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author | Pieter Wuille <pieter.wuille@gmail.com> | 2020-03-29 17:29:01 -0700 |
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committer | Pieter Wuille <pieter.wuille@gmail.com> | 2020-05-12 14:12:46 -0700 |
commit | ab303a16d114b1e94c6cf0e4c5db5389dfa197f6 (patch) | |
tree | 804c4048b8e5061d4e4e6c19d8f8b095ac882025 | |
parent | bb3d38fc061d8482e68cd335a45c9cd8bb66a475 (diff) | |
download | bitcoin-ab303a16d114b1e94c6cf0e4c5db5389dfa197f6.tar.xz |
Add Span constructors for arrays and vectors
-rw-r--r-- | src/span.h | 34 |
1 files changed, 21 insertions, 13 deletions
diff --git a/src/span.h b/src/span.h index f83c1a5349..d5fec462e1 100644 --- a/src/span.h +++ b/src/span.h @@ -56,6 +56,20 @@ public: /** 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; } @@ -79,19 +93,13 @@ public: 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> |