102c6a61bc
The former is very similar to the latter, but less general (mostly in naming). This CL, which is the first to use Maybe at scale, also removes the implicit conversion from T to Maybe<T>, since it was agreed that the increased verbosity increased legibility. Review URL: https://codereview.webrtc.org/1430433004 Cr-Commit-Position: refs/heads/master@{#10461}
111 lines
3.5 KiB
C++
111 lines
3.5 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_MAYBE_H_
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#define WEBRTC_BASE_MAYBE_H_
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#include <algorithm>
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#include <utility>
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#include "webrtc/base/checks.h"
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namespace rtc {
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// Simple std::experimental::optional-wannabe. It either contains a T or not.
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// In order to keep the implementation simple and portable, this implementation
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// actually contains a (default-constructed) T even when it supposedly doesn't
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// contain a value; use e.g. rtc::scoped_ptr<T> instead if that's too
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// expensive.
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//
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// A moved-from Maybe<T> may only be destroyed, and assigned to if T allows
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// being assigned to after having been moved from. Specifically, you may not
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// assume that it just doesn't contain a value anymore.
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//
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// TODO(kwiberg): Get rid of this class when the standard library has
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// std::optional (and we're allowed to use it).
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template <typename T>
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class Maybe final {
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public:
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// Construct an empty Maybe.
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Maybe() : has_value_(false) {}
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// Construct a Maybe that contains a value.
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explicit Maybe(const T& val) : value_(val), has_value_(true) {}
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explicit Maybe(T&& val) : value_(static_cast<T&&>(val)), has_value_(true) {}
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// Copy and move constructors.
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// TODO(kwiberg): =default the move constructor when MSVC supports it.
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Maybe(const Maybe&) = default;
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Maybe(Maybe&& m)
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: value_(static_cast<T&&>(m.value_)), has_value_(m.has_value_) {}
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// Assignment.
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// TODO(kwiberg): =default the move assignment op when MSVC supports it.
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Maybe& operator=(const Maybe&) = default;
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Maybe& operator=(Maybe&& m) {
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value_ = static_cast<T&&>(m.value_);
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has_value_ = m.has_value_;
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return *this;
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}
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friend void swap(Maybe& m1, Maybe& m2) {
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using std::swap;
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swap(m1.value_, m2.value_);
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swap(m1.has_value_, m2.has_value_);
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}
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// Conversion to bool to test if we have a value.
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explicit operator bool() const { return has_value_; }
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// Dereferencing. Only allowed if we have a value.
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const T* operator->() const {
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RTC_DCHECK(has_value_);
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return &value_;
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}
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T* operator->() {
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RTC_DCHECK(has_value_);
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return &value_;
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}
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const T& operator*() const {
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RTC_DCHECK(has_value_);
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return value_;
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}
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T& operator*() {
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RTC_DCHECK(has_value_);
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return value_;
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}
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// Dereference with a default value in case we don't have a value.
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const T& value_or(const T& default_val) const {
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return has_value_ ? value_ : default_val;
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}
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// Equality tests. Two Maybes are equal if they contain equivalent values, or
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// if they're both empty.
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friend bool operator==(const Maybe& m1, const Maybe& m2) {
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return m1.has_value_ && m2.has_value_ ? m1.value_ == m2.value_
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: m1.has_value_ == m2.has_value_;
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}
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friend bool operator!=(const Maybe& m1, const Maybe& m2) {
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return m1.has_value_ && m2.has_value_ ? m1.value_ != m2.value_
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: m1.has_value_ != m2.has_value_;
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}
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private:
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// Invariant: Unless *this has been moved from, value_ is default-initialized
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// (or copied or moved from a default-initialized T) if !has_value_.
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T value_;
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bool has_value_;
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};
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} // namespace rtc
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#endif // WEBRTC_BASE_MAYBE_H_
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