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/*
* Copyright 2015 The WebRTC Project Authors. All rights reserved.
*
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
*/
#ifndef API_ARRAY_VIEW_H_
#define API_ARRAY_VIEW_H_
#include <algorithm>
#include <array>
#include <cstddef>
#include <iterator>
#include <type_traits>
#include "rtc_base/checks.h"
#include "rtc_base/type_traits.h"
namespace rtc {
// tl;dr: rtc::ArrayView is the same thing as gsl::span from the Guideline
// Support Library.
//
// Many functions read from or write to arrays. The obvious way to do this is
// to use two arguments, a pointer to the first element and an element count:
//
// bool Contains17(const int* arr, size_t size) {
// for (size_t i = 0; i < size; ++i) {
// if (arr[i] == 17)
// return true;
// }
// return false;
// }
//
// This is flexible, since it doesn't matter how the array is stored (C array,
// std::vector, rtc::Buffer, ...), but it's error-prone because the caller has
// to correctly specify the array length:
//
// Contains17(arr, arraysize(arr)); // C array
// Contains17(arr.data(), arr.size()); // std::vector
// Contains17(arr, size); // pointer + size
// ...
//
// It's also kind of messy to have two separate arguments for what is
// conceptually a single thing.
//
// Enter rtc::ArrayView<T>. It contains a T pointer (to an array it doesn't
// own) and a count, and supports the basic things you'd expect, such as
// indexing and iteration. It allows us to write our function like this:
//
// bool Contains17(rtc::ArrayView<const int> arr) {
// for (auto e : arr) {
// if (e == 17)
// return true;
// }
// return false;
// }
//
// And even better, because a bunch of things will implicitly convert to
// ArrayView, we can call it like this:
//
// Contains17(arr); // C array
// Contains17(arr); // std::vector
// Contains17(rtc::ArrayView<int>(arr, size)); // pointer + size
// Contains17(nullptr); // nullptr -> empty ArrayView
// ...
//
// ArrayView<T> stores both a pointer and a size, but you may also use
// ArrayView<T, N>, which has a size that's fixed at compile time (which means
// it only has to store the pointer).
//
// One important point is that ArrayView<T> and ArrayView<const T> are
// different types, which allow and don't allow mutation of the array elements,
// respectively. The implicit conversions work just like you'd hope, so that
// e.g. vector<int> will convert to either ArrayView<int> or ArrayView<const
// int>, but const vector<int> will convert only to ArrayView<const int>.
// (ArrayView itself can be the source type in such conversions, so
// ArrayView<int> will convert to ArrayView<const int>.)
//
// Note: ArrayView is tiny (just a pointer and a count if variable-sized, just
// a pointer if fix-sized) and trivially copyable, so it's probably cheaper to
// pass it by value than by const reference.
namespace array_view_internal {
// Magic constant for indicating that the size of an ArrayView is variable
// instead of fixed.
enum : std::ptrdiff_t { kArrayViewVarSize = -4711 };
// Base class for ArrayViews of fixed nonzero size.
template <typename T, std::ptrdiff_t Size>
class ArrayViewBase {
static_assert(Size > 0, "ArrayView size must be variable or non-negative");
public:
ArrayViewBase(T* data, size_t size) : data_(data) {}
static constexpr size_t size() { return Size; }
static constexpr bool empty() { return false; }
T* data() const { return data_; }
protected:
static constexpr bool fixed_size() { return true; }
private:
T* data_;
};
// Specialized base class for ArrayViews of fixed zero size.
template <typename T>
class ArrayViewBase<T, 0> {
public:
explicit ArrayViewBase(T* data, size_t size) {}
static constexpr size_t size() { return 0; }
static constexpr bool empty() { return true; }
T* data() const { return nullptr; }
protected:
static constexpr bool fixed_size() { return true; }
};
// Specialized base class for ArrayViews of variable size.
