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// 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 <algorithm>
#include <array>
#include <iterator>
#include <limits>
#include <type_traits>
#include <utility>
#include "base/containers/checked_iterators.h"
#include "base/logging.h"
#include "base/macros.h"
#include "base/stl_util.h"
namespace base {
// [views.constants]
constexpr size_t dynamic_extent = std::numeric_limits<size_t>::max();
template <typename T, size_t Extent = dynamic_extent>
class span;
namespace internal {
template <typename T>
struct ExtentImpl : std::integral_constant<size_t, dynamic_extent> {};
template <typename T, size_t N>
struct ExtentImpl<T[N]> : std::integral_constant<size_t, N> {};
template <typename T, size_t N>
struct ExtentImpl<std::array<T, N>> : std::integral_constant<size_t, N> {};
template <typename T, size_t N>
struct ExtentImpl<base::span<T, N>> : std::integral_constant<size_t, N> {};
template <typename T>
using Extent = ExtentImpl<std::remove_cv_t<std::remove_reference_t<T>>>;
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(base::data(std::declval<Container>()))>,
T>;
template <typename Container>
using ContainerHasIntegralSize =
std::is_integral<decltype(base::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, typename T, size_t Extent>
using EnableIfSpanCompatibleArray =
std::enable_if_t<(Extent == dynamic_extent ||
Extent == internal::Extent<Array>::value) &&
ContainerHasConvertibleData<Array, T>::value>;
// SFINAE check if Container can be converted to a span<T>.
template <typename Container, typename T>
using IsSpanCompatibleContainer =
std::conditional_t<!IsSpan<Container>::value &&
!IsStdArray<Container>::value &&
!IsCArray<Container>::value &&
ContainerHasConvertibleData<Container, T>::value &&
ContainerHasIntegralSize<Container>::value,
std::true_type,
std::false_type>;
template <typename Container, typename T>
using EnableIfSpanCompatibleContainer =
std::enable_if_t<IsSpanCompatibleContainer<Container, T>::value>;
template <typename Container, typename T, size_t Extent>
using EnableIfSpanCompatibleContainerAndSpanIsDynamic =
std::enable_if_t<IsSpanCompatibleContainer<Container, T>::value &&
Extent == dynamic_extent>;
// A helper template for storing the size of a span. Spans with static extents
// don't require additional storage, since the extent itself is specified in the
// template parameter.
template <size_t Extent>
class ExtentStorage {
public:
constexpr explicit ExtentStorage(size_t size) noexcept {}
constexpr size_t size() const noexcept { return Extent; }
};
// Specialization of ExtentStorage for dynamic extents, which do require
// explicit storage for the size.
template <>
struct ExtentStorage<dynamic_extent> {
constexpr explicit ExtentStorage(size_t size) noexcept : size_(size) {}
constexpr size_t size() const noexcept { return size_; }
private:
size_t size_;
};
} // 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 StringPiece, 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 C++20 draft
// --------------------------------
//
// Chromium tries to follow the draft as close as possible. Differences between
// the draft 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 (std::byte is a C++17 feature)
//
// Differences from [span.cons]:
// - Constructing a static span (i.e. Extent != dynamic_extent) from a dynamic
// sized container (e.g. std::vector) requires an explicit conversion (in the
// C++20 draft this is simply UB)
//
// Differences from [span.obs]:
// - empty() is marked with WARN_UNUSED_RESULT instead of [[nodiscard]]
// ([[nodiscard]] is a C++17 feature)
//
// 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 internal::ExtentStorage<Extent> {
private:
using ExtentStorage = internal::ExtentStorage<Extent>;
public:
using element_type = T;
using value_type = std::remove_cv_t<T>;
using size_type = size_t;
using difference_type = ptrdiff_t;
using pointer = T*;
using reference = T&;
using iterator = CheckedContiguousIterator<T>;
using const_iterator = CheckedContiguousConstIterator<T>;
using reverse_iterator = std::reverse_iterator<iterator>;
using const_reverse_iterator = std::reverse_iterator<const_iterator>;
static constexpr size_t extent = Extent;
// [span.cons], span constructors, copy, assignment, and destructor
constexpr span() noexcept : ExtentStorage(0), data_(nullptr) {
static_assert(Extent == dynamic_extent || Extent == 0, "Invalid Extent");
}
constexpr span(T* data, size_t size) noexcept
: ExtentStorage(size), data_(data) {
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], T, Extent>>
constexpr span(T (&array)[N]) noexcept : span(base::data(array), N) {}
template <
size_t N,
typename = internal::
EnableIfSpanCompatibleArray<std::array<value_type, N>&, T, Extent>>
constexpr span(std::array<value_type, N>& array) noexcept
: span(base::data(array), N) {}
template <size_t N,
typename = internal::EnableIfSpanCompatibleArray<
const std::array<value_type, N>&,
T,
Extent>>
constexpr span(const std::array<value_type, N>& array) noexcept
: span(base::data(array), N) {}
// Conversion from a container that has compatible base::data() and integral
// base::size().
