//===- 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 OtherT> friend class ErrorOr;
static constexpr bool isRef = std::is_reference<T>::value;
using wrap = std::reference_wrapper<std::remove_reference_t<T>>;
public:
using storage_type = std::conditional_t<isRef, wrap, T>;
private:
using reference = std::remove_reference_t<T> &;
using const_reference = const std::remove_reference_t<T> &;
using pointer = std::remove_reference_t<T> *;
using const_pointer = const std::remove_reference_t<T> *;
public:
template <class E>
ErrorOr(E ErrorCode,
std::enable_if_t<std::is_error_code_enum<E>::value ||
std::is_error_condition_enum<E>::value,
void *> = nullptr)
: HasError(true) {
new (getErrorStorage()) std::error_code(make_error_code(ErrorCode));
}
ErrorOr(std::error_code EC) : HasError(true) {
new (getErrorStorage()) std::error_code(EC);
}
template <class OtherT>
ErrorOr(OtherT &&Val,
std::enable_if_t<std::is_convertible<OtherT, T>::value> * = nullptr)
: HasError(false) {
new (getStorage()) storage_type(std::forward<OtherT>(Val));
}
ErrorOr(const ErrorOr &Other) {
copyConstruct(Other);
}
template <class OtherT>
ErrorOr(const ErrorOr<OtherT> &Other,
std::enable_if_t<std::is_convertible<OtherT, T>::value> * = nullptr) {
copyConstruct(Other);
}
template <class OtherT>
explicit ErrorOr(
const ErrorOr<OtherT> &Other,
std::enable_if_t<!std::is_convertible<OtherT, const T &>::value> * =
nullptr) {
copyConstruct(Other);
}
ErrorOr(ErrorOr &&Other) {
moveConstruct(std::move(Other));
}
template <class OtherT>
ErrorOr(ErrorOr<OtherT> &&Other,
std::enable_if_t<std::is_convertible<OtherT, T>::value> * = nullptr) {
moveConstruct(std::move(Other));
}
// This might eventually need SFINAE but it's more complex than is_convertible
// & I'm too lazy to write it right now.
template <class OtherT>
explicit ErrorOr(
ErrorOr<OtherT> &&Other,
std::enable_if_t<!std::is_convertible<OtherT, T>::value> * = nullptr) {
moveConstruct(std::move(Other));
}
ErrorOr &operator=(const ErrorOr &Other) {
copyAssign(Other);
return *this;
}
ErrorOr &operator=(ErrorOr &&Other) {
moveAssign(std::move(Other));
return *this;
}
~ErrorOr() {
if (!HasError)
getStorage()->~storage_type();
}
/// Return false if there is an error.
explicit operator bool() const {
return !HasError;
}
reference get() { return *getStorage(); }
const_reference get() const { return const_cast<ErrorOr<T> *>(this)->get(); }
std::error_code getError() const {
return HasError ? *getErrorStorage() : std::error_code();
}
pointer operator ->() {
return toPointer(getStorage());
}
const_pointer operator->() const { return toPointer(getStorage()); }
reference operator *() {
return *getStorage();
}
const_reference operator*() const { return *getStorage(); }
private:
template <class OtherT>
void copyConstruct(const ErrorOr<OtherT> &Other) {
if (!Other.HasError) {
// Get the other value.
HasError = false;
new (getStorage()) storage_type(*Other.getStorage());
} else {
// Get other's error.
HasError = true;
new (getErrorStorage()) std::error_code(Other.getError());
}
}
template <class T1>
static bool compareThisIfSameType(const T1 &a, const T1 &b) {
return &a == &b;
}
template <class T1, class T2>
static bool compareThisIfSameType(const T1 &a, const T2 &b) {
return false;
}
template <class OtherT>
void copyAssign(const ErrorOr<OtherT> &Other) {
if (compareThisIfSameType(*this, Other))
return;
this->~ErrorOr();
new (this) ErrorOr(Other);
}
template <class OtherT>
void moveConstruct(ErrorOr<OtherT> &&Other) {
if (!Other.HasError) {
// Get the other value.
HasError = false;
new (getStorage()) storage_type(std::move(*Other.getStorage()));
} else {
// Get other's error.
HasError = true;
new (getErrorStorage()) std::error_code(Other.getError());
}
}
template <class OtherT>
void moveAssign(ErrorOr<OtherT> &&Other) {
if (compareThisIfSameType(*this, Other))
return;
this->~ErrorOr();
new (this) ErrorOr(std::move(Other));
}
pointer toPointer(pointer Val) {
return Val;
}
const_pointer toPointer(const_pointer Val) const { return Val; }
pointer toPointer(wrap *Val) {
return &Val->get();
}
const_pointer toPointer(const wrap *Val) const { return &Val->get(); }
storage_type *getStorage() {
assert(!HasError && "Cannot get value when an error exists!");
return reinterpret_cast<storage_type *>(&TStorage);
}
const storage_type *getStorage() const {
assert(!HasError && "Cannot get value when an error exists!");
return reinterpret_cast<const storage_type *>(&TStorage);
}
std::error_code *getErrorStorage() {
assert(HasError && "Cannot get error when a value exists!");
return reinterpret_cast<std::error_code *>(&ErrorStorage);
}
const std::error_code *getErrorStorage() const {
return const_cast<ErrorOr<T> *>(this)->getErrorStorage();
}
union {
AlignedCharArrayUnion<storage_type> TStorage;
AlignedCharArrayUnion<std::error_code> ErrorStorage;
};
bool HasError : 1;
};
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) {
return Err.getError() == Code;
}
} // end namespace llvm
#endif // LLVM_SUPPORT_ERROROR_H