/* Copyright 2017 The TensorFlow Authors. All Rights Reserved.

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

    http://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.
==============================================================================*/

// StatusOr<T> is the union of a Status object and a T
// object. StatusOr models the concept of an object that is either a
// usable value, or an error Status explaining why such a value is
// not present. To this end, StatusOr<T> does not allow its Status
// value to be Status::OK. Furthermore, the value of a StatusOr<T*>
// must not be null. This is enforced by a debug check in most cases,
// but even when it is not, clients must not set the value to null.
//
// The primary use-case for StatusOr<T> is as the return value of a
// function which may fail.
//
// Example client usage for a StatusOr<T>, where T is not a pointer:
//
//  StatusOr<float> result = DoBigCalculationThatCouldFail();
//  if (result.ok()) {
//    float answer = result.ValueOrDie();
//    printf("Big calculation yielded: %f", answer);
//  } else {
//    LOG(ERROR) << result.status();
//  }
//
// Example client usage for a StatusOr<T*>:
//
//  StatusOr<Foo*> result = FooFactory::MakeNewFoo(arg);
//  if (result.ok()) {
//    std::unique_ptr<Foo> foo(result.ValueOrDie());
//    foo->DoSomethingCool();
//  } else {
//    LOG(ERROR) << result.status();
//  }
//
// Example client usage for a StatusOr<std::unique_ptr<T>>:
//
//  StatusOr<std::unique_ptr<Foo>> result = FooFactory::MakeNewFoo(arg);
//  if (result.ok()) {
//    std::unique_ptr<Foo> foo = std::move(result.ValueOrDie());
//    foo->DoSomethingCool();
//  } else {
//    LOG(ERROR) << result.status();
//  }
//
// Example factory implementation returning StatusOr<T*>:
//
//  StatusOr<Foo*> FooFactory::MakeNewFoo(int arg) {
//    if (arg <= 0) {
//      return tensorflow::InvalidArgument("Arg must be positive");
//    } else {
//      return new Foo(arg);
//    }
//  }
//
// Note that the assignment operators require that destroying the currently
// stored value cannot invalidate the argument; in other words, the argument
// cannot be an alias for the current value, or anything owned by the current
// value.
#ifndef TENSORFLOW_COMPILER_XLA_STATUSOR_H_
#define TENSORFLOW_COMPILER_XLA_STATUSOR_H_

#include "tensorflow/compiler/xla/status.h"
#include "tensorflow/compiler/xla/statusor_internals.h"
#include "tensorflow/core/platform/macros.h"

namespace xla {

#if defined(__clang__)
// Only clang supports warn_unused_result as a type annotation.
template <typename T>
class TF_MUST_USE_RESULT StatusOr;
#endif

template <typename T>
class StatusOr : private internal_statusor::StatusOrData<T>,
                 private internal_statusor::TraitsBase<
                     std::is_copy_constructible<T>::value,
                     std::is_move_constructible<T>::value> {
  template <typename U>
  friend class StatusOr;

  typedef internal_statusor::StatusOrData<T> Base;

 public:
  typedef T element_type;

  // Constructs a new StatusOr with Status::UNKNOWN status.  This is marked
  // 'explicit' to try to catch cases like 'return {};', where people think
  // StatusOr<std::vector<int>> will be initialized with an empty vector,
  // instead of a Status::UNKNOWN status.
  explicit StatusOr();

  // StatusOr<T> will be copy constructible/assignable if T is copy
  // constructible.
  StatusOr(const StatusOr&) = default;
  StatusOr& operator=(const StatusOr&) = default;

  // StatusOr<T> will be move constructible/assignable if T is move
  // constructible.
  StatusOr(StatusOr&&) = default;
  StatusOr& operator=(StatusOr&&) = default;

  // Conversion copy/move constructor, T must be convertible from U.
  // TODO(b/62186717): These should not participate in overload resolution if U
  // is not convertible to T.
  template <typename U>
  StatusOr(const StatusOr<U>& other);
  template <typename U>
  StatusOr(StatusOr<U>&& other);

  // Conversion copy/move assignment operator, T must be convertible from U.
  template <typename U>
  StatusOr& operator=(const StatusOr<U>& other);
  template <typename U>
  StatusOr& operator=(StatusOr<U>&& other);

  // Constructs a new StatusOr with the given value. After calling this
  // constructor, calls to ValueOrDie() will succeed, and calls to status() will
  // return OK.
  //
  // NOTE: Not explicit - we want to use StatusOr<T> as a return type
  // so it is convenient and sensible to be able to do 'return T()'
  // when the return type is StatusOr<T>.
  //
  // REQUIRES: T is copy constructible.
  StatusOr(const T& value);

  // Constructs a new StatusOr with the given non-ok status. After calling
  // this constructor, calls to ValueOrDie() will CHECK-fail.
  //
  // NOTE: Not explicit - we want to use StatusOr<T> as a return
  // value, so it is convenient and sensible to be able to do 'return
  // Status()' when the return type is StatusOr<T>.
  //
  // REQUIRES: !status.ok(). This requirement is DCHECKed.
  // In optimized builds, passing Status::OK() here will have the effect
  // of passing tensorflow::error::INTERNAL as a fallback.
  StatusOr(const Status& status);
  StatusOr& operator=(const Status& status);

  // TODO(b/62186997): Add operator=(T) overloads.

