| // Copyright 2017 The Chromium Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style license that can be |
| // found in the LICENSE file. |
| |
| #ifndef BASE_CONTAINERS_SPAN_H_ |
| #define BASE_CONTAINERS_SPAN_H_ |
| |
| #include <stddef.h> |
| #include <stdint.h> |
| |
| #include <algorithm> |
| #include <array> |
| #include <iterator> |
| #include <string_view> |
| #include <type_traits> |
| #include <utility> |
| |
| #include "base/logging.h" |
| #include "base/stl_util.h" |
| |
| namespace base { |
| |
| // [views.constants] |
| constexpr size_t dynamic_extent = static_cast<size_t>(-1); |
| |
| template <typename T, size_t Extent = dynamic_extent> |
| class span; |
| |
| namespace internal { |
| |
| template <typename T> |
| struct IsSpanImpl : std::false_type {}; |
| |
| template <typename T, size_t Extent> |
| struct IsSpanImpl<span<T, Extent>> : std::true_type {}; |
| |
| template <typename T> |
| using IsSpan = IsSpanImpl<std::decay_t<T>>; |
| |
| template <typename T> |
| struct IsStdArrayImpl : std::false_type {}; |
| |
| template <typename T, size_t N> |
| struct IsStdArrayImpl<std::array<T, N>> : std::true_type {}; |
| |
| template <typename T> |
| using IsStdArray = IsStdArrayImpl<std::decay_t<T>>; |
| |
| template <typename T> |
| using IsCArray = std::is_array<std::remove_reference_t<T>>; |
| |
| template <typename From, typename To> |
| using IsLegalDataConversion = std::is_convertible<From (*)[], To (*)[]>; |
| |
| template <typename Container, typename T> |
| using ContainerHasConvertibleData = IsLegalDataConversion< |
| std::remove_pointer_t<decltype(std::data(std::declval<Container>()))>, |
| T>; |
| |
| template <typename Container> |
| using ContainerHasIntegralSize = |
| std::is_integral<decltype(std::size(std::declval<Container>()))>; |
| |
| template <typename From, size_t FromExtent, typename To, size_t ToExtent> |
| using EnableIfLegalSpanConversion = |
| std::enable_if_t<(ToExtent == dynamic_extent || ToExtent == FromExtent) && |
| IsLegalDataConversion<From, To>::value>; |
| |
| // SFINAE check if Array can be converted to a span<T>. |
| template <typename Array, size_t N, typename T, size_t Extent> |
| using EnableIfSpanCompatibleArray = |
| std::enable_if_t<(Extent == dynamic_extent || Extent == N) && |
| ContainerHasConvertibleData<Array, T>::value>; |
| |
| // SFINAE check if Container can be converted to a span<T>. |
| template <typename Container, typename T> |
| using EnableIfSpanCompatibleContainer = |
| std::enable_if_t<!internal::IsSpan<Container>::value && |
| !internal::IsStdArray<Container>::value && |
| !internal::IsCArray<Container>::value && |
| ContainerHasConvertibleData<Container, T>::value && |
| ContainerHasIntegralSize<Container>::value>; |
| |
| } // namespace internal |
| |
| // A span is a value type that represents an array of elements of type T. Since |
| // it only consists of a pointer to memory with an associated size, it is very |
| // light-weight. It is cheap to construct, copy, move and use spans, so that |
| // users are encouraged to use it as a pass-by-value parameter. A span does not |
| // own the underlying memory, so care must be taken to ensure that a span does |
| // not outlive the backing store. |
| // |
| // span is somewhat analogous to std::string_view, but with arbitrary element |
| // types, allowing mutation if T is non-const. |
| // |
| // span is implicitly convertible from C++ arrays, as well as most [1] |
| // container-like types that provide a data() and size() method (such as |
| // std::vector<T>). A mutable span<T> can also be implicitly converted to an |
| // immutable span<const T>. |
| // |
| // Consider using a span for functions that take a data pointer and size |
| // parameter: it allows the function to still act on an array-like type, while |
| // allowing the caller code to be a bit more concise. |
| // |
| // For read-only data access pass a span<const T>: the caller can supply either |
| // a span<const T> or a span<T>, while the callee will have a read-only view. |
| // For read-write access a mutable span<T> is required. |
| // |
| // Without span: |
| // Read-Only: |
