| // 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_ |