template <typename T>
class ArrayViewBase<T, array_view_internal::kArrayViewVarSize> {
public:
ArrayViewBase(T* data, size_t size)
: data_(size == 0 ? nullptr : data), size_(size) {}
size_t size() const { return size_; }
bool empty() const { return size_ == 0; }
T* data() const { return data_; }
protected:
static constexpr bool fixed_size() { return false; }
private:
T* data_;
size_t size_;
};
} // namespace array_view_internal
template <typename T,
std::ptrdiff_t Size = array_view_internal::kArrayViewVarSize>
class ArrayView final : public array_view_internal::ArrayViewBase<T, Size> {
public:
using value_type = T;
using reference = value_type&;
using const_reference = const value_type&;
using pointer = value_type*;
using const_pointer = const value_type*;
using const_iterator = const T*;
// Construct an ArrayView from a pointer and a length.
template <typename U>
ArrayView(U* data, size_t size)
: array_view_internal::ArrayViewBase<T, Size>::ArrayViewBase(data, size) {
RTC_DCHECK_EQ(size == 0 ? nullptr : data, this->data());
RTC_DCHECK_EQ(size, this->size());
RTC_DCHECK_EQ(!this->data(),
this->size() == 0); // data is null iff size == 0.
}
// Construct an empty ArrayView. Note that fixed-size ArrayViews of size > 0
// cannot be empty.
ArrayView() : ArrayView(nullptr, 0) {}
ArrayView(std::nullptr_t) // NOLINT
: ArrayView() {}
ArrayView(std::nullptr_t, size_t size)
: ArrayView(static_cast<T*>(nullptr), size) {
static_assert(Size == 0 || Size == array_view_internal::kArrayViewVarSize,
"");
RTC_DCHECK_EQ(0, size);
}
// Construct an ArrayView from a C-style array.
template <typename U, size_t N>
ArrayView(U (&array)[N]) // NOLINT
: ArrayView(array, N) {
static_assert(Size == N || Size == array_view_internal::kArrayViewVarSize,
"Array size must match ArrayView size");
}
// (Only if size is fixed.) Construct a fixed size ArrayView<T, N> from a
// non-const std::array instance. For an ArrayView with variable size, the
// used ctor is ArrayView(U& u) instead.
template <typename U,
size_t N,
typename std::enable_if<
Size == static_cast<std::ptrdiff_t>(N)>::type* = nullptr>
ArrayView(std::array<U, N>& u) // NOLINT
: ArrayView(u.data(), u.size()) {}
// (Only if size is fixed.) Construct a fixed size ArrayView<T, N> where T is
// const from a const(expr) std::array instance. For an ArrayView with
// variable size, the used ctor is ArrayView(U& u) instead.
template <typename U,
size_t N,
typename std::enable_if<
Size == static_cast<std::ptrdiff_t>(N)>::type* = nullptr>
ArrayView(const std::array<U, N>& u) // NOLINT
: ArrayView(u.data(), u.size()) {}
// (Only if size is fixed.) Construct an ArrayView from any type U that has a
// static constexpr size() method whose return value is equal to Size, and a
// data() method whose return value converts implicitly to T*. In particular,
// this means we allow conversion from ArrayView<T, N> to ArrayView<const T,
// N>, but not the other way around. We also don't allow conversion from
// ArrayView<T> to ArrayView<T, N>, or from ArrayView<T, M> to ArrayView<T,
// N> when M != N.
template <
typename U,
typename std::enable_if<Size != array_view_internal::kArrayViewVarSize &&
HasDataAndSize<U, T>::value>::type* = nullptr>
ArrayView(U& u) // NOLINT
: ArrayView(u.data(), u.size()) {
static_assert(U::size() == Size, "Sizes must match exactly");
}
template <
typename U,
typename std::enable_if<Size != array_view_internal::kArrayViewVarSize &&
HasDataAndSize<U, T>::value>::type* = nullptr>
ArrayView(const U& u) // NOLINT(runtime/explicit)
: ArrayView(u.data(), u.size()) {
static_assert(U::size() == Size, "Sizes must match exactly");
}
// (Only if size is variable.) Construct an ArrayView from any type U that
// has a size() method whose return value converts implicitly to size_t, and
// a data() method whose return value converts implicitly to T*. In
// particular, this means we allow conversion from ArrayView<T> to
// ArrayView<const T>, but not the other way around. Other allowed
// conversions include
// ArrayView<T, N> to ArrayView<T> or ArrayView<const T>,
// std::vector<T> to ArrayView<T> or ArrayView<const T>,
// const std::vector<T> to ArrayView<const T>,
// rtc::Buffer to ArrayView<uint8_t> or ArrayView<const uint8_t>, and
// const rtc::Buffer to ArrayView<const uint8_t>.
