134 lines
4.9 KiB
C++
134 lines
4.9 KiB
C++
/*
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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 WEBRTC_BASE_ARRAY_VIEW_H_
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#define WEBRTC_BASE_ARRAY_VIEW_H_
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#include "webrtc/base/checks.h"
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namespace rtc {
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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[0], 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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// ...
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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) and trivially copyable,
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// so it's probably cheaper to pass it by value than by const reference.
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template <typename T>
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class ArrayView final {
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public:
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// Construct an empty ArrayView.
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ArrayView() : ArrayView(static_cast<T*>(nullptr), 0) {}
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// Construct an ArrayView for a (pointer,size) pair.
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template <typename U>
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ArrayView(U* data, size_t size)
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: data_(size == 0 ? nullptr : data), size_(size) {
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CheckInvariant();
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}
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// Construct an ArrayView for an array.
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template <typename U, size_t N>
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ArrayView(U (&array)[N]) : ArrayView(&array[0], N) {}
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// Construct an ArrayView for any type U that has a size() method whose
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// return value converts implicitly to size_t, and a data() method whose
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// return value converts implicitly to T*. In particular, this means we allow
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// conversion from ArrayView<T> to ArrayView<const T>, but not the other way
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// around. Other allowed conversions include std::vector<T> to ArrayView<T>
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// or ArrayView<const T>, const std::vector<T> to ArrayView<const T>, and
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// rtc::Buffer to ArrayView<uint8_t> (with the same const behavior as
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// std::vector).
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template <typename U>
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ArrayView(U& u) : ArrayView(u.data(), u.size()) {}
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// Indexing, size, and iteration. These allow mutation even if the ArrayView
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// is const, because the ArrayView doesn't own the array. (To prevent
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// mutation, use ArrayView<const T>.)
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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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T& operator[](size_t idx) const {
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RTC_DCHECK_LT(idx, size_);
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RTC_DCHECK(data_); // Follows from size_ > idx and the class invariant.
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return data_[idx];
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}
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T* begin() const { return data_; }
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T* end() const { return data_ + size_; }
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const T* cbegin() const { return data_; }
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const T* cend() const { return data_ + size_; }
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// Comparing two ArrayViews compares their (pointer,size) pairs; it does
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// *not* dereference the pointers.
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friend bool operator==(const ArrayView& a, const ArrayView& b) {
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return a.data_ == b.data_ && a.size_ == b.size_;
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}
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friend bool operator!=(const ArrayView& a, const ArrayView& b) {
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return !(a == b);
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}
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private:
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// Invariant: !data_ iff size_ == 0.
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void CheckInvariant() const { RTC_DCHECK_EQ(!data_, size_ == 0); }
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T* data_;
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size_t size_;
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};
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} // namespace rtc
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#endif // WEBRTC_BASE_ARRAY_VIEW_H_
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