| // // std::string HexEncode(const uint8_t* data, size_t size); |
| // std::vector<uint8_t> data_buffer = GenerateData(); |
| // std::string r = HexEncode(data_buffer.data(), data_buffer.size()); |
| // |
| // Mutable: |
| // // ssize_t SafeSNPrintf(char* buf, size_t N, const char* fmt, Args...); |
| // char str_buffer[100]; |
| // SafeSNPrintf(str_buffer, sizeof(str_buffer), "Pi ~= %lf", 3.14); |
| // |
| // With span: |
| // Read-Only: |
| // // std::string HexEncode(base::span<const uint8_t> data); |
| // std::vector<uint8_t> data_buffer = GenerateData(); |
| // std::string r = HexEncode(data_buffer); |
| // |
| // Mutable: |
| // // ssize_t SafeSNPrintf(base::span<char>, const char* fmt, Args...); |
| // char str_buffer[100]; |
| // SafeSNPrintf(str_buffer, "Pi ~= %lf", 3.14); |
| // |
| // Spans with "const" and pointers |
| // ------------------------------- |
| // |
| // Const and pointers can get confusing. Here are vectors of pointers and their |
| // corresponding spans: |
| // |
| // const std::vector<int*> => base::span<int* const> |
| // std::vector<const int*> => base::span<const int*> |
| // const std::vector<const int*> => base::span<const int* const> |
| // |
| // Differences from the working group proposal |
| // ------------------------------------------- |
| // |
| // https://wg21.link/P0122 is the latest working group proposal, Chromium |
| // currently implements R7. Differences between the proposal and the |
| // implementation are documented in subsections below. |
| // |
| // Differences from [span.objectrep]: |
| // - as_bytes() and as_writable_bytes() return spans of uint8_t instead of |
| // std::byte |
| // |
| // Differences in constants and types: |
| // - index_type is aliased to size_t |
| // |
| // Differences from [span.sub]: |
| // - using size_t instead of ptrdiff_t for indexing |
| // |
| // Differences from [span.obs]: |
| // - using size_t instead of ptrdiff_t to represent size() |
| // |
| // Differences from [span.elem]: |
| // - using size_t instead of ptrdiff_t for indexing |
| // |
| // Furthermore, all constructors and methods are marked noexcept due to the lack |
| // of exceptions in Chromium. |
| // |
| // Due to the lack of class template argument deduction guides in C++14 |
| // appropriate make_span() utility functions are provided. |
| |
| // [span], class template span |
| template <typename T, size_t Extent> |
| class span { |
| public: |
| using element_type = T; |
| using value_type = std::remove_cv_t<T>; |
| using index_type = size_t; |
| using difference_type = ptrdiff_t; |
| using pointer = T*; |
| using reference = T&; |
| using iterator = T*; |
| using const_iterator = const T*; |
| using reverse_iterator = std::reverse_iterator<iterator>; |
| using const_reverse_iterator = std::reverse_iterator<const_iterator>; |
| static constexpr index_type extent = Extent; |
| |
| // [span.cons], span constructors, copy, assignment, and destructor |
| constexpr span() noexcept : data_(nullptr), size_(0) { |
| static_assert(Extent == dynamic_extent || Extent == 0, "Invalid Extent"); |
| } |
| |
| constexpr span(T* data, size_t size) noexcept : data_(data), size_(size) { |
| CHECK(Extent == dynamic_extent || Extent == size); |
| } |
| |
| // Artificially templatized to break ambiguity for span(ptr, 0). |
| template <typename = void> |
| constexpr span(T* begin, T* end) noexcept : span(begin, end - begin) { |
| // Note: CHECK_LE is not constexpr, hence regular CHECK must be used. |
| CHECK(begin <= end); |
| } |
| |
| template < |
| size_t N, |
| typename = internal::EnableIfSpanCompatibleArray<T (&)[N], N, T, Extent>> |
| constexpr span(T (&array)[N]) noexcept : span(std::data(array), N) {} |
| |
| template < |
| size_t N, |
| typename = internal:: |
| EnableIfSpanCompatibleArray<std::array<value_type, N>&, N, T, Extent>> |
| constexpr span(std::array<value_type, N>& array) noexcept |
| : span(std::data(array), N) {} |
| |
| template <size_t N, |
| typename = internal::EnableIfSpanCompatibleArray< |
| const std::array<value_type, N>&, |
| N, |
| T, |
| Extent>> |
| constexpr span(const std::array<value_type, N>& array) noexcept |
| : span(std::data(array), N) {} |
| |