template <
typename U,
typename std::enable_if<Size == array_view_internal::kArrayViewVarSize &&
HasDataAndSize<U, T>::value>::type* = nullptr>
ArrayView(U& u) // NOLINT
: ArrayView(u.data(), u.size()) {}
template <
typename U,
typename std::enable_if<Size == array_view_internal::kArrayViewVarSize &&
HasDataAndSize<U, T>::value>::type* = nullptr>
ArrayView(const U& u) // NOLINT(runtime/explicit)
: ArrayView(u.data(), u.size()) {}
// Indexing and iteration. These allow mutation even if the ArrayView is
// const, because the ArrayView doesn't own the array. (To prevent mutation,
// use a const element type.)
T& operator[](size_t idx) const {
RTC_DCHECK_LT(idx, this->size());
RTC_DCHECK(this->data());
return this->data()[idx];
}
T* begin() const { return this->data(); }
T* end() const { return this->data() + this->size(); }
const T* cbegin() const { return this->data(); }
const T* cend() const { return this->data() + this->size(); }
std::reverse_iterator<T*> rbegin() const {
return std::make_reverse_iterator(end());
}
std::reverse_iterator<T*> rend() const {
return std::make_reverse_iterator(begin());
}
std::reverse_iterator<const T*> crbegin() const {
return std::make_reverse_iterator(cend());
}
std::reverse_iterator<const T*> crend() const {
return std::make_reverse_iterator(cbegin());
}
ArrayView<T> subview(size_t offset, size_t size) const {
return offset < this->size()
? ArrayView<T>(this->data() + offset,
std::min(size, this->size() - offset))
: ArrayView<T>();
}
ArrayView<T> subview(size_t offset) const {
return subview(offset, this->size());
}
};
// Comparing two ArrayViews compares their (pointer,size) pairs; it does *not*
// dereference the pointers.
template <typename T, std::ptrdiff_t Size1, std::ptrdiff_t Size2>
bool operator==(const ArrayView<T, Size1>& a, const ArrayView<T, Size2>& b) {
return a.data() == b.data() && a.size() == b.size();
}
template <typename T, std::ptrdiff_t Size1, std::ptrdiff_t Size2>
bool operator!=(const ArrayView<T, Size1>& a, const ArrayView<T, Size2>& b) {
return !(a == b);
}
// Variable-size ArrayViews are the size of two pointers; fixed-size ArrayViews
// are the size of one pointer. (And as a special case, fixed-size ArrayViews
// of size 0 require no storage.)
static_assert(sizeof(ArrayView<int>) == 2 * sizeof(int*), "");
static_assert(sizeof(ArrayView<int, 17>) == sizeof(int*), "");
static_assert(std::is_empty<ArrayView<int, 0>>::value, "");
template <typename T>
inline ArrayView<T> MakeArrayView(T* data, size_t size) {
return ArrayView<T>(data, size);
}
// Only for primitive types that have the same size and aligment.
// Allow reinterpret cast of the array view to another primitive type of the
// same size.
// Template arguments order is (U, T, Size) to allow deduction of the template
// arguments in client calls: reinterpret_array_view<target_type>(array_view).
template <typename U, typename T, std::ptrdiff_t Size>
inline ArrayView<U, Size> reinterpret_array_view(ArrayView<T, Size> view) {
static_assert(sizeof(U) == sizeof(T) && alignof(U) == alignof(T),
"ArrayView reinterpret_cast is only supported for casting "
"between views that represent the same chunk of memory.");
static_assert(
std::is_fundamental<T>::value && std::is_fundamental<U>::value,
"ArrayView reinterpret_cast is only supported for casting between "
"fundamental types.");
return ArrayView<U, Size>(reinterpret_cast<U*>(view.data()), view.size());
}
} // namespace rtc
#endif // API_ARRAY_VIEW_H_