basic_result<T, E, NoValuePolicy>
A sum type carrying either a T or an E, with NoValuePolicy specifying what to do if one tries to read state which isn’t there, and enabling injection of hooks to trap when lifecycle events occur. Either or both of T and E can be void to indicate no value for that state is present. Note that E = void makes basic result into effectively an optional<T>, but with NoValuePolicy configurable behaviour. Detectable using is_basic_result<T>
.
Requires: Concept requirements if C++ 20, else static asserted:
- That trait
type_can_be_used_in_basic_result<R>is true for bothTandE. - That either
EisvoidorDefaultConstructible(Outcome v2.1 and earlier only).
Namespace: BOOST_OUTCOME_V2_NAMESPACE
Header: <boost/outcome/basic_result.hpp>
Inclusions: The very lightest weight of C and C++ header files:
<cstdint><initializer_list><iosfwd><new><type_traits>- If
BOOST_OUTCOME_USE_STD_IN_PLACE_TYPEis1,<utility>(defaults to1for C++ 17 or later only) - If C++ exceptions disabled and
BOOST_OUTCOME_DISABLE_EXECINFOundefined only (used to print stack backtraces on “exception throw”):<sal.h>(Windows only)<stddef.h>(Windows only)<string.h>(Windows only)<execinfo.h>(POSIX only)
<cstdio><cstdlib><cassert>
This very light weight set of inclusion dependencies makes basic result suitable for use in global header files of very large C++ codebases.
Design rationale
The basic result type is the main workhorse type of the Outcome library, providing a simple sum type with optional values representing success or disappointment. Unlike
P0323 std::expected<T, E>
, Outcome’s result type is designed specifically for convenience when implementing failure handling across very large codebases, and it has a number of API differences to facilitate that.
The first major design difference is that basic result models its constructor design on
std::variant<...>
, rather than modelling
std::optional<T>
’s constructor design like std::expected<T, E> does. This means that basic result will implicitly construct either a T or an E if doing so is unambiguous, same as variant does. Where implicit construction is ambiguous, the implicit constructors disable and a T or E can be specified via in_place_type_t<T>
, or via success_type<T>
or failure_type<T>
. We implement a subset of variant’s constructors for improved compile time impact, so the implicit and explicit constructor design is split into fixed subsets to reduce SFINAE execution.
The second major design difference is that union storage is ONLY used when both T and E are trivially copyable or void, otherwise struct storage is used. This is usually not a problem, as it is assumed that sizeof(E) will be small for failure handling. The choice to only use union storage for trivially copyable types only very considerably reduces load on the compiler, and substantially improves compile times in very large C++ 14 codebases, because copies and moves do not need to jump through complex ceremony in order to implement the never-empty guarantees which would be required in a union storage based implementation (C++ 17 onwards does far fewer copy and move constructor instantiations, but it all adds up – work avoided is always the fastest).
Public member type aliases
value_typeisT.error_typeisE.no_value_policy_typeisNoValuePolicy.value_type_if_enabledisTif construction fromTis available, else it is a usefully named unusable internal type.error_type_if_enabledisEif construction fromEis available, else it is a usefully named unusable internal type.rebind<A, B = E, C = NoValuePolicy>isbasic_result<A, B, C>.
Protected member predicate booleans
predicate::constructors_enabledis constexpr boolean true if decayedvalue_typeand decayederror_typeare not the same type.predicate::implicit_constructors_enabledis constexpr boolean true if:predicate::constructors_enabledis true.- Trait
is_error_type<E>is not true for both decayedvalue_typeand decayederror_typeat the same time. value_typeis not implicitly constructible fromerror_typeanderror_typeis not implicitly constructible fromvalue_type.
OR
traitis_error_type<E>is true for decayederror_typeanderror_typeis not implicitly constructible fromvalue_typeandvalue_typeis an integral type.
predicate::enable_value_converting_constructor<A>is constexpr boolean true if:predicate::constructors_enabledis true.- Decayed
Ais not thisbasic_resulttype. predicate::implicit_constructors_enabledis true.- Decayed
Ais not anin_place_type_t. - Trait
is_error_type_enum<E, Enum>is false forerror_typeand decayedA. value_typeis implicitly constructible fromAanderror_typeis not implicitly constructible fromA.
ORvalue_typeis the exact same type as decayedAandvalue_typeis implicitly constructible fromA.
predicate::enable_error_converting_constructor<A>is constexpr boolean true if:predicate::constructors_enabledis true.- Decayed
Ais not thisbasic_resulttype. predicate::implicit_constructors_enabledis true.- Decayed
Ais not anin_place_type_t. - Trait
is_error_type_enum<E, Enum>is false forerror_typeand decayedA. value_typeis not implicitly constructible fromAanderror_typeis implicitly constructible fromA.
