refactor: Make Span an alias of std::span

This uses a macro, which can be a bit more brittle than an alias
template. However, class template argument deduction for alias templates
is only implemented in clang-19.

Author: MarcoFalke

doc/developer-notes.md

@@ -856,14 +856,14 @@ class A
   - *Rationale*: Easier to understand what is happening, thus easier to spot mistakes, even for those
   that are not language lawyers.
 
-- Use `Span` as function argument when it can operate on any range-like container.
+- Use `std::span` as function argument when it can operate on any range-like container.
 
   - *Rationale*: Compared to `Foo(const vector<int>&)` this avoids the need for a (potentially expensive)
     conversion to vector if the caller happens to have the input stored in another type of container.
     However, be aware of the pitfalls documented in [span.h](../src/span.h).
 
 ```cpp
-void Foo(Span<const int> data);
+void Foo(std::span<const int> data);
 
 std::vector<int> vec{1,2,3};
 Foo(vec);

src/span.h

@@ -11,31 +11,7 @@
 #include <type_traits>
 #include <utility>
 
-#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
-
-#if defined(__clang__)
-#if __has_attribute(lifetimebound)
-#define SPAN_ATTR_LIFETIMEBOUND [[clang::lifetimebound]]
-#else
-#define SPAN_ATTR_LIFETIMEBOUND
-#endif
-#else
-#define SPAN_ATTR_LIFETIMEBOUND
-#endif
-
 /** A Span is an object that can refer to a contiguous sequence of objects.
- *
- * This file implements a subset of C++20's std::span.  It can be considered
- * temporary compatibility code until C++20 and is designed to be a
- * self-contained abstraction without depending on other project files. For this
- * reason, Clang lifetimebound is defined here instead of including
- * <attributes.h>, which also defines it.
  *
  * Things to be aware of when writing code that deals with Spans:
  *
@@ -93,155 +69,7 @@
  *   result will be present in that variable after the call. Passing a temporary
  *   is useless in that context.
  */
-template<typename C>
-class Span
-{
-    C* m_data;
-    std::size_t m_size{0};
-
-    template <class T>
-    struct is_Span_int : public std::false_type {};
-    template <class T>
-    struct is_Span_int<Span<T>> : public std::true_type {};
-    template <class T>
-    struct is_Span : public is_Span_int<typename std::remove_cv<T>::type>{};
-
-
-public:
-    constexpr Span() noexcept : m_data(nullptr) {}
-
-    /** 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.
-     *
-     *  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;
-
-    /** 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>
-    constexpr Span(V& other SPAN_ATTR_LIFETIMEBOUND,
-        typename std::enable_if<!is_Span<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, std::nullptr_t>::type = nullptr)
-        : m_data(other.data()), m_size(other.size()){}
-
-    template <typename V>
-    constexpr Span(const V& other SPAN_ATTR_LIFETIMEBOUND,
-        typename std::enable_if<!is_Span<V>::value &&
-                                std::is_convertible<typename std::remove_pointer<decltype(std::declval<const V&>().data())>::type (*)[], C (*)[]>::value &&
-                                std::is_convertible<decltype(std::declval<const V&>().size()), std::size_t>::value, std::nullptr_t>::type = nullptr)
-        : m_data(other.data()), m_size(other.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_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 std::size_t size_bytes() const noexcept { return sizeof(C) * m_size; }
-    constexpr bool empty() const noexcept { return size() == 0; }
-    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);
-    }
-
-    template <typename O> friend class Span;
-};
-
-// Return result of calling .data() method on type T. This is used to be able to
-// write template deduction guides for the single-parameter Span constructor
-// below that will work if the value that is passed has a .data() method, and if
-// the data method does not return a void pointer.
-//
-// It is important to check for the void type specifically below, so the
-// deduction guides can be used in SFINAE contexts to check whether objects can
-// be converted to spans. If the deduction guides did not explicitly check for
-// void, and an object was passed that returned void* from data (like
-// std::vector<bool>), the template deduction would succeed, but the Span<void>
-// object instantiation would fail, resulting in a hard error, rather than a
-// SFINAE error.
-// https://stackoverflow.com/questions/68759148/sfinae-to-detect-the-explicitness-of-a-ctad-deduction-guide
-// https://stackoverflow.com/questions/16568986/what-happens-when-you-call-data-on-a-stdvectorbool
-template<typename T>
-using DataResult = std::remove_pointer_t<decltype(std::declval<T&>().data())>;
-
-// Deduction guides for Span
-// For the pointer/size based and iterator based constructor:
-template <typename T, typename EndOrSize> Span(T*, EndOrSize) -> Span<T>;
-// For the array constructor:
-template <typename T, std::size_t N> Span(T (&)[N]) -> Span<T>;
-// For the temporaries/rvalue references constructor, only supporting const output.
-template <typename T> Span(T&&) -> Span<std::enable_if_t<!std::is_lvalue_reference_v<T> && !std::is_void_v<DataResult<T&&>>, const DataResult<T&&>>>;
-// For (lvalue) references, supporting mutable output.
-template <typename T> Span(T&) -> Span<std::enable_if_t<!std::is_void_v<DataResult<T&>>, DataResult<T&>>>;
+#define Span std::span
 
 /** Pop the last element off a span, and return a reference to that element. */
 template <typename T>

src/test/span_tests.cpp

@@ -47,13 +47,6 @@ BOOST_AUTO_TEST_SUITE(span_tests)
 // don't work. This makes it is possible to use the Span constructor in a SFINAE
 // contexts like in the Spannable function above to detect whether types are or
 // aren't compatible with Spans at compile time.
-//
-// Previously there was a bug where writing a SFINAE check for vector<bool> was
-// not possible, because in libstdc++ vector<bool> has a data() member
-// returning void, and the Span template guide ignored the data() return value,
-// so the template substitution would succeed, but the constructor would fail,
-// resulting in a fatal compile error, rather than a SFINAE error that could be
-// handled.
 BOOST_AUTO_TEST_CASE(span_constructor_sfinae)
 {
     BOOST_CHECK(Spannable(std::vector<int>{}));