  // Similar to the `const T&` overload.
  //
  // REQUIRES: T is move constructible.
  StatusOr(T&& value);

  // RValue versions of the operations declared above.
  StatusOr(Status&& status);
  StatusOr& operator=(Status&& status);

  // Returns this->status().ok()
  bool ok() const { return this->status_.ok(); }

  // Returns a reference to our status. If this contains a T, then
  // returns Status::OK().
  const Status& status() const &;
  Status status() &&;

  // Returns a reference to our current value, or CHECK-fails if !this->ok().
  //
  // Note: for value types that are cheap to copy, prefer simple code:
  //
  //   T value = statusor.ValueOrDie();
  //
  // Otherwise, if the value type is expensive to copy, but can be left
  // in the StatusOr, simply assign to a reference:
  //
  //   T& value = statusor.ValueOrDie();  // or `const T&`
  //
  // Otherwise, if the value type supports an efficient move, it can be
  // used as follows:
  //
  //   T value = std::move(statusor).ValueOrDie();
  //
  // The std::move on statusor instead of on the whole expression enables
  // warnings about possible uses of the statusor object after the move.
  // C++ style guide waiver for ref-qualified overloads granted in cl/143176389
  // See go/ref-qualifiers for more details on such overloads.
  const T& ValueOrDie() const &;
  T& ValueOrDie() &;
  const T&& ValueOrDie() const &&;
  T&& ValueOrDie() &&;

  T ConsumeValueOrDie() { return std::move(ValueOrDie()); }

  // Ignores any errors. This method does nothing except potentially suppress
  // complaints from any tools that are checking that errors are not dropped on
  // the floor.
  void IgnoreError() const;
};

////////////////////////////////////////////////////////////////////////////////
// Implementation details for StatusOr<T>

template <typename T>
StatusOr<T>::StatusOr() : Base(Status(tensorflow::error::UNKNOWN, "")) {}

template <typename T>
StatusOr<T>::StatusOr(const T& value) : Base(value) {}

template <typename T>
StatusOr<T>::StatusOr(const Status& status) : Base(status) {}

template <typename T>
StatusOr<T>& StatusOr<T>::operator=(const Status& status) {
  this->Assign(status);
  return *this;
}

template <typename T>
StatusOr<T>::StatusOr(T&& value) : Base(std::move(value)) {}

template <typename T>
StatusOr<T>::StatusOr(Status&& status) : Base(std::move(status)) {}

template <typename T>
StatusOr<T>& StatusOr<T>::operator=(Status&& status) {
  this->Assign(std::move(status));
  return *this;
}

template <typename T>
template <typename U>
inline StatusOr<T>::StatusOr(const StatusOr<U>& other)
    : Base(static_cast<const typename StatusOr<U>::Base&>(other)) {}

template <typename T>
template <typename U>
inline StatusOr<T>& StatusOr<T>::operator=(const StatusOr<U>& other) {
  if (other.ok())
    this->Assign(other.ValueOrDie());
  else
    this->Assign(other.status());
  return *this;
}

template <typename T>
template <typename U>
inline StatusOr<T>::StatusOr(StatusOr<U>&& other)
    : Base(static_cast<typename StatusOr<U>::Base&&>(other)) {}

template <typename T>
template <typename U>
inline StatusOr<T>& StatusOr<T>::operator=(StatusOr<U>&& other) {
  if (other.ok()) {
    this->Assign(std::move(other).ValueOrDie());
  } else {
    this->Assign(std::move(other).status());
  }
  return *this;
}

template <typename T>
const Status& StatusOr<T>::status() const & {
  return this->status_;
}
template <typename T>
Status StatusOr<T>::status() && {
  return ok() ? Status::OK() : std::move(this->status_);
}

template <typename T>
const T& StatusOr<T>::ValueOrDie() const & {
  this->EnsureOk();
  return this->data_;
}

template <typename T>
T& StatusOr<T>::ValueOrDie() & {
  this->EnsureOk();
  return this->data_;
}

template <typename T>
const T&& StatusOr<T>::ValueOrDie() const && {
  this->EnsureOk();
  return std::move(this->data_);
}

template <typename T>
T&& StatusOr<T>::ValueOrDie() && {
  this->EnsureOk();
  return std::move(this->data_);
}

template <typename T>
void StatusOr<T>::IgnoreError() const {
  // no-op
}

}  // namespace xla

#endif  // TENSORFLOW_COMPILER_XLA_STATUSOR_H_