template <
typename Container,
typename =
internal::EnableIfSpanCompatibleContainerAndSpanIsDynamic<Container&,
T,
Extent>>
constexpr span(Container& container) noexcept
: span(base::data(container), base::size(container)) {}
template <
typename Container,
typename = internal::EnableIfSpanCompatibleContainerAndSpanIsDynamic<
const Container&,
T,
Extent>>
constexpr span(const Container& container) noexcept
: span(base::data(container), base::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 ExtentStorage::size(); }
constexpr size_t size_bytes() const noexcept { return size() * sizeof(T); }
constexpr bool empty() const noexcept WARN_UNUSED_RESULT {
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& front() const noexcept {
static_assert(Extent == dynamic_extent || Extent > 0,
"Extent must not be 0");
CHECK(Extent != dynamic_extent || !empty());
return *data();
}
constexpr T& back() const noexcept {
static_assert(Extent == dynamic_extent || Extent > 0,
"Extent must not be 0");
CHECK(Extent != dynamic_extent || !empty());
return *(data() + size() - 1);
}
constexpr T* data() const noexcept { return data_; }
// [span.iter], span iterator support
constexpr iterator begin() const noexcept {
return iterator(data_, data_ + size());
}
constexpr iterator end() const noexcept {
return iterator(data_, data_ + size(), 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_;
};
// 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.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 <int&... ExplicitArgumentBarrier, typename T>
constexpr span<T> make_span(T* data, size_t size) noexcept {
return {data, size};
}
template <int&... ExplicitArgumentBarrier, typename T>
constexpr span<T> make_span(T* begin, T* end) noexcept {
return {begin, end};
}
// make_span utility function that deduces both the span's value_type and extent
// from the passed in argument.
//
// Usage: auto span = base::make_span(...);
template <int&... ExplicitArgumentBarrier, typename Container>
constexpr auto make_span(Container&& container) noexcept {
using T =
std::remove_pointer_t<decltype(base::data(std::declval<Container>()))>;
using Extent = internal::Extent<Container>;
return span<T, Extent::value>(std::forward<Container>(container));
}
// make_span utility function that allows callers to explicit specify the span's
// extent, the value_type is deduced automatically. This is useful when passing
// a dynamically sized container to a method expecting static spans, when the
// container is known to have the correct size.
//
// Note: This will CHECK that N indeed matches size(container).
//
// Usage: auto static_span = base::make_span<N>(...);
template <size_t N, int&... ExplicitArgumentBarrier, typename Container>
constexpr auto make_span(Container&& container) noexcept {
using T =
std::remove_pointer_t<decltype(base::data(std::declval<Container>()))>;
return span<T, N>(base::data(container), base::size(container));
}
} // namespace base
// Note: std::tuple_size, std::tuple_element and std::get are specialized for
// static spans, so that they can be used in C++17's structured bindings. While
// we don't support C++17 yet, there is no harm in providing these
// specializations already.
namespace std {
// [span.tuple], tuple interface
#if defined(__clang__)
// respective fixes different versions of libc++ declare std::tuple_size and
// std::tuple_element either as classes or structs. In order to be able to
// specialize std::tuple_size and std::tuple_element for custom base types we
// thus need to disable -Wmismatched-tags in order to support all build
// configurations. Note that this is blessed by the standard in
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wmismatched-tags"
#endif
template <typename T, size_t X>
struct tuple_size<base::span<T, X>> : public integral_constant<size_t, X> {};
template <typename T>
struct tuple_size<base::span<T, base::dynamic_extent>>; // not defined
template <size_t I, typename T, size_t X>
struct tuple_element<I, base::span<T, X>> {
static_assert(
base::dynamic_extent != X,
"std::tuple_element<> not supported for base::span<T, dynamic_extent>");
static_assert(I < X,
"Index out of bounds in std::tuple_element<> (base::span)");
using type = T;
};
#if defined(__clang__)
#pragma clang diagnostic pop // -Wmismatched-tags
#endif
template <size_t I, typename T, size_t X>
constexpr T& get(base::span<T, X> s) noexcept {
static_assert(base::dynamic_extent != X,
"std::get<> not supported for base::span<T, dynamic_extent>");
static_assert(I < X, "Index out of bounds in std::get<> (base::span)");
return s[I];
}
} // namespace std
#endif // BASE_CONTAINERS_SPAN_H_