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//
// Copyright 2017 The Abseil Authors.
//
// Licensed 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.
//
// -----------------------------------------------------------------------------
// span.h
// -----------------------------------------------------------------------------
//
// This header file defines a `Span<T>` type for holding a view of an existing
// array of data. The `Span` object, much like the `absl::string_view` object,
// does not own such data itself. A span provides a lightweight way to pass
// around view of such data.
//
// Additionally, this header file defines `MakeSpan()` and `MakeConstSpan()`
// factory functions, for clearly creating spans of type `Span<T>` or read-only
// `Span<const T>` when such types may be difficult to identify due to issues
// with implicit conversion.
//
// The C++ standards committee currently has a proposal for a `std::span` type,
// (http://wg21.link/p0122), which is not yet part of the standard (though may
// become part of C++20). As of August 2017, the differences between
// `absl::Span` and this proposal are:
// * `absl::Span` uses `size_t` for `size_type`
// * `absl::Span` has no `operator()`
// * `absl::Span` has no constructors for `std::unique_ptr` or
// `std::shared_ptr`
// * `absl::span` has the factory functions `MakeSpan()` and
// `MakeConstSpan()`
// * `absl::Span` has `front()` and `back()` methods
// * bounds-checked access to `absl::Span` is accomplished with `at()`
// * `absl::Span` has compiler-provided move and copy constructors and
// assignment. This is due to them being specified as `constexpr`, but that
// implies const in C++11.
// * `absl::Span` has no `element_type` or `index_type` typedefs
// * A read-only `absl::Span<const T>` can be implicitly constructed from an
// initializer list.
// * `absl::Span` has no `bytes()`, `size_bytes()`, `as_bytes()`, or
// `as_mutable_bytes()` methods
// * `absl::Span` has no static extent template parameter, nor constructors
// which exist only because of the static extent parameter.
// * `absl::Span` has an explicit mutable-reference constructor
//
// For more information, see the class comments below.
#ifndef ABSL_TYPES_SPAN_H_
#define ABSL_TYPES_SPAN_H_
#include <algorithm>
#include <cassert>
#include <cstddef>
#include <initializer_list>
#include <iterator>
#include <string>
#include <type_traits>
#include <utility>
#include "absl/algorithm/algorithm.h"
#include "absl/base/internal/throw_delegate.h"
#include "absl/base/macros.h"
#include "absl/base/optimization.h"
#include "absl/base/port.h"
#include "absl/meta/type_traits.h"
namespace absl {
template <typename T>
class Span;
namespace span_internal {
// A constexpr min function
constexpr size_t Min(size_t a, size_t b) noexcept { return a < b ? a : b; }
// Wrappers for access to container data pointers.
template <typename C>
constexpr auto GetDataImpl(C& c, char) noexcept // NOLINT(runtime/references)
-> decltype(c.data()) {
return c.data();
}
// Before C++17, std::string::data returns a const char* in all cases.
inline char* GetDataImpl(std::string& s, // NOLINT(runtime/references)
int) noexcept {
return &s[0];
}
template <typename C>
constexpr auto GetData(C& c) noexcept // NOLINT(runtime/references)
-> decltype(GetDataImpl(c, 0)) {
return GetDataImpl(c, 0);
}
// Detection idioms for size() and data().
template <typename C>
using HasSize =
std::is_integral<absl::decay_t<decltype(std::declval<C&>().size())>>;
// We want to enable conversion from vector<T*> to Span<const T* const> but
// disable conversion from vector<Derived> to Span<Base>. Here we use
// the fact that U** is convertible to Q* const* if and only if Q is the same
// type or a more cv-qualified version of U. We also decay the result type of
// data() to avoid problems with classes which have a member function data()
// which returns a reference.