ORerror_typeis the exact same type as decayedAanderror_typeis implicitly constructible fromA.
predicate::enable_error_condition_converting_constructor<ErrorCondEnum>is constexpr boolean true if:predicate::constructors_enabledis true.- Decayed
ErrorCondEnumis not thisbasic_resulttype. - Decayed
ErrorCondEnumis not anin_place_type_t. - Trait
is_error_type_enum<E, Enum>is true forerror_typeand decayedErrorCondEnum.
predicate::enable_compatible_conversion<A, B, C>is constexpr boolean true if:predicate::constructors_enabledis true.basic_result<A, B, C>is not thisbasic_resulttype.AisvoidORvalue_typeis explicitly constructible fromA.BisvoidORerror_typeis explicitly constructible fromB.
predicate::enable_make_error_code_compatible_conversion<A, B, C>is constexpr boolean true if:predicate::constructors_enabledis true.basic_result<A, B, C>is not thisbasic_resulttype.- Trait
is_error_code_available<T>is true for decayederror_type. predicate::enable_compatible_conversion<A, B, C>is not true.AisvoidORvalue_typeis explicitly constructible fromA.error_typeis explicitly constructible frommake_error_code(B).
predicate::enable_make_exception_ptr_compatible_conversion<A, B, C>is constexpr boolean true if:predicate::constructors_enabledis true.basic_result<A, B, C>is not thisbasic_resulttype.- Trait
is_exception_ptr_available<T>is true for decayederror_type. predicate::enable_compatible_conversion<A, B, C>is not true.AisvoidORvalue_typeis explicitly constructible fromA.error_typeis explicitly constructible frommake_exception_ptr(B).
predicate::enable_inplace_value_constructor<Args...>is constexpr boolean true if:predicate::constructors_enabledis true.value_typeisvoidORvalue_typeis explicitly constructible fromArgs....
predicate::enable_inplace_error_constructor<Args...>is constexpr boolean true if:predicate::constructors_enabledis true.error_typeisvoidORerror_typeis explicitly constructible fromArgs....
predicate::enable_inplace_value_error_constructor<Args...>is constexpr boolean true if:predicate::constructors_enabledis true.predicate::implicit_constructors_enabledis true.- Either, but not both, of
value_typeis explicitly constructible fromArgs...orerror_typeis explicitly constructible fromArgs....
Summary of standard requirements provided
, always deleted to force user to choose valued or errored for every result instanced.DefaultConstructibleMoveConstructible, if bothvalue_typeanderror_typeimplement move constructors.CopyConstructible, if bothvalue_typeanderror_typeimplement copy constructors.MoveAssignable, if bothvalue_typeanderror_typeimplement move constructors and move assignment.CopyAssignable, if bothvalue_typeanderror_typeimplement copy constructors and copy assignment.Destructible.Swappable, with the strong rather than weak guarantee. Seevoid swap(basic_result &)for more information.TriviallyCopyable, if bothvalue_typeanderror_typeare trivially copyable.TrivialType, if bothvalue_typeanderror_typeare trivial types.LiteralType, if bothvalue_typeanderror_typeare literal types.StandardLayoutType, if bothvalue_typeanderror_typeare standard layout types. If so, layout ofbasic_resultin C is guaranteed to be one of:struct trivially_copyable_result_layout { union { value_type value; error_type error; }; unsigned int flags; };… if both
value_typeanderror_typeareTriviallyCopyable, otherwise:struct non_trivially_copyable_result_layout { value_type value; unsigned int flags; error_type error; };Obviously, all C compatible C++ types are
TriviallyCopyable, so if you are passing non-trivially copyable types from C++ to C, you are doing undefined behaviour.EqualityComparable, if bothvalue_typeanderror_typeimplement equality comparisons with one another., not implemented due to availability of implicit conversions fromLessThanComparablevalue_typeanderror_type, this can cause major surprise (i.e. hard to diagnose bugs), so we don’t implement these at all., not implemented as a generic implementation of a unique hash for non-valued items which are unequal would require a dependency on RTTI being enabled.Hash
Thus basic_result meets the Regular concept if both value_type and error_type are Regular, except for the lack of a default constructor. Often where one needs a default constructor, wrapping basic_result into
std::optional<T>
will suffice.
Public member functions
Disabling constructors
-
basic_result(Args...) = deleteDisabling catchall constructor used to give useful diagnostic error when trying to use non-inplace constructors when
predicate::constructors_enabledis false. -
basic_result(X &&) = deleteDisabling implicit constructor used to give useful diagnostic error when trying to use implicit constructors when
predicate::implicit_constructors_enabledis false.
Copy and move constructors and assignment, and destructor
-
basic_result() = deleteThe default constructor (disabled).
-
basic_result(basic_result &&)Move constructor. Constexpr, triviality and noexcept propagating.
