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// Copyright 2019 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// Definition of the Maybe<T> template class.
#ifndef EMBOSS_PUBLIC_EMBOSS_MAYBE_H_
#define EMBOSS_PUBLIC_EMBOSS_MAYBE_H_
#include <utility>
#include "runtime/cpp/emboss_defines.h"
namespace emboss {
// TODO(bolms): Should Maybe be a public type (i.e., live in ::emboss)?
namespace support {
// Maybe<T> is similar to, but much more restricted than, C++17's std::optional.
// It is intended for use in Emboss's expression system, wherein a non-Known()
// Maybe<T> will usually (but not always) poison the result of an operation.
//
// As such, Maybe<> is intended for use with small, copyable T's: specifically,
// integers, enums, and booleans. It may not perform well with other types.
template <typename T>
class Maybe final {
public:
constexpr Maybe() : value_(), known_(false) {}
constexpr explicit Maybe(T value)
: value_(::std::move(value)), known_(true) {}
constexpr Maybe(const Maybe<T> &) = default;
~Maybe() = default;
Maybe &operator=(const Maybe &) = default;
Maybe &operator=(T value) {
value_ = ::std::move(value);
known_ = true;
return *this;
}
Maybe &operator=(const T &value) {
value_ = value;
known_ = true;
return *this;
}
constexpr bool Known() const { return known_; }
T Value() const {
EMBOSS_CHECK(Known());
return value_;
}
constexpr T ValueOr(T default_value) const {
return known_ ? value_ : default_value;
}
// A non-Ok() Maybe value-initializes value_ to a default (by explicitly
// calling the nullary constructor on value_ in the initializer list), so it
// is safe to just return value_ here. For integral types and enums, value_
// will be 0, for bool it will be false, and for other types it depends on the
// constructor's behavior.
constexpr T ValueOrDefault() const { return value_; }
private:
T value_;
bool known_;
};
} // namespace support
} // namespace emboss
#endif // EMBOSS_PUBLIC_EMBOSS_MAYBE_H_