template <typename T, typename C>
using HasData =
std::is_convertible<absl::decay_t<decltype(GetData(std::declval<C&>()))>*,
T* const*>;
// Extracts value type from a Container
template <typename C>
struct ElementType {
using type = typename absl::remove_reference_t<C>::value_type;
};
template <typename T, size_t N>
struct ElementType<T (&)[N]> {
using type = T;
};
template <typename C>
using ElementT = typename ElementType<C>::type;
template <typename T>
using EnableIfMutable =
typename std::enable_if<!std::is_const<T>::value, int>::type;
template <typename T>
bool EqualImpl(Span<T> a, Span<T> b) {
static_assert(std::is_const<T>::value, "");
return absl::equal(a.begin(), a.end(), b.begin(), b.end());
}
template <typename T>
bool LessThanImpl(Span<T> a, Span<T> b) {
static_assert(std::is_const<T>::value, "");
return std::lexicographical_compare(a.begin(), a.end(), b.begin(), b.end());
}
// The `IsConvertible` classes here are needed because of the
// `std::is_convertible` bug in libcxx when compiled with GCC. This build
// configuration is used by Android NDK toolchain. Reference link:
// https://bugs.llvm.org/show_bug.cgi?id=27538.
template <typename From, typename To>
struct IsConvertibleHelper {
private:
static std::true_type test(To);
static std::false_type test(...);
public:
using type = decltype(test(std::declval<From>()));
};
template <typename From, typename To>
struct IsConvertible : IsConvertibleHelper<From, To>::type {};
// TODO(zhangxy): replace `IsConvertible` with `std::is_convertible` once the
// older version of libcxx is not supported.
template <typename From, typename To>
using EnableIfConvertibleToSpanConst =
typename std::enable_if<IsConvertible<From, Span<const To>>::value>::type;
} // namespace span_internal
//------------------------------------------------------------------------------
// Span
//------------------------------------------------------------------------------
//
// A `Span` is an "array view" type for holding a view of a contiguous data
// array; the `Span` object does not and cannot own such data itself. A span
// provides an easy way to provide overloads for anything operating on
// contiguous sequences without needing to manage pointers and array lengths
// manually.
// A span is conceptually a pointer (ptr) and a length (size) into an already
// existing array of contiguous memory; the array it represents references the
// elements "ptr[0] .. ptr[size-1]". Passing a properly-constructed `Span`
// instead of raw pointers avoids many issues related to index out of bounds
// errors.
//
// Spans may also be constructed from containers holding contiguous sequences.
// Such containers must supply `data()` and `size() const` methods (e.g
// `std::vector<T>`, `absl::InlinedVector<T, N>`). All implicit conversions to
// `absl::Span` from such containers will create spans of type `const T`;
// spans which can mutate their values (of type `T`) must use explicit
// constructors.
//
// A `Span<T>` is somewhat analogous to an `absl::string_view`, but for an array
// of elements of type `T`. A user of `Span` must ensure that the data being
// pointed to outlives the `Span` itself.
//
// You can construct a `Span<T>` in several ways:
//
// * Explicitly from a reference to a container type
// * Explicitly from a pointer and size
// * Implicitly from a container type (but only for spans of type `const T`)
// * Using the `MakeSpan()` or `MakeConstSpan()` factory functions.
//
// Examples:
//
// // Construct a Span explicitly from a container:
// std::vector<int> v = {1, 2, 3, 4, 5};
// auto span = absl::Span<const int>(v);
//
// // Construct a Span explicitly from a C-style array:
// int a[5] = {1, 2, 3, 4, 5};
// auto span = absl::Span<const int>(a);
//
// // Construct a Span implicitly from a container
// void MyRoutine(absl::Span<const int> a) {
// ...
// };
// std::vector v = {1,2,3,4,5};
// MyRoutine(v) // convert to Span<const T>
//
// Note that `Span` objects, in addition to requiring that the memory they
// point to remains alive, must also ensure that such memory does not get
// reallocated. Therefore, to avoid undefined behavior, containers with
// associated span views should not invoke operations that may reallocate memory
// (such as resizing) or invalidate iterarors into the container.
