Simplified the implementation and features of span

contrib/epee/include/span.h

@@ -0,0 +1,130 @@
+// Copyright (c) 2017, The Monero Project
+//
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#pragma once
+
+#include <cstdint>
+#include <memory>
+#include <type_traits>
+
+namespace epee
+{
+  /*!
+    \brief Non-owning sequence of data. Does not deep copy
+
+    Inspired by `gsl::span` and/or `boost::iterator_range`. This class is
+    intended to be used as a parameter type for functions that need to take a
+    writable or read-only sequence of data. Most common cases are `span<char>`
+    and `span<std::uint8_t>`. Using as a class member is only recommended if
+    clearly documented as not doing a deep-copy. C-arrays are easily convertible
+    to this type.
+
+    \note Conversion from C string literal to `span<const char>` will include
+      the NULL-terminator.
+    \note Never allows derived-to-base pointer conversion; an array of derived
+      types is not an array of base types.
+   */
+  template<typename T>
+  class span
+  {
+    /* Supporting class types is tricky - the {ptr,len} constructor will allow
+       derived-to-base conversions. This is NOT desireable because an array of
+       derived types is not an array of base types. It is possible to handle
+       this case, implement when/if needed. */
+    static_assert(!std::is_class<T>(), "no class types are currently allowed");
+  public:
+    using value_type = T;
+    using size_type = std::size_t;
+    using difference_type = std::ptrdiff_t;
+    using pointer = T*;
+    using const_pointer = const T*;
+    using reference = T&;
+    using const_reference = const T&;
+    using iterator = pointer;
+    using const_iterator = const_pointer;
+
+    constexpr span() noexcept : ptr(nullptr), len(0) {}
+    constexpr span(std::nullptr_t) noexcept : span() {}
+
+    constexpr span(T* const src_ptr, const std::size_t count) noexcept
+      : ptr(src_ptr), len(count) {}
+
+    //! Conversion from C-array. Prevents common bugs with sizeof + arrays.
+    template<std::size_t N>
+    constexpr span(T (&src)[N]) noexcept : span(src, N) {}
+
+    constexpr span(const span&) noexcept = default;
+    span& operator=(const span&) noexcept = default;
+
+    constexpr iterator begin() const noexcept { return ptr; }
+    constexpr const_iterator cbegin() const noexcept { return ptr; }
+
+    constexpr iterator end() const noexcept { return begin() + size(); }
+    constexpr const_iterator cend() const noexcept { return cbegin() + size(); }
+
+    constexpr bool empty() const noexcept { return size() == 0; }
+    constexpr pointer data() const noexcept { return ptr; }
+    constexpr std::size_t size() const noexcept { return len; }
+    constexpr std::size_t size_bytes() const noexcept { return size() * sizeof(value_type); }
+
+  private:
+    T* ptr;
+    std::size_t len;
+  };
+
+  //! \return `span<const T::value_type>` from a STL compatible `src`.
+  template<typename T>
+  constexpr span<const typename T::value_type> to_span(const T& src)
+  {
+    // compiler provides diagnostic if size() is not size_t.
+    return {src.data(), src.size()};
+  }
+
+  template<typename T>
+  constexpr bool has_padding() noexcept
+  {
+    return !std::is_pod<T>() || alignof(T) != 1;
+  }
+
+  //! \return Cast data from `src` as `span<const std::uint8_t>`.
+  template<typename T>
+  span<const std::uint8_t> to_byte_span(const span<const T> src) noexcept
+  {
+    static_assert(!has_padding<T>(), "source type may have padding");
+    return {reinterpret_cast<const std::uint8_t*>(src.data()), src.size_bytes()}; 
+  }
+
+  //! \return `span<const std::uint8_t>` which represents the bytes at `&src`.
+  template<typename T>
+  span<const std::uint8_t> as_byte_span(const T& src) noexcept
+  {
+    static_assert(!std::is_empty<T>(), "empty types will not work -> sizeof == 1");
+    static_assert(!has_padding<T>(), "source type may have padding");
+    return {reinterpret_cast<const std::uint8_t*>(std::addressof(src)), sizeof(T)};
+  }
+}