BoundedInlineVector: Vector class of bounded size with inline allocation
Selling point is that it never touches the heap. Intended use case is cheaply returning a variable, bounded, and small number of things from a function. Specifically, there are situations where we'd like to return things like ArrayView<ArrayView<float>> where we currently have to allocate an array of ArrayView<float> for the outer ArrayView to point to, which is a bother; however, although the outer ArrayView is of variable size, that size is statically guaranteed to not exceed some small constant. After this CL, we'll be able to instead return BoundedInlineVector<ArrayView<float>, kSmallConstant> which is much more convenient. We already had the option of returning e.g. std::vector<ArrayView<float>> but that would bloat our binary with code to handle heap allocations in places we'd rather be lean and mean. https://godbolt.org/z/r-vcPj demonstrates that the overhead compared to a raw C array + a size is ~zero. Bug: webrtc:11391 Change-Id: Ifb6d937193052588be641aa62cc67ba0ec64ded6 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/168944 Commit-Queue: Karl Wiberg <kwiberg@webrtc.org> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org> Reviewed-by: Per Åhgren <peah@webrtc.org> Reviewed-by: Danil Chapovalov <danilchap@webrtc.org> Cr-Commit-Position: refs/heads/master@{#30663}
This commit is contained in:
Karl Wiberg
138
rtc_base/bounded_inline_vector.h
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138
rtc_base/bounded_inline_vector.h
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/*
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* Copyright 2020 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 RTC_BASE_BOUNDED_INLINE_VECTOR_H_
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#define RTC_BASE_BOUNDED_INLINE_VECTOR_H_
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#include <stdint.h>
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#include <memory>
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#include <type_traits>
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#include <utility>
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#include "rtc_base/bounded_inline_vector_impl.h"
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#include "rtc_base/checks.h"
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namespace webrtc {
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// A small std::vector-like type whose capacity is a compile-time constant. It
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// stores all data inline and never heap allocates (beyond what its element type
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// requires). Trying to grow it beyond its constant capacity is an error.
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//
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// TODO(bugs.webrtc.org/11391): Comparison operators.
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// TODO(bugs.webrtc.org/11391): Methods for adding and deleting elements.
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template <typename T, int fixed_capacity>
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class BoundedInlineVector {
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static_assert(!std::is_const<T>::value, "T may not be const");
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static_assert(fixed_capacity > 0, "Capacity must be strictly positive");
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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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BoundedInlineVector() = default;
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BoundedInlineVector(const BoundedInlineVector&) = default;
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BoundedInlineVector(BoundedInlineVector&&) = default;
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BoundedInlineVector& operator=(const BoundedInlineVector&) = default;
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BoundedInlineVector& operator=(BoundedInlineVector&&) = default;
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~BoundedInlineVector() = default;
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// This constructor is implicit, to make it possible to write e.g.
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//
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// BoundedInlineVector<double, 7> x = {2.72, 3.14};
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//
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// and
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//
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// BoundedInlineVector<double, 7> GetConstants() {
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// return {2.72, 3.14};
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// }
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template <typename... Ts,
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typename std::enable_if_t<
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bounded_inline_vector_impl::AllConvertible<T, Ts...>::value>* =
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nullptr>
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BoundedInlineVector(Ts&&... elements) // NOLINT(runtime/explicit)
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: storage_(std::forward<Ts>(elements)...) {
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static_assert(sizeof...(Ts) <= fixed_capacity, "");
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}
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template <
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int other_capacity,
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typename std::enable_if_t<other_capacity != fixed_capacity>* = nullptr>
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BoundedInlineVector(const BoundedInlineVector<T, other_capacity>& other) {
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RTC_DCHECK_LE(other.size(), fixed_capacity);
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bounded_inline_vector_impl::CopyElements(other.data(), other.size(),
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storage_.data, &storage_.size);
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}
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template <
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int other_capacity,
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typename std::enable_if_t<other_capacity != fixed_capacity>* = nullptr>
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BoundedInlineVector(BoundedInlineVector<T, other_capacity>&& other) {
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RTC_DCHECK_LE(other.size(), fixed_capacity);
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bounded_inline_vector_impl::MoveElements(other.data(), other.size(),
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storage_.data, &storage_.size);
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}
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template <
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int other_capacity,
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typename std::enable_if_t<other_capacity != fixed_capacity>* = nullptr>
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BoundedInlineVector& operator=(
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const BoundedInlineVector<T, other_capacity>& other) {
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bounded_inline_vector_impl::DestroyElements(storage_.data, storage_.size);
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RTC_DCHECK_LE(other.size(), fixed_capacity);
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bounded_inline_vector_impl::CopyElements(other.data(), other.size(),
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storage_.data, &storage_.size);
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return *this;
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}
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template <
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int other_capacity,
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typename std::enable_if_t<other_capacity != fixed_capacity>* = nullptr>
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BoundedInlineVector& operator=(
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BoundedInlineVector<T, other_capacity>&& other) {
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bounded_inline_vector_impl::DestroyElements(storage_.data, storage_.size);
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RTC_DCHECK_LE(other.size(), fixed_capacity);
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bounded_inline_vector_impl::MoveElements(other.data(), other.size(),
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storage_.data, &storage_.size);
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return *this;
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}
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bool empty() const { return storage_.size == 0; }
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int size() const { return storage_.size; }
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constexpr int capacity() const { return fixed_capacity; }
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const T* data() const { return storage_.data; }
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T* data() { return storage_.data; }
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const T& operator[](int index) const {
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RTC_DCHECK_GE(index, 0);
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RTC_DCHECK_LT(index, storage_.size);
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return storage_.data[index];
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}
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T& operator[](int index) {
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RTC_DCHECK_GE(index, 0);
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RTC_DCHECK_LT(index, storage_.size);
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return storage_.data[index];
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}
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T* begin() { return storage_.data; }
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T* end() { return storage_.data + storage_.size; }
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const T* begin() const { return storage_.data; }
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const T* end() const { return storage_.data + storage_.size; }
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const T* cbegin() const { return storage_.data; }
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const T* cend() const { return storage_.data + storage_.size; }
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private:
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bounded_inline_vector_impl::Storage<T, fixed_capacity> storage_;
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};
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} // namespace webrtc
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#endif // RTC_BASE_BOUNDED_INLINE_VECTOR_H_
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