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/*
* Copyright (C) 2017 The Android Open Source Project
*
* 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.
*/
// This file contains classes for returning a successful result along with an optional
// arbitrarily typed return value or for returning a failure result along with an optional string
// indicating why the function failed.
// There are 3 classes that implement this functionality and one additional helper type.
//
// Result<T> either contains a member of type T that can be accessed using similar semantics as
// std::optional<T> or it contains a ResultError describing an error, which can be accessed via
// Result<T>::error().
//
// ResultError is a type that contains both a std::string describing the error and a copy of errno
// from when the error occurred. ResultError can be used in an ostream directly to print its
// string value.
//
// Success is a typedef that aids in creating Result<T> that do not contain a return value.
// Result<Success> is the correct return type for a function that either returns successfully or
// returns an error value. Returning Success() from a function that returns Result<Success> is the
// correct way to indicate that a function without a return type has completed successfully.
//
// A successful Result<T> is constructed implicitly from any type that can be implicitly converted
// to T or from the constructor arguments for T. This allows you to return a type T directly from
// a function that returns Result<T>.
//
// Error and ErrnoError are used to construct a Result<T> that has failed. The Error class takes
// an ostream as an input and are implicitly cast to a Result<T> containing that failure.
// ErrnoError() is a helper function to create an Error class that appends ": " + strerror(errno)
// to the end of the failure string to aid in interacting with C APIs. Alternatively, an errno
// value can be directly specified via the Error() constructor.
//
// ResultError can be used in the ostream when using Error to construct a Result<T>. In this case,
// the string that the ResultError takes is passed through the stream normally, but the errno is
// passed to the Result<T>. This can be used to pass errno from a failing C function up multiple
// callers.
//
// ResultError can also directly construct a Result<T>. This is particularly useful if you have a
// function that return Result<T> but you have a Result<U> and want to return its error. In this
// case, you can return the .error() from the Result<U> to construct the Result<T>.
// An example of how to use these is below:
// Result<U> CalculateResult(const T& input) {
// U output;
// if (!SomeOtherCppFunction(input, &output)) {
// return Error() << "SomeOtherCppFunction(" << input << ") failed";
// }
// if (!c_api_function(output)) {
// return ErrnoError() << "c_api_function(" << output << ") failed";
// }
// return output;
// }
//
// auto output = CalculateResult(input);
// if (!output) return Error() << "CalculateResult failed: " << output.error();
// UseOutput(*output);
#ifndef _INIT_RESULT_H
#define _INIT_RESULT_H
#include <errno.h>
#include <sstream>
#include <string>
#include <variant>
namespace android {
namespace init {
struct ResultError {
template <typename T>
ResultError(T&& error_string, int error_errno)
: error_string(std::forward<T>(error_string)), error_errno(error_errno) {}
std::string error_string;
int error_errno;
};
inline std::ostream& operator<<(std::ostream& os, const ResultError& t) {
os << t.error_string;
return os;
}
inline std::ostream& operator<<(std::ostream& os, ResultError&& t) {
os << std::move(t.error_string);
return os;
}
class Error {
public:
Error() : errno_(0), append_errno_(false) {}
Error(int errno_to_append) : errno_(errno_to_append), append_errno_(true) {}
template <typename T>
Error&& operator<<(T&& t) {
ss_ << std::forward<T>(t);
return std::move(*this);
}
Error&& operator<<(const ResultError& result_error) {
ss_ << result_error.error_string;
errno_ = result_error.error_errno;
return std::move(*this);
}
Error&& operator<<(ResultError&& result_error) {
ss_ << std::move(result_error.error_string);
errno_ = result_error.error_errno;
return std::move(*this);
}
const std::string str() const {
std::string str = ss_.str();
if (append_errno_) {
if (str.empty()) {
return strerror(errno_);
}
return str + ": " + strerror(errno_);
}
return str;
}
int get_errno() const { return errno_; }
Error(const Error&) = delete;
Error(Error&&) = delete;
Error& operator=(const Error&) = delete;
Error& operator=(Error&&) = delete;
private:
std::stringstream ss_;
int errno_;
bool append_errno_;
};
inline Error ErrnoError() {
return Error(errno);
}
template <typename T>
class Result {
public:
Result() {}
template <typename U, typename... V,
typename = std::enable_if_t<!(std::is_same_v<std::decay_t<U>, Result<T>> &&
sizeof...(V) == 0)>>
Result(U&& result, V&&... results)
: contents_(std::in_place_index_t<0>(), std::forward<U>(result),
std::forward<V>(results)...) {}
Result(Error&& error) : contents_(std::in_place_index_t<1>(), error.str(), error.get_errno()) {}
Result(const ResultError& result_error)
: contents_(std::in_place_index_t<1>(), result_error.error_string,
result_error.error_errno) {}
Result(ResultError&& result_error)
: contents_(std::in_place_index_t<1>(), std::move(result_error.error_string),
result_error.error_errno) {}
bool has_value() const { return contents_.index() == 0; }
T& value() & { return std::get<0>(contents_); }
const T& value() const & { return std::get<0>(contents_); }
T&& value() && { return std::get<0>(std::move(contents_)); }
const T&& value() const && { return std::get<0>(std::move(contents_)); }
const ResultError& error() const & { return std::get<1>(contents_); }
ResultError&& error() && { return std::get<1>(std::move(contents_)); }
const ResultError&& error() const && { return std::get<1>(std::move(contents_)); }
const std::string& error_string() const & { return std::get<1>(contents_).error_string; }
std::string&& error_string() && { return std::get<1>(std::move(contents_)).error_string; }
const std::string&& error_string() const && {
return std::get<1>(std::move(contents_)).error_string;
}
int error_errno() const { return std::get<1>(contents_).error_errno; }
explicit operator bool() const { return has_value(); }
T& operator*() & { return value(); }
const T& operator*() const & { return value(); }
T&& operator*() && { return std::move(value()); }
const T&& operator*() const && { return std::move(value()); }
T* operator->() { return &value(); }
const T* operator->() const { return &value(); }
private:
std::variant<T, ResultError> contents_;
};
using Success = std::monostate;
} // namespace init
} // namespace android
#endif
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