//
// One common use for a `Span` is when passing arguments to a routine that can
// accept a variety of array types (e.g. a `std::vector`, `absl::InlinedVector`,
// a C-style array, etc.). Instead of creating overloads for each case, you
// can simply specify a `Span` as the argument to such a routine.
//
// Example:
//
// void MyRoutine(absl::Span<const int> a) {
// ...
// };
//
// std::vector v = {1,2,3,4,5};
// MyRoutine(v);
//
// absl::InlinedVector<int, 4> my_inline_vector;
// MyRoutine(my_inline_vector);
//
// // Explicit constructor from pointer,size
// int* my_array = new int[10];
// MyRoutine(absl::Span<const int>(my_array, 10));
template <typename T>
class Span {
private:
// Used to determine whether a Span can be constructed from a container of
// type C.
template <typename C>
using EnableIfConvertibleFrom =
typename std::enable_if<span_internal::HasData<T, C>::value &&
span_internal::HasSize<C>::value>::type;
// Used to SFINAE-enable a function when the slice elements are const.
template <typename U>
using EnableIfConstView =
typename std::enable_if<std::is_const<T>::value, U>::type;
// Used to SFINAE-enable a function when the slice elements are mutable.
template <typename U>
using EnableIfMutableView =
typename std::enable_if<!std::is_const<T>::value, U>::type;
public:
using value_type = absl::remove_cv_t<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;
using reverse_iterator = std::reverse_iterator<iterator>;
using const_reverse_iterator = std::reverse_iterator<const_iterator>;
using size_type = size_t;
using difference_type = ptrdiff_t;
static const size_type npos = -1;
constexpr Span() noexcept : Span(nullptr, 0) {}
constexpr Span(pointer array, size_type length) noexcept
: ptr_(array), len_(length) {}
// Implicit conversion constructors
template <size_t N>
constexpr Span(T (&a)[N]) noexcept // NOLINT(runtime/explicit)
: Span(a, N) {}
// Explicit reference constructor for a mutable `Span<T>` type
template <typename V, typename = EnableIfConvertibleFrom<V>,
typename = EnableIfMutableView<V>>
explicit Span(V& v) noexcept // NOLINT(runtime/references)
: Span(span_internal::GetData(v), v.size()) {}
// Implicit reference constructor for a read-only `Span<const T>` type
template <typename V, typename = EnableIfConvertibleFrom<V>,
typename = EnableIfConstView<V>>
constexpr Span(const V& v) noexcept // NOLINT(runtime/explicit)
: Span(span_internal::GetData(v), v.size()) {}
// Implicit constructor from an initializer list, making it possible to pass a
// brace-enclosed initializer list to a function expecting a `Span`. Such
// spans constructed from an initializer list must be of type `Span<const T>`.
//
// void Process(absl::Span<const int> x);
// Process({1, 2, 3});
//
// Note that as always the array referenced by the span must outlive the span.
// Since an initializer list constructor acts as if it is fed a temporary
// array (cf. C++ standard [dcl.init.list]/5), it's safe to use this
// constructor only when the `std::initializer_list` itself outlives the span.
// In order to meet this requirement it's sufficient to ensure that neither
// the span nor a copy of it is used outside of the expression in which it's
// created:
//
// // Assume that this function uses the array directly, not retaining any
// // copy of the span or pointer to any of its elements.
// void Process(absl::Span<const int> ints);
//
// // Okay: the std::initializer_list<int> will reference a temporary array
// // that isn't destroyed until after the call to Process returns.
// Process({ 17, 19 });
//
// // Not okay: the storage used by the std::initializer_list<int> is not
// // allowed to be referenced after the first line.
// absl::Span<const int> ints = { 17, 19 };
// Process(ints);
//
// // Not okay for the same reason as above: even when the elements of the
// // initializer list expression are not temporaries the underlying array
// // is, so the initializer list must still outlive the span.