-
basic_result(const basic_result &)Copy constructor. Constexpr, triviality and noexcept propagating.
-
basic_result &operator=(basic_result &&)Move assignment. Constexpr, triviality and noexcept propagating.
-
basic_result &operator=(const basic_result &)Copy assignment. Constexpr, triviality and noexcept propagating.
-
~basic_result()Destructor. Constexpr, triviality and noexcept propagating.
Converting constructors
-
basic_result(R &&)Implicit
value_typeconstructor. Available ifpredicate::enable_value_converting_constructor<R>is true. Constexpr, triviality and noexcept propagating. -
basic_result(S &&)Implicit
error_typeconstructor. Available ifpredicate::enable_error_converting_constructor<S>is true. Constexpr, triviality and noexcept propagating. -
basic_result(ErrorCondEnum &&)Implicit
error_typefromErrorCondEnumconstructor. Available ifpredicate::enable_error_condition_converting_constructor<ErrorCondEnum>is true. Constexpr, triviality and noexcept propagating. -
explicit basic_result(concepts::value_or_error<T, E> &&)Explicit converting constructor from
concepts::value_or_error<T, E>concept matching types. Available ifconvert::value_or_error<>permits it. Constexpr, triviality and noexcept propagating. -
explicit basic_result(const basic_result<R, S, P> &)Explicit converting copy constructor from compatible
basic_result. Available ifpredicate::enable_compatible_conversion<R, S, P>is true. Constexpr, triviality and noexcept propagating. -
explicit basic_result(basic_result<R, S, P> &&)Explicit converting move constructor from compatible
basic_result. Available ifpredicate::enable_compatible_conversion<R, S, P>is true. Constexpr, triviality and noexcept propagating. -
explicit basic_result(const basic_result<R, S, P> &)Explicit converting copy constructor from compatible
basic_result. Available ifpredicate::enable_make_error_code_compatible_conversion<R, S, P>is true. Constexpr, triviality and noexcept propagating. -
explicit basic_result(basic_result<R, S, P> &&)Explicit converting move constructor from compatible
basic_result. Available ifpredicate::enable_make_error_code_compatible_conversion<R, S, P>is true. Constexpr, triviality and noexcept propagating. -
explicit basic_result(const basic_result<R, S, P> &)Explicit converting copy constructor from compatible
basic_result. Available ifpredicate::enable_make_exception_ptr_compatible_conversion<R, S, P>is true. Constexpr, triviality and noexcept propagating. -
explicit basic_result(basic_result<R, S, P> &&)Explicit converting move constructor from compatible
basic_result. Available ifpredicate::enable_make_exception_ptr_compatible_conversion<R, S, P>is true. Constexpr, triviality and noexcept propagating.
Inplace constructors
-
explicit basic_result(in_place_type_t<value_type_if_enabled>, Args ...)Explicit inplace value constructor. Available if
predicate::enable_inplace_value_constructor<Args ...>is true. Constexpr, triviality and noexcept propagating. -
explicit basic_result(in_place_type_t<value_type_if_enabled>, std::initializer_list<U>, Args ...)Explicit inplace value constructor. Available if
predicate::enable_inplace_value_constructor<std::initializer_list<U>, Args ...>is true. Constexpr, triviality and noexcept propagating. -
explicit basic_result(in_place_type_t<error_type_if_enabled>, Args ...)Explicit inplace error constructor. Available if
predicate::enable_inplace_error_constructor<Args ...>is true. Constexpr, triviality and noexcept propagating. -
explicit basic_result(in_place_type_t<error_type_if_enabled>, std::initializer_list<U>, Args ...)Explicit inplace error constructor. Available if
predicate::enable_inplace_error_constructor<std::initializer_list<U>, Args ...>is true. Constexpr, triviality and noexcept propagating. -
basic_result(A1 &&, A2 &&, Args ...)Implicit inplace value or error constructor. Available if
predicate::enable_inplace_value_error_constructor<A1, A2, Args ...>is true. Constexpr, triviality and noexcept propagating.
