llvm/llvm/include/llvm/Support/ErrorOr.h

//===- llvm/Support/ErrorOr.h - Error Smart Pointer -------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
///
/// \file
///
/// Provides ErrorOr<T> smart pointer.
///
//===----------------------------------------------------------------------===//

#ifndef LLVM_SUPPORT_ERROROR_H
#define LLVM_SUPPORT_ERROROR_H

#include "llvm/Support/AlignOf.h"
#include <cassert>
#include <system_error>
#include <type_traits>
#include <utility>

namespace llvm {

/// Represents either an error or a value T.
///
/// ErrorOr<T> is a pointer-like class that represents the result of an
/// operation. The result is either an error, or a value of type T. This is
/// designed to emulate the usage of returning a pointer where nullptr indicates
/// failure. However instead of just knowing that the operation failed, we also
/// have an error_code and optional user data that describes why it failed.
///
/// It is used like the following.
/// \code
///   ErrorOr<Buffer> getBuffer();
///
///   auto buffer = getBuffer();
///   if (error_code ec = buffer.getError())
///     return ec;
///   buffer->write("adena");
/// \endcode
///
///
/// Implicit conversion to bool returns true if there is a usable value. The
/// unary * and -> operators provide pointer like access to the value. Accessing
/// the value when there is an error has undefined behavior.
///
/// When T is a reference type the behavior is slightly different. The reference
/// is held in a std::reference_wrapper<std::remove_reference<T>::type>, and
/// there is special handling to make operator -> work as if T was not a
/// reference.
///
/// T cannot be a rvalue reference.
template<class T>
class ErrorOr { … };

template <class T, class E>
std::enable_if_t<std::is_error_code_enum<E>::value ||
                     std::is_error_condition_enum<E>::value,
                 bool>
operator==(const ErrorOr<T> &Err, E Code) { … }

} // end namespace llvm

#endif // LLVM_SUPPORT_ERROROR_H