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### Rationale for this change basic_string_view was never really intended to support types other than chars; the template which the STL intends for use in this situation is std::span ### What changes are included in this PR? Since std::span is added in c++20, a minimal polyfill is added named util::span. - doesn't include support for static extents - some accessors like `front()` are not present in util::span ### User facing changes: - The broken alias bytes_view and its header are removed. I don't expect this to trouble anybody much since it isn't used in much of our API, and if it's really necessary it will be trivial to add it back with `using bytes_view = span<uint8_t const>;` or so * Closes: #15281 Lead-authored-by: Benjamin Kietzman <[email protected]> Co-authored-by: Antoine Pitrou <[email protected]> Signed-off-by: Antoine Pitrou <[email protected]>
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| // Licensed to the Apache Software Foundation (ASF) under one | ||
| // or more contributor license agreements. See the NOTICE file | ||
| // distributed with this work for additional information | ||
| // regarding copyright ownership. The ASF licenses this file | ||
| // to you under the Apache License, Version 2.0 (the | ||
| // "License"); you may not use this file except in compliance | ||
| // with the License. You may obtain a copy of the License at | ||
| // | ||
| // http://www.apache.org/licenses/LICENSE-2.0 | ||
| // | ||
| // Unless required by applicable law or agreed to in writing, | ||
| // software distributed under the License is distributed on an | ||
| // "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY | ||
| // KIND, either express or implied. See the License for the | ||
| // specific language governing permissions and limitations | ||
| // under the License. | ||
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| #pragma once | ||
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| #include <cstddef> | ||
| #include <cstdint> | ||
| #include <cstring> | ||
| #include <iterator> | ||
| #include <type_traits> | ||
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| namespace arrow::util { | ||
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| /// std::span polyfill. | ||
| /// | ||
| /// Does not support static extents. | ||
| template <typename T> | ||
| class span { | ||
| static_assert(sizeof(T), | ||
| R"( | ||
| std::span allows contiguous_iterators instead of just pointers, the enforcement | ||
| of which requires T to be a complete type. arrow::util::span does not support | ||
| contiguous_iterators, but T is still required to be a complete type to prevent | ||
| writing code which would break when it is replaced by std::span.)"); | ||
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| public: | ||
| using element_type = T; | ||
| using value_type = std::remove_cv_t<T>; | ||
| using iterator = T*; | ||
| using const_iterator = T const*; | ||
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| span() = default; | ||
| span(const span&) = default; | ||
| span& operator=(const span&) = default; | ||
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| template <typename M, typename = std::enable_if_t<std::is_same_v<T, M const>>> | ||
| // NOLINTNEXTLINE runtime/explicit | ||
| constexpr span(span<M> mut) : span{mut.data(), mut.size()} {} | ||
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| constexpr span(T* data, size_t count) : data_{data}, size_{count} {} | ||
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| constexpr span(T* begin, T* end) | ||
| : data_{begin}, size_{static_cast<size_t>(end - begin)} {} | ||
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| template < | ||
| typename R, | ||
| typename DisableUnlessConstructibleFromDataAndSize = | ||
| decltype(span<T>(std::data(std::declval<R>()), std::size(std::declval<R>()))), | ||
| typename DisableUnlessSimilarTypes = std::enable_if_t<std::is_same_v< | ||
| std::decay_t<std::remove_pointer_t<decltype(std::data(std::declval<R>()))>>, | ||
| std::decay_t<T>>>> | ||
| // NOLINTNEXTLINE runtime/explicit, non-const reference | ||
| constexpr span(R&& range) : span{std::data(range), std::size(range)} {} | ||
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| constexpr T* begin() const { return data_; } | ||
| constexpr T* end() const { return data_ + size_; } | ||
| constexpr T* data() const { return data_; } | ||
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| constexpr size_t size() const { return size_; } | ||
| constexpr size_t size_bytes() const { return size_ * sizeof(T); } | ||
| constexpr bool empty() const { return size_ == 0; } | ||
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| constexpr T& operator[](size_t i) { return data_[i]; } | ||
| constexpr const T& operator[](size_t i) const { return data_[i]; } | ||
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| constexpr span subspan(size_t offset) const { | ||
| if (offset > size_) return {data_, data_}; | ||
| return {data_ + offset, size_ - offset}; | ||
| } | ||
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| constexpr span subspan(size_t offset, size_t count) const { | ||
| auto out = subspan(offset); | ||
| if (count < out.size_) { | ||
| out.size_ = count; | ||
| } | ||
| return out; | ||
| } | ||
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| constexpr bool operator==(span const& other) const { | ||
| if (size_ != other.size_) return false; | ||
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| if constexpr (std::is_integral_v<T>) { | ||
| if (size_ == 0) { | ||
| return true; // memcmp does not handle null pointers, even if size_ == 0 | ||
| } | ||
| return std::memcmp(data_, other.data_, size_bytes()) == 0; | ||
| } else { | ||
| T* ptr = data_; | ||
| for (T const& e : other) { | ||
| if (*ptr++ != e) return false; | ||
| } | ||
| return true; | ||
| } | ||
| } | ||
| constexpr bool operator!=(span const& other) const { return !(*this == other); } | ||
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| private: | ||
| T* data_{}; | ||
| size_t size_{}; | ||
| }; | ||
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| template <typename R> | ||
| span(R& range) -> span<std::remove_pointer_t<decltype(std::data(range))>>; | ||
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| template <typename T> | ||
| span(T*, size_t) -> span<T>; | ||
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| template <typename T> | ||
| constexpr span<std::byte const> as_bytes(span<T> s) { | ||
| return {reinterpret_cast<std::byte const*>(s.data()), s.size_bytes()}; | ||
| } | ||
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| template <typename T> | ||
| constexpr span<std::byte> as_writable_bytes(span<T> s) { | ||
| return {reinterpret_cast<std::byte*>(s.data()), s.size_bytes()}; | ||
| } | ||
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| } // namespace arrow::util |
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