Tagged constructors
-
basic_result(const success_type<T> &)Implicit value-from-success-type-sugar copy constructor. Available if
predicate::enable_compatible_conversion<T, void, void>is true, orTisvoid. Constexpr, triviality and noexcept propagating. -
basic_result(success_type<T> &&)Implicit value-from-success-type-sugar move constructor. Available if
predicate::enable_compatible_conversion<T, void, void>is true, orTisvoid. Constexpr, triviality and noexcept propagating. -
basic_result(const failure_type<T> &)Implicit error-from-failure-type-sugar copy constructor. Available if
predicate::enable_compatible_conversion<void, T, void>is true, orTisvoid. Constexpr, triviality and noexcept propagating. -
basic_result(failure_type<T> &&)Implicit error-from-failure-type-sugar move constructor. Available if
predicate::enable_compatible_conversion<void, T, void>is true, orTisvoid. Constexpr, triviality and noexcept propagating. -
basic_result(const failure_type<T> &)Implicit error-from-failure-type-sugar copy constructor. Available if
predicate::enable_make_error_code_compatible_conversion<void, T, void>is true, orTisvoid. Constexpr, triviality and noexcept propagating. -
basic_result(failure_type<T> &&)Implicit error-from-failure-type-sugar move constructor. Available if
predicate::enable_make_error_code_compatible_conversion<void, T, void>is true, orTisvoid. Constexpr, triviality and noexcept propagating. -
basic_result(const failure_type<T> &)Implicit error-from-failure-type-sugar copy constructor. Available if
predicate::enable_make_exception_ptr_compatible_conversion<void, T, void>is true, orTisvoid. Constexpr, triviality and noexcept propagating. -
basic_result(failure_type<T> &&)Implicit error-from-failure-type-sugar move constructor. Available if
predicate::enable_make_exception_ptr_compatible_conversion<void, T, void>is true, orTisvoid. Constexpr, triviality and noexcept propagating.
Observers
-
explicit operator bool() const noexceptReturns true if a value is present. Constexpr, never throws.
-
bool has_value() const noexceptReturns true if a value is present. Constexpr, never throws.
-
bool has_error() const noexceptReturns true if an error is present. Constexpr, never throws.
-
bool has_exception() const noexceptAlways returns false for
basic_result. Constexpr, never throws. -
bool has_failure() const noexceptReturns true if there is either an error or an exception. Constexpr, never throws.
-
bool has_lost_consistency() const noexceptReturns true if a preceding swap involving this object failed to preserve the strong guarantee. Constexpr, never throws.
-
value_type &assume_value() & noexceptNarrow contract lvalue reference observer of any value present. Constexpr propagating, never throws.
-
const value_type &assume_value() const & noexceptNarrow contract const lvalue reference observer of any value present. Constexpr propagating, never throws.
-
value_type &&assume_value() && noexceptNarrow contract rvalue reference observer of any value present. Constexpr propagating, never throws.
-
const value_type &&assume_value() const && noexceptNarrow contract const rvalue reference observer of any value present. Constexpr propagating, never throws.
-
value_type &value() &Wide contract lvalue reference observer of any value present. Constexpr propagating.
-
const value_type &value() const &Wide contract const lvalue reference observer of any value present. Constexpr propagating.
-
value_type &&value() &&Wide contract rvalue reference observer of any value present. Constexpr propagating.
-
const value_type &&value() const &&Wide contract const rvalue reference observer of any value present. Constexpr propagating.
-
error_type &assume_error() & noexceptNarrow contract lvalue reference observer of the stored error. Constexpr propagating, never throws.
-
const error_type &assume_error() const & noexceptNarrow contract const lvalue reference observer of the stored error. Constexpr propagating, never throws.
-
error_type &&assume_error() && noexceptNarrow contract rvalue reference observer of the stored error. Constexpr propagating, never throws.
-
const error_type &&assume_error() const && noexceptNarrow contract const rvalue reference observer of the stored error. Constexpr propagating, never throws.
-
error_type &error() &Wide contract lvalue reference observer of the stored error. Constexpr propagating.
-
const error_type &error() const &Wide contract const lvalue reference observer of the stored error. Constexpr propagating.
-
error_type &&error() &&Wide contract rvalue reference observer of the stored error. Constexpr propagating.
-
const error_type &&error() const &&Wide contract const rvalue reference observer of the stored error. Constexpr propagating.
-
auto as_failure() const &Return the output from free function
failure()containing a copy of any errored state.
Modifiers
-
void swap(basic_result &)Swap one basic_result with another, with the strong guarantee. Noexcept propagating.
-
auto as_failure() &&Return the output from free function
failure()containing a move of any errored state.
Comparisons
See above for why LessThanComparable is not implemented.
-
bool operator==(const basic_result<A, B, C> &) constReturns true if this result compares equal to the other result. Constexpr and noexcept propagating.
-
bool operator==(const success_type<A> &) constReturns true if this result compares equal to the success type sugar. Constexpr and noexcept propagating.
-
bool operator==(const failure_type<A, void> &) constReturns true if this result compares equal to the failure type sugar. Constexpr and noexcept propagating.
-
bool operator!=(const basic_result<A, B, C> &) constReturns true if this result does not compare equal to the other result. Constexpr and noexcept propagating.
-
bool operator!=(const success_type<A> &) constReturns true if this result does not compare equal to the success type sugar. Constexpr and noexcept propagating.
-
bool operator!=(const failure_type<A, void> &) constReturns true if this result does not compare equal to the failure type sugar. Constexpr and noexcept propagating.



