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// Copyright (c) 2022 The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or https://www.opensource.org/licenses/mit-license.php.
#ifndef BITCOIN_UTIL_RESULT_H
#define BITCOIN_UTIL_RESULT_H
#include <attributes.h>
#include <util/translation.h>
#include <variant>
namespace util {
struct Error {
bilingual_str message;
};
//! The util::Result class provides a standard way for functions to return
//! either error messages or result values.
//!
//! It is intended for high-level functions that need to report error strings to
//! end users. Lower-level functions that don't need this error-reporting and
//! only need error-handling should avoid util::Result and instead use standard
//! classes like std::optional, std::variant, and std::tuple, or custom structs
//! and enum types to return function results.
//!
//! Usage examples can be found in \example ../test/result_tests.cpp, but in
//! general code returning `util::Result<T>` values is very similar to code
//! returning `std::optional<T>` values. Existing functions returning
//! `std::optional<T>` can be updated to return `util::Result<T>` and return
//! error strings usually just replacing `return std::nullopt;` with `return
//! util::Error{error_string};`.
template <class M>
class Result
{
private:
using T = std::conditional_t<std::is_same_v<M, void>, std::monostate, M>;
std::variant<bilingual_str, T> m_variant;
template <typename FT>
friend bilingual_str ErrorString(const Result<FT>& result);
public:
Result() : m_variant{std::in_place_index_t<1>{}, std::monostate{}} {} // constructor for void
Result(T obj) : m_variant{std::in_place_index_t<1>{}, std::move(obj)} {}
Result(Error error) : m_variant{std::in_place_index_t<0>{}, std::move(error.message)} {}
//! std::optional methods, so functions returning optional<T> can change to
//! return Result<T> with minimal changes to existing code, and vice versa.
bool has_value() const noexcept { return m_variant.index() == 1; }
const T& value() const LIFETIMEBOUND
{
assert(has_value());
return std::get<1>(m_variant);
}
T& value() LIFETIMEBOUND
{
assert(has_value());
return std::get<1>(m_variant);
}
template <class U>
T value_or(U&& default_value) const&
{
return has_value() ? value() : std::forward<U>(default_value);
}
template <class U>
T value_or(U&& default_value) &&
{
return has_value() ? std::move(value()) : std::forward<U>(default_value);
}
explicit operator bool() const noexcept { return has_value(); }
const T* operator->() const LIFETIMEBOUND { return &value(); }
const T& operator*() const LIFETIMEBOUND { return value(); }
T* operator->() LIFETIMEBOUND { return &value(); }
T& operator*() LIFETIMEBOUND { return value(); }
};
template <typename T>
bilingual_str ErrorString(const Result<T>& result)
{
return result ? bilingual_str{} : std::get<0>(result.m_variant);
}
} // namespace util
#endif // BITCOIN_UTIL_RESULT_H
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