basic_outcome<T, EC, EP, NoValuePolicy>

A type carrying one of (i) a successful T (ii) a disappointment EC (iii) a failure EP (iv) both a disappointment EC and a failure EP, with NoValuePolicy specifying what to do if one tries to read state which isn’t there. Any one, two, or all of T, EC and EP can be void to indicate no value for that state is present. Detectable using is_basic_outcome<T> .

Requires: Concept requirements if C++ 20, else static asserted:

Namespace: OUTCOME_V2_NAMESPACE

Header: <outcome/basic_outcome.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 OUTCOME_USE_STD_IN_PLACE_TYPE is 1, <utility> (defaults to 1 for C++ 17 or later only)
  7. If C++ exceptions disabled and 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 outcome suitable for use in global header files of very large C++ codebases.

Design rationale

basic_outcome extends basic_result<T, E, NoValuePolicy> with a third state to transport, conventionally (but not necessarily) some sort of “abort” or “exceptional” state which a function can return to indicate that not only did the operation fail, but it did so catastrophically i.e. please abort any attempt to retry the operation.

A perfect alternative is to throw a C++ exception for the abort code path, and indeed most programs ought to do exactly that instead of using basic_outcome. However there are a number of use cases where choosing basic_outcome shines:

  1. Where C++ exceptions or RTTI is not available, but the ability to fail catastrophically without terminating the program is important.
  2. Where deterministic behaviour is required even in the catastrophic failure situation.
  3. In unit test suites of code using Outcome it is extremely convenient to accumulate test failures into a basic_outcome for later reporting. A similar convenience applies to RPC situations, where C++ exception throws need to be accumulated for reporting back to the initiating endpoint.
  4. Where a function is “dual use deterministic” i.e. it can be used deterministically, in which case one switches control flow based on .error(), or it can be used non-deterministically by throwing an exception perhaps carrying a custom payload.

Public member type aliases

Protected member predicate booleans

Summary of standard requirements provided

Thus basic_outcome meets the Regular concept if all of value_type, error_type and exception_type are Regular, except for the lack of a default constructor. Often where one needs a default constructor, wrapping basic_outcome into std::optional<T> will suffice.

Public member functions

Disabling constructors

  1. basic_outcome(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_outcome(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_outcome() = delete

    The default constructor (disabled).

  2. basic_outcome(basic_outcome &&)

    Move constructor. Constexpr, triviality and noexcept propagating.

  3. basic_outcome(const basic_outcome &)

    Copy constructor. Constexpr, triviality and noexcept propagating.

  4. basic_outcome &operator=(basic_outcome &&)

    Move assignment. Constexpr, triviality and noexcept propagating.

  5. basic_outcome &operator=(const basic_outcome &)

    Copy assignment. Constexpr, triviality and noexcept propagating.

  6. ~basic_outcome()

    Destructor. Constexpr, triviality and noexcept propagating.

Converting constructors

  1. basic_outcome(R &&)

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

  2. basic_outcome(S &&)

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

  3. basic_outcome(ErrorCondEnum &&)

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

  4. basic_outcome(P &&)

    Implicit exception_type constructor. Available if predicate::enable_exception_converting_constructor<P> is true. Constexpr, triviality and noexcept propagating.

  5. basic_outcome(S &&, P &&)

    Implicit error_type + exception_type constructor. Available if predicate::enable_error_exception_converting_constructor<S, P> is true. Constexpr, triviality and noexcept propagating.

  6. explicit basic_outcome(ValueOrError<T, E> &&)

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

  7. explicit basic_outcome(const basic_outcome<A, B, C, D> &)

    Explicit converting copy constructor from compatible basic_outcome. Available if predicate::enable_compatible_conversion<A, B, C, D> is true. Constexpr, triviality and noexcept propagating.

