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