// const int foo = 17;
// absl::Span<const int> ints = { foo };
// Process(ints);
//
template <typename LazyT = T,
typename = EnableIfConstView<LazyT>>
Span(
std::initializer_list<value_type> v) noexcept // NOLINT(runtime/explicit)
: Span(v.begin(), v.size()) {}
// Accessors
// Span::data()
//
// Returns a pointer to the span's underlying array of data (which is held
// outside the span).
constexpr pointer data() const noexcept { return ptr_; }
// Span::size()
//
// Returns the size of this span.
constexpr size_type size() const noexcept { return len_; }
// Span::length()
//
// Returns the length (size) of this span.
constexpr size_type length() const noexcept { return size(); }
// Span::empty()
//
// Returns a boolean indicating whether or not this span is considered empty.
constexpr bool empty() const noexcept { return size() == 0; }
// Span::operator[]
//
// Returns a reference to the i'th element of this span.
constexpr reference operator[](size_type i) const noexcept {
// MSVC 2015 accepts this as constexpr, but not ptr_[i]
return *(data() + i);
}
// Span::at()
//
// Returns a reference to the i'th element of this span.
constexpr reference at(size_type i) const {
return ABSL_PREDICT_FALSE(i < size())
? ptr_[i]
: (base_internal::ThrowStdOutOfRange(
"Span::at failed bounds check"),
ptr_[i]);
}
// Span::front()
//
// Returns a reference to the first element of this span.
reference front() const noexcept { return ABSL_ASSERT(size() > 0), ptr_[0]; }
// Span::back()
//
// Returns a reference to the last element of this span.
reference back() const noexcept {
return ABSL_ASSERT(size() > 0), ptr_[size() - 1];
}
// Span::begin()
//
// Returns an iterator to the first element of this span.
constexpr iterator begin() const noexcept { return ptr_; }
// Span::cbegin()
//
// Returns a const iterator to the first element of this span.
constexpr const_iterator cbegin() const noexcept { return ptr_; }
// Span::end()
//
// Returns an iterator to the last element of this span.
iterator end() const noexcept { return ptr_ + len_; }
// Span::cend()
//
// Returns a const iterator to the last element of this span.
const_iterator cend() const noexcept { return end(); }
// Span::rbegin()
//
// Returns a reverse iterator starting at the last element of this span.
reverse_iterator rbegin() const noexcept { return reverse_iterator(end()); }
// Span::crbegin()
//
// Returns a reverse const iterator starting at the last element of this span.
const_reverse_iterator crbegin() const noexcept { return rbegin(); }
// Span::rend()
//
// Returns a reverse iterator starting at the first element of this span.
reverse_iterator rend() const noexcept { return reverse_iterator(begin()); }
// Span::crend()
//
// Returns a reverse iterator starting at the first element of this span.
const_reverse_iterator crend() const noexcept { return rend(); }
// Span mutations
// Span::remove_prefix()
//
// Removes the first `n` elements from the span.
void remove_prefix(size_type n) noexcept {
assert(len_ >= n);
ptr_ += n;
len_ -= n;
}
// Span::remove_suffix()
//
// Removes the last `n` elements from the span.
void remove_suffix(size_type n) noexcept {
assert(len_ >= n);
len_ -= n;
}
// Span::subspan()
//
// Returns a `Span` starting at element `pos` and of length `len`, with
// proper bounds checking to ensure `len` does not exceed the ptr+size of the
// original array. (Spans whose `len` would point past the end of the array
// will throw a `std::out_of_range`.)
constexpr Span subspan(size_type pos = 0, size_type len = npos) const {
return (pos <= len_)
? Span(ptr_ + pos, span_internal::Min(len_ - pos, len))
: (base_internal::ThrowStdOutOfRange("pos > size()"), Span());
}
private:
pointer ptr_;
size_type len_;
};
template <typename T>
const typename Span<T>::size_type Span<T>::npos;
// Span relationals
// Equality is compared element-by-element, while ordering is lexicographical.
// We provide three overloads for each operator to cover any combination on the
// left or right hand side of mutable Span<T>, read-only Span<const T>, and
// convertible-to-read-only Span<T>.
