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The new variant combines the attempt with unique-pointer semantics and is thereby usable for returning non-copyable objects such as RAM 'Allocation'. Issue #5502
152 lines
4.2 KiB
C++
152 lines
4.2 KiB
C++
/*
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* \brief Utility for passing return values
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* \author Norman Feske
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* \date 2021-11-09
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*/
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/*
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* Copyright (C) 2021 Genode Labs GmbH
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*
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* This file is part of the Genode OS framework, which is distributed
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* under the terms of the GNU Affero General Public License version 3.
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*/
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#ifndef _INCLUDE__UTIL__ATTEMPT_H_
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#define _INCLUDE__UTIL__ATTEMPT_H_
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#include <util/reconstructible.h>
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namespace Genode {
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template <typename, typename> struct Attempt;
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template <typename, typename> struct Unique_attempt;
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}
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/**
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* Option type for return values
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*
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* The 'Attempt' type addresses the C++ limitation to only a single return
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* value, which makes it difficult to propagate error information in addition
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* to an actual return value from a called function back to the caller. Hence,
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* errors have to be propagated as exceptions, results have to be returned via
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* out parameters, or error codes have to be encoded in the form of magic
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* values. Each of these approaches create its own set of robustness problems.
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*
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* An 'Attempt' represents the result of a function call that is either a
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* meaningful value or an error code, but never both. The result value and
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* error are distinct types. To consume the return value of a call, the caller
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* needs to specify two functors, one for handing the value if the value exists
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* (the call was successful), and one for handling the error value if the call
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* failed. Thereby the use of an 'Attempt' return type reinforces the explicit
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* handling of all possible error conditions at the caller site.
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*/
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template <typename RESULT, typename ERROR>
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class Genode::Attempt
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{
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private:
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bool _ok;
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RESULT _result { };
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ERROR _error { };
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public:
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Attempt(RESULT result) : _ok(true), _result(result) { }
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Attempt(ERROR error) : _ok(false), _error(error) { }
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Attempt(Attempt const &) = default;
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Attempt &operator = (Attempt const &) = default;
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template <typename RET>
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RET convert(auto const &access_fn, auto const &fail_fn) const
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{
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return _ok ? RET { access_fn(_result) }
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: RET { fail_fn(_error) };
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}
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void with_result(auto const &access_fn, auto const &fail_fn) const
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{
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_ok ? access_fn(_result) : fail_fn(_error);
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}
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void with_error(auto const &fail_fn) const
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{
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if (!_ok)
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fail_fn(_error);
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}
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bool operator == (ERROR const &rhs) const {
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return failed() && (_error == rhs); }
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bool operator == (RESULT const &rhs) const {
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return ok() && (_result == rhs); }
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bool ok() const { return _ok; }
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bool failed() const { return !_ok; }
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};
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/**
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* Base type for the result of constrained RAM allocations
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*
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* The type is a unique pointer to the allocated RAM while also holding
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* allocation-error information, following the conventions of 'Attempt'.
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*/
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template <typename RESULT, typename ERROR>
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class Genode::Unique_attempt : Noncopyable
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{
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private:
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Constructible<RESULT> _result { };
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ERROR _error { };
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protected:
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void _construct(auto &&... args) { _result.construct(args...); }
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void _destruct (ERROR e) { _result.destruct(); _error = e; }
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public:
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Unique_attempt(auto &&... args) { _result.construct(args...); }
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Unique_attempt(ERROR error) { _destruct(error); }
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template <typename RET>
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RET convert(auto const &access_fn, auto const &fail_fn)
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{
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return _result.constructed() ? RET { access_fn(*_result) }
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: RET { fail_fn(_error) };
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}
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template <typename RET>
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RET convert(auto const &access_fn, auto const &fail_fn) const
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{
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return _result.constructed() ? RET { access_fn(*_result) }
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: RET { fail_fn(_error) };
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}
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void with_result(auto const &access_fn, auto const &fail_fn)
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{
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_result.constructed() ? access_fn(*_result) : fail_fn(_error);
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}
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void with_result(auto const &access_fn, auto const &fail_fn) const
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{
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_result.constructed() ? access_fn(*_result) : fail_fn(_error);
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}
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void with_error(auto const &fail_fn) const
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{
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if (!_result.constructed())
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fail_fn(_error);
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}
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bool operator == (ERROR const &rhs) const {
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return failed() && (_error == rhs); }
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bool ok() const { return _result.constructed(); }
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bool failed() const { return !_result.constructed(); }
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};
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#endif /* _INCLUDE__UTIL__ATTEMPT_H_ */
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