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