// TODO(zhangxy): Due to MSVC overload resolution bug with partial ordering
// template functions, 5 overloads per operator is needed as a workaround. We
// should update them to 3 overloads per operator using non-deduced context like
// string_view, i.e.
// - (Span<T>, Span<T>)
// - (Span<T>, non_deduced<Span<const T>>)
// - (non_deduced<Span<const T>>, Span<T>)
// operator==
template <typename T>
bool operator==(Span<T> a, Span<T> b) {
return span_internal::EqualImpl<const T>(a, b);
}
template <typename T>
bool operator==(Span<const T> a, Span<T> b) {
return span_internal::EqualImpl<const T>(a, b);
}
template <typename T>
bool operator==(Span<T> a, Span<const T> b) {
return span_internal::EqualImpl<const T>(a, b);
}
template <typename T, typename U,
typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
bool operator==(const U& a, Span<T> b) {
return span_internal::EqualImpl<const T>(a, b);
}
template <typename T, typename U,
typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
bool operator==(Span<T> a, const U& b) {
return span_internal::EqualImpl<const T>(a, b);
}
// operator!=
template <typename T>
bool operator!=(Span<T> a, Span<T> b) {
return !(a == b);
}
template <typename T>
bool operator!=(Span<const T> a, Span<T> b) {
return !(a == b);
}
template <typename T>
bool operator!=(Span<T> a, Span<const T> b) {
return !(a == b);
}
template <typename T, typename U,
typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
bool operator!=(const U& a, Span<T> b) {
return !(a == b);
}
template <typename T, typename U,
typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
bool operator!=(Span<T> a, const U& b) {
return !(a == b);
}
// operator<
template <typename T>
bool operator<(Span<T> a, Span<T> b) {
return span_internal::LessThanImpl<const T>(a, b);
}
template <typename T>
bool operator<(Span<const T> a, Span<T> b) {
return span_internal::LessThanImpl<const T>(a, b);
}
template <typename T>
bool operator<(Span<T> a, Span<const T> b) {
return span_internal::LessThanImpl<const T>(a, b);
}
template <typename T, typename U,
typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
bool operator<(const U& a, Span<T> b) {
return span_internal::LessThanImpl<const T>(a, b);
}
template <typename T, typename U,
typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
bool operator<(Span<T> a, const U& b) {
return span_internal::LessThanImpl<const T>(a, b);
}
// operator>
template <typename T>
bool operator>(Span<T> a, Span<T> b) {
return b < a;
}
template <typename T>
bool operator>(Span<const T> a, Span<T> b) {
return b < a;
}
template <typename T>
bool operator>(Span<T> a, Span<const T> b) {
return b < a;
}
template <typename T, typename U,
typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
bool operator>(const U& a, Span<T> b) {
return b < a;
}
template <typename T, typename U,
typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
bool operator>(Span<T> a, const U& b) {
return b < a;
}
// operator<=
template <typename T>
bool operator<=(Span<T> a, Span<T> b) {
return !(b < a);
}
template <typename T>
bool operator<=(Span<const T> a, Span<T> b) {
return !(b < a);
}
template <typename T>
bool operator<=(Span<T> a, Span<const T> b) {
return !(b < a);
}
template <typename T, typename U,
typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
bool operator<=(const U& a, Span<T> b) {
return !(b < a);
}
template <typename T, typename U,
typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
bool operator<=(Span<T> a, const U& b) {
return !(b < a);
}
// operator>=
template <typename T>
bool operator>=(Span<T> a, Span<T> b) {
return !(a < b);
}
template <typename T>
bool operator>=(Span<const T> a, Span<T> b) {
return !(a < b);
}
template <typename T>
bool operator>=(Span<T> a, Span<const T> b) {
return !(a < b);
}
template <typename T, typename U,
typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
bool operator>=(const U& a, Span<T> b) {
return !(a < b);
}
template <typename T, typename U,
typename = span_internal::EnableIfConvertibleToSpanConst<U, T>>
bool operator>=(Span<T> a, const U& b) {
return !(a < b);
}
// MakeSpan()
//
// Constructs a mutable `Span<T>`, deducing `T` automatically from either a
// container or pointer+size.
