blob: e98ab2e3ce7e9765386bcb8cbb47c92f4a7b42fa [file] [log] [blame]
/*
* 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_OPTIONAL_H_
#define WEBRTC_BASE_OPTIONAL_H_
#include <algorithm>
#include <memory>
#include <utility>
#include "webrtc/base/checks.h"
namespace rtc {
// Simple std::optional-wannabe. It either contains a T or not.
//
// A moved-from Optional<T> may only be destroyed, and assigned to if T allows
// being assigned to after having been moved from. Specifically, you may not
// assume that it just doesn't contain a value anymore.
//
// Examples of good places to use Optional:
//
// - As a class or struct member, when the member doesn't always have a value:
// struct Prisoner {
// std::string name;
// Optional<int> cell_number; // Empty if not currently incarcerated.
// };
//
// - As a return value for functions that may fail to return a value on all
// allowed inputs. For example, a function that searches an array might
// return an Optional<size_t> (the index where it found the element, or
// nothing if it didn't find it); and a function that parses numbers might
// return Optional<double> (the parsed number, or nothing if parsing failed).
//
// Examples of bad places to use Optional:
//
// - As a return value for functions that may fail because of disallowed
// inputs. For example, a string length function should not return
// Optional<size_t> so that it can return nothing in case the caller passed
// it a null pointer; the function should probably use RTC_[D]CHECK instead,
// and return plain size_t.
//
// - As a return value for functions that may fail to return a value on all
// allowed inputs, but need to tell the caller what went wrong. Returning
// Optional<double> when parsing a single number as in the example above
// might make sense, but any larger parse job is probably going to need to
// tell the caller what the problem was, not just that there was one.
//
// TODO(kwiberg): Get rid of this class when the standard library has
// std::optional (and we're allowed to use it).
template <typename T>
class Optional final {
public:
// Construct an empty Optional.
Optional() : has_value_(false), empty_('\0') {}
// Construct an Optional that contains a value.
explicit Optional(const T& value) : has_value_(true) {
new (&value_) T(value);
}
explicit Optional(T&& value) : has_value_(true) {
new (&value_) T(std::move(value));
}
// Copy constructor: copies the value from m if it has one.
Optional(const Optional& m) : has_value_(m.has_value_) {
if (has_value_)
new (&value_) T(m.value_);
}
// Move constructor: if m has a value, moves the value from m, leaving m
// still in a state where it has a value, but a moved-from one (the
// properties of which depends on T; the only general guarantee is that we
// can destroy m).
Optional(Optional&& m) : has_value_(m.has_value_) {
if (has_value_)
new (&value_) T(std::move(m.value_));
}
~Optional() {
if (has_value_)
value_.~T();
}
// Copy assignment. Uses T's copy assignment if both sides have a value, T's
// copy constructor if only the right-hand side has a value.
Optional& operator=(const Optional& m) {
if (m.has_value_) {
if (has_value_) {
value_ = m.value_; // T's copy assignment.
} else {
new (&value_) T(m.value_); // T's copy constructor.
has_value_ = true;
}
} else if (has_value_) {
value_.~T();
has_value_ = false;
}
return *this;
}
// Move assignment. Uses T's move assignment if both sides have a value, T's
// move constructor if only the right-hand side has a value. The state of m
// after it's been moved from is as for the move constructor.
Optional& operator=(Optional&& m) {
if (m.has_value_) {
if (has_value_) {
value_ = std::move(m.value_); // T's move assignment.
} else {
new (&value_) T(std::move(m.value_)); // T's move constructor.
has_value_ = true;
}
} else if (has_value_) {
value_.~T();
has_value_ = false;
}
return *this;
}
// Swap the values if both m1 and m2 have values; move the value if only one
// of them has one.
friend void swap(Optional& m1, Optional& m2) {
if (m1.has_value_) {
if (m2.has_value_) {
// Both have values: swap.
using std::swap;
swap(m1.value_, m2.value_);
} else {
// Only m1 has a value: move it to m2.
new (&m2.value_) T(std::move(m1.value_));
m1.value_.~T(); // Destroy the moved-from value.
m1.has_value_ = false;
m2.has_value_ = true;
}
} else if (m2.has_value_) {
// Only m2 has a value: move it to m1.
new (&m1.value_) T(std::move(m2.value_));
m2.value_.~T(); // Destroy the moved-from value.
m1.has_value_ = true;
m2.has_value_ = false;
}
}
// Conversion to bool to test if we have a value.
explicit operator bool() const { return has_value_; }
// Dereferencing. Only allowed if we have a value.
const T* operator->() const {
RTC_DCHECK(has_value_);
return &value_;
}
T* operator->() {
RTC_DCHECK(has_value_);
return &value_;
}
const T& operator*() const {
RTC_DCHECK(has_value_);
return value_;
}
T& operator*() {
RTC_DCHECK(has_value_);
return value_;
}
// Dereference with a default value in case we don't have a value.
const T& value_or(const T& default_val) const {
return has_value_ ? value_ : default_val;
}
// Equality tests. Two Optionals are equal if they contain equivalent values,
// or
// if they're both empty.
friend bool operator==(const Optional& m1, const Optional& m2) {
return m1.has_value_ && m2.has_value_ ? m1.value_ == m2.value_
: m1.has_value_ == m2.has_value_;
}
friend bool operator!=(const Optional& m1, const Optional& m2) {
return m1.has_value_ && m2.has_value_ ? m1.value_ != m2.value_
: m1.has_value_ != m2.has_value_;
}
private:
bool has_value_; // True iff value_ contains a live value.
union {
// empty_ exists only to make it possible to initialize the union, even when
// it doesn't contain any data. If the union goes uninitialized, it may
// trigger compiler warnings.
char empty_;
// By placing value_ in a union, we get to manage its construction and
// destruction manually: the Optional constructors won't automatically
// construct it, and the Optional destructor won't automatically destroy
// it. Basically, this just allocates a properly sized and aligned block of
// memory in which we can manually put a T with placement new.
T value_;
};
};
} // namespace rtc
#endif // WEBRTC_BASE_OPTIONAL_H_