| // Conversion from a container that has compatible std::data() and integral |
| // std::size(). |
| template <typename Container, |
| typename = internal::EnableIfSpanCompatibleContainer<Container&, T>> |
| constexpr span(Container& container) noexcept |
| : span(std::data(container), std::size(container)) {} |
| |
| template < |
| typename Container, |
| typename = internal::EnableIfSpanCompatibleContainer<const Container&, T>> |
| span(const Container& container) noexcept |
| : span(std::data(container), std::size(container)) {} |
| |
| constexpr span(const span& other) noexcept = default; |
| |
| // Conversions from spans of compatible types and extents: this allows a |
| // span<T> to be seamlessly used as a span<const T>, but not the other way |
| // around. If extent is not dynamic, OtherExtent has to be equal to Extent. |
| template < |
| typename U, |
| size_t OtherExtent, |
| typename = |
| internal::EnableIfLegalSpanConversion<U, OtherExtent, T, Extent>> |
| constexpr span(const span<U, OtherExtent>& other) |
| : span(other.data(), other.size()) {} |
| |
| constexpr span& operator=(const span& other) noexcept = default; |
| ~span() noexcept = default; |
| |
| // [span.sub], span subviews |
| template <size_t Count> |
| constexpr span<T, Count> first() const noexcept { |
| static_assert(Extent == dynamic_extent || Count <= Extent, |
| "Count must not exceed Extent"); |
| CHECK(Extent != dynamic_extent || Count <= size()); |
| return {data(), Count}; |
| } |
| |
| template <size_t Count> |
| constexpr span<T, Count> last() const noexcept { |
| static_assert(Extent == dynamic_extent || Count <= Extent, |
| "Count must not exceed Extent"); |
| CHECK(Extent != dynamic_extent || Count <= size()); |
| return {data() + (size() - Count), Count}; |
| } |
| |
| template <size_t Offset, size_t Count = dynamic_extent> |
| constexpr span<T, |
| (Count != dynamic_extent |
| ? Count |
| : (Extent != dynamic_extent ? Extent - Offset |
| : dynamic_extent))> |
| subspan() const noexcept { |
| static_assert(Extent == dynamic_extent || Offset <= Extent, |
| "Offset must not exceed Extent"); |
| static_assert(Extent == dynamic_extent || Count == dynamic_extent || |
| Count <= Extent - Offset, |
| "Count must not exceed Extent - Offset"); |
| CHECK(Extent != dynamic_extent || Offset <= size()); |
| CHECK(Extent != dynamic_extent || Count == dynamic_extent || |
| Count <= size() - Offset); |
| return {data() + Offset, Count != dynamic_extent ? Count : size() - Offset}; |
| } |
| |
| constexpr span<T, dynamic_extent> first(size_t count) const noexcept { |
| // Note: CHECK_LE is not constexpr, hence regular CHECK must be used. |
| CHECK(count <= size()); |
| return {data(), count}; |
| } |
| |
| constexpr span<T, dynamic_extent> last(size_t count) const noexcept { |
| // Note: CHECK_LE is not constexpr, hence regular CHECK must be used. |
| CHECK(count <= size()); |
| return {data() + (size() - count), count}; |
| } |
| |
| constexpr span<T, dynamic_extent> subspan( |
| size_t offset, |
| size_t count = dynamic_extent) const noexcept { |
| // Note: CHECK_LE is not constexpr, hence regular CHECK must be used. |
| CHECK(offset <= size()); |
| CHECK(count == dynamic_extent || count <= size() - offset); |
| return {data() + offset, count != dynamic_extent ? count : size() - offset}; |
| } |
| |
| // [span.obs], span observers |
| constexpr size_t size() const noexcept { return size_; } |
| constexpr size_t size_bytes() const noexcept { return size() * sizeof(T); } |
| constexpr bool empty() const noexcept { return size() == 0; } |
| |
| // [span.elem], span element access |
| constexpr T& operator[](size_t idx) const noexcept { |
| // Note: CHECK_LT is not constexpr, hence regular CHECK must be used. |
| CHECK(idx < size()); |
| return *(data() + idx); |
| } |
| |
| constexpr T& operator()(size_t idx) const noexcept { |
| // Note: CHECK_LT is not constexpr, hence regular CHECK must be used. |
| CHECK(idx < size()); |
| return *(data() + idx); |
| } |
| |
| constexpr T* data() const noexcept { return data_; } |
| |
| // [span.iter], span iterator support |
| constexpr iterator begin() const noexcept { return data(); } |
