3d38f
bitcoin@ab303a1
bitcoin@26acc8d

src/span.h

@@ -10,6 +10,14 @@
 #include <algorithm>
 #include <assert.h>
 
+#ifdef DEBUG
+#define CONSTEXPR_IF_NOT_DEBUG
+#define ASSERT_IF_DEBUG(x) assert((x))
+#else
+#define CONSTEXPR_IF_NOT_DEBUG constexpr
+#define ASSERT_IF_DEBUG(x)
+#endif
+
 /** A Span is an object that can refer to a contiguous sequence of objects.
  *
  * It implements a subset of C++20's std::span.
@@ -18,12 +26,29 @@ template<typename C>
 class Span
 {
     C* m_data;
-    std::ptrdiff_t m_size;
+    std::size_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 Span(C* data, C* end) noexcept : m_data(data), m_size(end - data) {}
+
+    /** Construct a span from a begin pointer and a size.
+     *
+     * This implements a subset of the iterator-based std::span constructor in C++20,
+     * which is hard to implement without std::address_of.
+     */
+    template <typename T, typename std::enable_if<std::is_convertible<T (*)[], C (*)[]>::value, int>::type = 0>
+    constexpr Span(T* begin, std::size_t size) noexcept : m_data(begin), m_size(size) {}
+
+    /** Construct a span from a begin and end pointer.
+     *
+     * This implements a subset of the iterator-based std::span constructor in C++20,
+     * which is hard to implement without std::address_of.
+     */
+    template <typename T, typename std::enable_if<std::is_convertible<T (*)[], C (*)[]>::value, int>::type = 0>
+    CONSTEXPR_IF_NOT_DEBUG Span(T* begin, T* end) noexcept : m_data(begin), m_size(end - begin)
+    {
+        ASSERT_IF_DEBUG(end >= begin);
+    }
 
     /** Implicit conversion of spans between compatible types.
      *
@@ -42,18 +67,59 @@ class Span
     /** Default assignment operator. */
     Span& operator=(const Span& other) noexcept = default;
 
+    /** Construct a Span from an array. This matches the corresponding C++20 std::span constructor. */
+    template <int N>
+    constexpr Span(C (&a)[N]) noexcept : m_data(a), m_size(N) {}
+
+    /** Construct a Span for objects with .data() and .size() (std::string, std::array, std::vector, ...).
+     *
+     * This implements a subset of the functionality provided by the C++20 std::span range-based constructor.
+     *
+     * To prevent surprises, only Spans for constant value types are supported when passing in temporaries.
+     * Note that this restriction does not exist when converting arrays or other Spans (see above).
+     */
+    template <typename V, typename std::enable_if<(std::is_const<C>::value || std::is_lvalue_reference<V>::value) && std::is_convertible<typename std::remove_pointer<decltype(std::declval<V&>().data())>::type (*)[], C (*)[]>::value && std::is_convertible<decltype(std::declval<V&>().size()), std::size_t>::value, int>::type = 0>
+    constexpr Span(V&& v) noexcept : m_data(v.data()), m_size(v.size()) {}
+
     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]; }
-
-    constexpr Span<C> subspan(std::ptrdiff_t offset) const noexcept { return Span<C>(m_data + offset, m_size - offset); }
-    constexpr Span<C> subspan(std::ptrdiff_t offset, std::ptrdiff_t count) const noexcept { return Span<C>(m_data + offset, count); }
-    constexpr Span<C> first(std::ptrdiff_t count) const noexcept { return Span<C>(m_data, count); }
-    constexpr Span<C> last(std::ptrdiff_t count) const noexcept { return Span<C>(m_data + m_size - count, count); }
+    CONSTEXPR_IF_NOT_DEBUG C& front() const noexcept
+    {
+        ASSERT_IF_DEBUG(size() > 0);
+        return m_data[0];
+    }
+    CONSTEXPR_IF_NOT_DEBUG C& back() const noexcept
+    {
+        ASSERT_IF_DEBUG(size() > 0);
+        return m_data[m_size - 1];
+    }
+    constexpr std::size_t size() const noexcept { return m_size; }
+    CONSTEXPR_IF_NOT_DEBUG C& operator[](std::size_t pos) const noexcept
+    {
+        ASSERT_IF_DEBUG(size() > pos);
+        return m_data[pos];
+    }
+    CONSTEXPR_IF_NOT_DEBUG Span<C> subspan(std::size_t offset) const noexcept
+    {
+        ASSERT_IF_DEBUG(size() >= offset);
+        return Span<C>(m_data + offset, m_size - offset);
+    }
+    CONSTEXPR_IF_NOT_DEBUG Span<C> subspan(std::size_t offset, std::size_t count) const noexcept
+    {
+        ASSERT_IF_DEBUG(size() >= offset + count);
+        return Span<C>(m_data + offset, count);
+    }
+    CONSTEXPR_IF_NOT_DEBUG Span<C> first(std::size_t count) const noexcept
+    {
+        ASSERT_IF_DEBUG(size() >= count);
+        return Span<C>(m_data, count);
+    }
+    CONSTEXPR_IF_NOT_DEBUG Span<C> last(std::size_t count) const noexcept
+    {
+         ASSERT_IF_DEBUG(size() >= count);
+         return Span<C>(m_data + m_size - count, count);
+    }
 
     friend constexpr bool operator==(const Span& a, const Span& b) noexcept { return a.size() == b.size() && std::equal(a.begin(), a.end(), b.begin()); }
     friend constexpr bool operator!=(const Span& a, const Span& b) noexcept { return !(a == b); }
@@ -65,26 +131,20 @@ class Span
     template <typename O> friend class Span;
 };
 
-/** 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 A, int N>
-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()); }
+// MakeSpan helps constructing a Span of the right type automatically.
+/** MakeSpan for arrays: */
+template <typename A, int N> Span<A> constexpr MakeSpan(A (&a)[N]) { return Span<A>(a, N); }
+/** MakeSpan for temporaries / rvalue references, only supporting const output. */
+template <typename V> constexpr auto MakeSpan(V&& v) -> typename std::enable_if<!std::is_lvalue_reference<V>::value, Span<const typename std::remove_pointer<decltype(v.data())>::type>>::type { return std::forward<V>(v); }
+/** MakeSpan for (lvalue) references, supporting mutable output. */
+template <typename V> constexpr auto MakeSpan(V& v) -> Span<typename std::remove_pointer<decltype(v.data())>::type> { return v; }
 
 /** 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);
+    ASSERT_IF_DEBUG(size > 0);
     T& back = span[size - 1];
     span = Span<T>(span.data(), size - 1);
     return back;