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 both T and E.
• That either E is void or DefaultConstructible (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:

1. <cstdint>
2. <initializer_list>
3. <iosfwd>
4. <new>
5. <type_traits>
6. If BOOST_OUTCOME_USE_STD_IN_PLACE_TYPE is 1, <utility> (defaults to 1 for C++ 17 or later only)
7. If C++ exceptions disabled and BOOST_OUTCOME_DISABLE_EXECINFO undefined only (used to print stack backtraces on “exception throw”):
   1. <sal.h> (Windows only)
   2. <stddef.h> (Windows only)
   3. <string.h> (Windows only)
   4. <execinfo.h> (POSIX only)
8. <cstdio>
9. <cstdlib>
10. <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_type is T.
• error_type is E.
• no_value_policy_type is NoValuePolicy.
• value_type_if_enabled is T if construction from T is available, else it is a usefully named unusable internal type.
• error_type_if_enabled is E if construction from E is available, else it is a usefully named unusable internal type.
• rebind<A, B = E, C = NoValuePolicy> is basic_result<A, B, C>.

Protected member predicate booleans

• predicate::constructors_enabled is constexpr boolean true if decayed value_type and decayed error_type are not the same type.

• predicate::implicit_constructors_enabled is constexpr boolean true if:

  1. predicate::constructors_enabled is true.
  2. Trait is_error_type<E> is not true for both decayed value_type and decayed error_type at the same time.
  3. value_type is not implicitly constructible from error_type and error_type is not implicitly constructible from value_type.
     OR
     trait is_error_type<E> is true for decayed error_type and error_type is not implicitly constructible from value_type and value_type is an integral type.

• predicate::enable_value_converting_constructor<A> is constexpr boolean true if:

  1. predicate::constructors_enabled is true.
  2. Decayed A is not this basic_result type.
  3. predicate::implicit_constructors_enabled is true.
  4. Decayed A is not an in_place_type_t.
  5. Trait is_error_type_enum<E, Enum> is false for error_type and decayed A.
  6. value_type is implicitly constructible from A and error_type is not implicitly constructible from A.
     OR
     value_type is the exact same type as decayed A and value_type is implicitly constructible from A.

• predicate::enable_error_converting_constructor<A> is constexpr boolean true if:

  1. predicate::constructors_enabled is true.
  2. Decayed A is not this basic_result type.
  3. predicate::implicit_constructors_enabled is true.
  4. Decayed A is not an in_place_type_t.
  5. Trait is_error_type_enum<E, Enum> is false for error_type and decayed A.
  6. value_type is not implicitly constructible from A and error_type is implicitly constructible from A.
     OR
     error_type is the exact same type as decayed A and error_type is implicitly constructible from A.

• predicate::enable_error_condition_converting_constructor<ErrorCondEnum> is constexpr boolean true if:

  1. predicate::constructors_enabled is true.
  2. Decayed ErrorCondEnum is not this basic_result type.
  3. Decayed ErrorCondEnum is not an in_place_type_t.
  4. Trait is_error_type_enum<E, Enum> is true for error_type and decayed ErrorCondEnum.

• predicate::enable_compatible_conversion<A, B, C> is constexpr boolean true if:

  1. predicate::constructors_enabled is true.
  2. basic_result<A, B, C> is not this basic_result type.
  3. A is void OR value_type is explicitly constructible from A.
  4. B is void OR error_type is explicitly constructible from B.

• predicate::enable_make_error_code_compatible_conversion<A, B, C> is constexpr boolean true if:

  1. predicate::constructors_enabled is true.
  2. basic_result<A, B, C> is not this basic_result type.
  3. Trait is_error_code_available<T> is true for decayed error_type.
  4. predicate::enable_compatible_conversion<A, B, C> is not true.
  5. A is void OR value_type is explicitly constructible from A.
  6. error_type is explicitly constructible from make_error_code(B).

• predicate::enable_make_exception_ptr_compatible_conversion<A, B, C> is constexpr boolean true if:

  1. predicate::constructors_enabled is true.
  2. basic_result<A, B, C> is not this basic_result type.
  3. Trait is_exception_ptr_available<T> is true for decayed error_type.
  4. predicate::enable_compatible_conversion<A, B, C> is not true.
  5. A is void OR value_type is explicitly constructible from A.
  6. error_type is explicitly constructible from make_exception_ptr(B).

• predicate::enable_inplace_value_constructor<Args...> is constexpr boolean true if:

  1. predicate::constructors_enabled is true.
  2. value_type is void OR value_type is explicitly constructible from Args....

• predicate::enable_inplace_error_constructor<Args...> is constexpr boolean true if:

  1. predicate::constructors_enabled is true.
  2. error_type is void OR error_type is explicitly constructible from Args....