| constexpr iterator end() const noexcept { return data() + size(); } |
| |
| constexpr const_iterator cbegin() const noexcept { return begin(); } |
| constexpr const_iterator cend() const noexcept { return end(); } |
| |
| constexpr reverse_iterator rbegin() const noexcept { |
| return reverse_iterator(end()); |
| } |
| constexpr reverse_iterator rend() const noexcept { |
| return reverse_iterator(begin()); |
| } |
| |
| constexpr const_reverse_iterator crbegin() const noexcept { |
| return const_reverse_iterator(cend()); |
| } |
| constexpr const_reverse_iterator crend() const noexcept { |
| return const_reverse_iterator(cbegin()); |
| } |
| |
| private: |
| T* data_; |
| size_t size_; |
| }; |
| |
| // span<T, Extent>::extent can not be declared inline prior to C++17, hence this |
| // definition is required. |
| template <class T, size_t Extent> |
| constexpr size_t span<T, Extent>::extent; |
| |
| // [span.comparison], span comparison operators |
| // Relational operators. Equality is a element-wise comparison. |
| template <typename T, size_t X, typename U, size_t Y> |
| constexpr bool operator==(span<T, X> lhs, span<U, Y> rhs) noexcept { |
| return std::equal(lhs.cbegin(), lhs.cend(), rhs.cbegin(), rhs.cend()); |
| } |
| |
| template <typename T, size_t X, typename U, size_t Y> |
| constexpr bool operator!=(span<T, X> lhs, span<U, Y> rhs) noexcept { |
| return !(lhs == rhs); |
| } |
| |
| template <typename T, size_t X, typename U, size_t Y> |
| constexpr bool operator<(span<T, X> lhs, span<U, Y> rhs) noexcept { |
| return std::lexicographical_compare(lhs.cbegin(), lhs.cend(), rhs.cbegin(), |
| rhs.cend()); |
| } |
| |
| template <typename T, size_t X, typename U, size_t Y> |
| constexpr bool operator<=(span<T, X> lhs, span<U, Y> rhs) noexcept { |
| return !(rhs < lhs); |
| } |
| |
| template <typename T, size_t X, typename U, size_t Y> |
| constexpr bool operator>(span<T, X> lhs, span<U, Y> rhs) noexcept { |
| return rhs < lhs; |
| } |
| |
| template <typename T, size_t X, typename U, size_t Y> |
| constexpr bool operator>=(span<T, X> lhs, span<U, Y> rhs) noexcept { |
| return !(lhs < rhs); |
| } |
| |
| // [span.objectrep], views of object representation |
| template <typename T, size_t X> |
| span<const uint8_t, (X == dynamic_extent ? dynamic_extent : sizeof(T) * X)> |
| as_bytes(span<T, X> s) noexcept { |
| return {reinterpret_cast<const uint8_t*>(s.data()), s.size_bytes()}; |
| } |
| |
| template <typename T, |
| size_t X, |
| typename = std::enable_if_t<!std::is_const<T>::value>> |
| span<uint8_t, (X == dynamic_extent ? dynamic_extent : sizeof(T) * X)> |
| as_writable_bytes(span<T, X> s) noexcept { |
| return {reinterpret_cast<uint8_t*>(s.data()), s.size_bytes()}; |
| } |
| |
| // Type-deducing helpers for constructing a span. |
| template <typename T> |
| constexpr span<T> make_span(T* data, size_t size) noexcept { |
| return {data, size}; |
| } |
| |
| template <typename T> |
| constexpr span<T> make_span(T* begin, T* end) noexcept { |
| return {begin, end}; |
| } |
| |
| template <typename T, size_t N> |
| constexpr span<T, N> make_span(T (&array)[N]) noexcept { |
| return array; |
| } |
| |
| template <typename T, size_t N> |
| constexpr span<T, N> make_span(std::array<T, N>& array) noexcept { |
| return array; |
| } |
| |
| template <typename T, size_t N> |
| constexpr span<const T, N> make_span(const std::array<T, N>& array) noexcept { |
| return array; |
| } |
| |
| template <typename Container, |
| typename T = typename Container::value_type, |
| typename = internal::EnableIfSpanCompatibleContainer<Container&, T>> |
| constexpr span<T> make_span(Container& container) noexcept { |
| return container; |
| } |
| |
| template < |
| typename Container, |
| typename T = const typename Container::value_type, |
| typename = internal::EnableIfSpanCompatibleContainer<const Container&, T>> |
| constexpr span<T> make_span(const Container& container) noexcept { |
| return container; |
| } |
| |
| template <typename T, size_t X> |
| constexpr span<T, X> make_span(const span<T, X>& span) noexcept { |
| return span; |
| } |
| |
| } // namespace base |
| |
| #endif // BASE_CONTAINERS_SPAN_H_ |