//
// Because a read-only `Span<const T>` is implicitly constructed from container
// types regardless of whether the container itself is a const container,
// constructing mutable spans of type `Span<T>` from containers requires
// explicit constructors. The container-accepting version of `MakeSpan()`
// deduces the type of `T` by the constness of the pointer received from the
// container's `data()` member. Similarly, the pointer-accepting version returns
// a `Span<const T>` if `T` is `const`, and a `Span<T>` otherwise.
//
// Examples:
//
// void MyRoutine(absl::Span<MyComplicatedType> a) {
// ...
// };
// // my_vector is a container of non-const types
// std::vector<MyComplicatedType> my_vector;
//
// // Constructing a Span implicitly attempts to create a Span of type
// // `Span<const T>`
// MyRoutine(my_vector); // error, type mismatch
//
// // Explicitly constructing the Span is verbose
// MyRoutine(absl::Span<MyComplicatedType>(my_vector);
//
// // Use MakeSpan() to make an absl::Span<T>
// MyRoutine(absl::MakeSpan(my_vector));
//
// // Construct a span from an array ptr+size
// absl::Span<T> my_span() {
// return absl::MakeSpan(&array[0], num_elements_);
// }
//
template <int&... ExplicitArgumentBarrier, typename T>
constexpr Span<T> MakeSpan(T* ptr, size_t size) noexcept {
return Span<T>(ptr, size);
}
template <int&... ExplicitArgumentBarrier, typename T>
Span<T> MakeSpan(T* begin, T* end) noexcept {
return ABSL_ASSERT(begin <= end), Span<T>(begin, end - begin);
}
template <int&... ExplicitArgumentBarrier, typename C>
constexpr auto MakeSpan(C& c) noexcept // NOLINT(runtime/references)
-> decltype(absl::MakeSpan(span_internal::GetData(c), c.size())) {
return MakeSpan(span_internal::GetData(c), c.size());
}
template <int&... ExplicitArgumentBarrier, typename T, size_t N>
constexpr Span<T> MakeSpan(T (&array)[N]) noexcept {
return Span<T>(array, N);
}
// MakeConstSpan()
//
// Constructs a `Span<const T>` as with `MakeSpan`, deducing `T` automatically,
// but always returning a `Span<const T>`.
//
// Examples:
//
// void ProcessInts(absl::Span<const int> some_ints);
//
// // Call with a pointer and size.
// int array[3] = { 0, 0, 0 };
// ProcessInts(absl::MakeConstSpan(&array[0], 3));
//
// // Call with a [begin, end) pair.
// ProcessInts(absl::MakeConstSpan(&array[0], &array[3]));
//
// // Call directly with an array.
// ProcessInts(absl::MakeConstSpan(array));
//
// // Call with a contiguous container.
// std::vector<int> some_ints = ...;
// ProcessInts(absl::MakeConstSpan(some_ints));
// ProcessInts(absl::MakeConstSpan(std::vector<int>{ 0, 0, 0 }));
//
template <int&... ExplicitArgumentBarrier, typename T>
constexpr Span<const T> MakeConstSpan(T* ptr, size_t size) noexcept {
return Span<const T>(ptr, size);
}
template <int&... ExplicitArgumentBarrier, typename T>
Span<const T> MakeConstSpan(T* begin, T* end) noexcept {
return ABSL_ASSERT(begin <= end), Span<const T>(begin, end - begin);
}
template <int&... ExplicitArgumentBarrier, typename C>
constexpr auto MakeConstSpan(const C& c) noexcept -> decltype(MakeSpan(c)) {
return MakeSpan(c);
}
template <int&... ExplicitArgumentBarrier, typename T, size_t N>
constexpr Span<const T> MakeConstSpan(const T (&array)[N]) noexcept {
return Span<const T>(array, N);
}
} // namespace absl
#endif // ABSL_TYPES_SPAN_H_