• predicate::enable_inplace_value_error_constructor<Args...> is constexpr boolean true if:

  1. predicate::constructors_enabled is true.
  2. predicate::implicit_constructors_enabled is true.
  3. Either, but not both, of value_type is explicitly constructible from Args... or error_type is explicitly constructible from Args....

Summary of standard requirements provided

• DefaultConstructible, always deleted to force user to choose valued or errored for every result instanced.
• MoveConstructible, if both value_type and error_type implement move constructors.
• CopyConstructible, if both value_type and error_type implement copy constructors.
• MoveAssignable, if both value_type and error_type implement move constructors and move assignment.
• CopyAssignable, if both value_type and error_type implement copy constructors and copy assignment.
• Destructible.
• Swappable, with the strong rather than weak guarantee. See void swap(basic_result &) for more information.
• TriviallyCopyable, if both value_type and error_type are trivially copyable.
• TrivialType, if both value_type and error_type are trivial types.
• LiteralType, if both value_type and error_type are literal types.

• StandardLayoutType, if both value_type and error_type are standard layout types. If so, layout of basic_result in 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_type and error_type are TriviallyCopyable, 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 both value_type and error_type implement equality comparisons with one another.

• LessThanComparable, not implemented due to availability of implicit conversions from value_type and error_type, this can cause major surprise (i.e. hard to diagnose bugs), so we don’t implement these at all.

• Hash, 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.

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

1. basic_result(Args...) = delete

   Disabling catchall constructor used to give useful diagnostic error when trying to use non-inplace constructors when predicate::constructors_enabled is false.

2. basic_result(X &&) = delete

   Disabling implicit constructor used to give useful diagnostic error when trying to use implicit constructors when predicate::implicit_constructors_enabled is false.

Copy and move constructors and assignment, and destructor

1. basic_result() = delete

   The default constructor (disabled).

2. basic_result(basic_result &&)

   Move constructor. Constexpr, triviality and noexcept propagating.

3. basic_result(const basic_result &)

   Copy constructor. Constexpr, triviality and noexcept propagating.

4. basic_result &operator=(basic_result &&)

   Move assignment. Constexpr, triviality and noexcept propagating.

5. basic_result &operator=(const basic_result &)

   Copy assignment. Constexpr, triviality and noexcept propagating.

6. ~basic_result()

   Destructor. Constexpr, triviality and noexcept propagating.

Converting constructors

1. basic_result(R &&)

   Implicit value_type constructor. Available if predicate::enable_value_converting_constructor<R> is true. Constexpr, triviality and noexcept propagating.

2. basic_result(S &&)

   Implicit error_type constructor. Available if predicate::enable_error_converting_constructor<S> is true. Constexpr, triviality and noexcept propagating.

3. basic_result(ErrorCondEnum &&)

   Implicit error_type from ErrorCondEnum constructor. Available if predicate::enable_error_condition_converting_constructor<ErrorCondEnum> is true. Constexpr, triviality and noexcept propagating.

4. explicit basic_result(concepts::value_or_error<T, E> &&)

   Explicit converting constructor from concepts::value_or_error<T, E> concept matching types. Available if convert::value_or_error<> permits it. Constexpr, triviality and noexcept propagating.

5. explicit basic_result(const basic_result<R, S, P> &)

   Explicit converting copy constructor from compatible basic_result. Available if predicate::enable_compatible_conversion<R, S, P> is true. Constexpr, triviality and noexcept propagating.

6. explicit basic_result(basic_result<R, S, P> &&)

   Explicit converting move constructor from compatible basic_result. Available if predicate::enable_compatible_conversion<R, S, P> is true. Constexpr, triviality and noexcept propagating.

7. explicit basic_result(const basic_result<R, S, P> &)

   Explicit converting copy constructor from compatible basic_result. Available if predicate::enable_make_error_code_compatible_conversion<R, S, P> is true. Constexpr, triviality and noexcept propagating.

8. explicit basic_result(basic_result<R, S, P> &&)

   Explicit converting move constructor from compatible basic_result. Available if predicate::enable_make_error_code_compatible_conversion<R, S, P> is true. Constexpr, triviality and noexcept propagating.

9. explicit basic_result(const basic_result<R, S, P> &)

   Explicit converting copy constructor from compatible basic_result. Available if predicate::enable_make_exception_ptr_compatible_conversion<R, S, P> is true. Constexpr, triviality and noexcept propagating.

10. explicit basic_result(basic_result<R, S, P> &&)

    Explicit converting move constructor from compatible basic_result. Available if predicate::enable_make_exception_ptr_compatible_conversion<R, S, P> is true. Constexpr, triviality and noexcept propagating.

Inplace constructors

1. 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.

2. 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.

3. 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.

4. 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.

5. 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

1. 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, or T is void. Constexpr, triviality and noexcept propagating.

2. basic_result(success_type<T> &&)

   Implicit value-from-success-type-sugar move constructor. Available if predicate::enable_compatible_conversion<T, void, void> is true, or T is void. Constexpr, triviality and noexcept propagating.

3. 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, or T is void. Constexpr, triviality and noexcept propagating.

4. basic_result(failure_type<T> &&)

   Implicit error-from-failure-type-sugar move constructor. Available if predicate::enable_compatible_conversion<void, T, void> is true, or T is void. Constexpr, triviality and noexcept propagating.

5. 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, or T is void. Constexpr, triviality and noexcept propagating.

6. 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, or T is void. Constexpr, triviality and noexcept propagating.

7. 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, or T is void. Constexpr, triviality and noexcept propagating.

8. 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, or T is void. Constexpr, triviality and noexcept propagating.

Observers

1. explicit operator bool() const noexcept

   Returns true if a value is present. Constexpr, never throws.

2. bool has_value() const noexcept

   Returns true if a value is present. Constexpr, never throws.

3. bool has_error() const noexcept

   Returns true if an error is present. Constexpr, never throws.

4. bool has_exception() const noexcept

   Always returns false for basic_result. Constexpr, never throws.

5. bool has_failure() const noexcept

   Returns true if there is either an error or an exception. Constexpr, never throws.

6. bool has_lost_consistency() const noexcept

   Returns true if a preceding swap involving this object failed to preserve the strong guarantee. Constexpr, never throws.

7. value_type &assume_value() & noexcept

   Narrow contract lvalue reference observer of any value present. Constexpr propagating, never throws.

8. const value_type &assume_value() const & noexcept

   Narrow contract const lvalue reference observer of any value present. Constexpr propagating, never throws.

9. value_type &&assume_value() && noexcept

   Narrow contract rvalue reference observer of any value present. Constexpr propagating, never throws.

10. const value_type &&assume_value() const && noexcept

    Narrow contract const rvalue reference observer of any value present. Constexpr propagating, never throws.

11. value_type &value() &

    Wide contract lvalue reference observer of any value present. Constexpr propagating.

12. const value_type &value() const &

    Wide contract const lvalue reference observer of any value present. Constexpr propagating.

13. value_type &&value() &&

    Wide contract rvalue reference observer of any value present. Constexpr propagating.

14. const value_type &&value() const &&

    Wide contract const rvalue reference observer of any value present. Constexpr propagating.

15. error_type &assume_error() & noexcept

    Narrow contract lvalue reference observer of the stored error. Constexpr propagating, never throws.

16. const error_type &assume_error() const & noexcept

    Narrow contract const lvalue reference observer of the stored error. Constexpr propagating, never throws.

17. error_type &&assume_error() && noexcept

    Narrow contract rvalue reference observer of the stored error. Constexpr propagating, never throws.

18. const error_type &&assume_error() const && noexcept

    Narrow contract const rvalue reference observer of the stored error. Constexpr propagating, never throws.

19. error_type &error() &

    Wide contract lvalue reference observer of the stored error. Constexpr propagating.

20. const error_type &error() const &

    Wide contract const lvalue reference observer of the stored error. Constexpr propagating.

21. error_type &&error() &&

    Wide contract rvalue reference observer of the stored error. Constexpr propagating.

22. const error_type &&error() const &&

    Wide contract const rvalue reference observer of the stored error. Constexpr propagating.

23. auto as_failure() const &

    Return the output from free function failure() containing a copy of any errored state.

Modifiers

1. void swap(basic_result &)

   Swap one basic_result with another, with the strong guarantee. Noexcept propagating.

2. 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.

1. bool operator==(const basic_result<A, B, C> &) const

   Returns true if this result compares equal to the other result. Constexpr and noexcept propagating.

2. bool operator==(const success_type<A> &) const

   Returns true if this result compares equal to the success type sugar. Constexpr and noexcept propagating.

3. bool operator==(const failure_type<A, void> &) const

   Returns true if this result compares equal to the failure type sugar. Constexpr and noexcept propagating.

4. bool operator!=(const basic_result<A, B, C> &) const

   Returns true if this result does not compare equal to the other result. Constexpr and noexcept propagating.

5. bool operator!=(const success_type<A> &) const

   Returns true if this result does not compare equal to the success type sugar. Constexpr and noexcept propagating.

6. bool operator!=(const failure_type<A, void> &) const

   Returns true if this result does not compare equal to the failure type sugar. Constexpr and noexcept propagating.