  8. explicit basic_outcome(basic_outcome<A, B, C, D> &&)

    Explicit converting move constructor from compatible basic_outcome. Available if predicate::enable_compatible_conversion<A, B, C, D> is true. Constexpr, triviality and noexcept propagating.

  9. explicit basic_outcome(const basic_result<A, B, C> &)

    Explicit converting copy constructor from compatible basic_result. Available if predicate::enable_compatible_conversion<A, B, void, C> is true. Constexpr, triviality and noexcept propagating.

  10. explicit basic_outcome(basic_result<A, B, C> &&)

    Explicit converting move constructor from compatible basic_result. Available if predicate::enable_compatible_conversion<A, B, void, C> is true. Constexpr, triviality and noexcept propagating.

  11. explicit basic_outcome(const basic_result<A, B, C> &)

    Explicit converting copy constructor from compatible basic_result. Available if predicate::enable_make_error_code_compatible_conversion<A, B, void, C> is true. Constexpr, triviality and noexcept propagating.

  12. explicit basic_outcome(basic_result<A, B, C> &&)

    Explicit converting move constructor from compatible basic_result. Available if predicate::enable_make_error_code_compatible_conversion<A, B, void, C> is true. Constexpr, triviality and noexcept propagating.

Inplace constructors

  1. explicit basic_outcome(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_outcome(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_outcome(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_outcome(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. explicit basic_outcome(in_place_type_t<exception_type_if_enabled>, Args ...)

    Explicit inplace exception constructor. Available if predicate::enable_inplace_exception_constructor<Args ...> is true. Constexpr, triviality and noexcept propagating.

  6. explicit basic_outcome(in_place_type_t<exception_type_if_enabled>, std::initializer_list<U>, Args ...)

    Explicit inplace exception constructor. Available if predicate::enable_inplace_exception_constructor<std::initializer_list<U>, Args ...> is true. Constexpr, triviality and noexcept propagating.

  7. basic_outcome(A1 &&, A2 &&, Args ...)

    Implicit inplace value or error or exception constructor. Available if predicate::enable_inplace_value_error_exception_constructor<A1, A2, Args ...> is true. Constexpr, triviality and noexcept propagating.

Tagged constructors

  1. basic_outcome(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_outcome(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_outcome(const failure_type<EC> &)

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

  4. basic_outcome(failure_type<EC> &&)

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

  5. basic_outcome(const failure_type<EC> &)

    Implicit error-from-failure-type-sugar copy constructor. Available if predicate::enable_make_error_code_compatible_conversion<void, EC, void, void> is true. Constexpr, triviality and noexcept propagating.

  6. basic_outcome(failure_type<EC> &&)

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

  7. basic_outcome(const failure_type<EP> &)

    Implicit exception-from-failure-type-sugar copy constructor. Available if predicate::enable_compatible_conversion<void, void, EP, void> is true. Constexpr, triviality and noexcept propagating.

  8. basic_outcome(failure_type<EP> &&)

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

  9. basic_outcome(const failure_type<EC, EP> &)

    Implicit error-and-exception-from-failure-type-sugar copy constructor. Available if predicate::enable_compatible_conversion<void, EC, EP, void> is true. Constexpr, triviality and noexcept propagating.

  10. basic_outcome(failure_type<EC, EP> &&)

    Implicit error-and-exception-from-failure-type-sugar move constructor. Available if predicate::enable_compatible_conversion<void, EC, EP, void> is true. 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

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

  5. bool has_failure() const noexcept

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

  6. value_type &assume_value() & noexcept

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

  7. const value_type &assume_value() const & noexcept

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

  8. value_type &&assume_value() && noexcept

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

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

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

  10. value_type &value() &

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

  11. const value_type &value() const &

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

  12. value_type &&value() &&

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

  13. const value_type &&value() const &&

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

  14. error_type &assume_error() & noexcept

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

  15. const error_type &assume_error() const & noexcept

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

  16. error_type &&assume_error() && noexcept

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

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

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

  18. error_type &error() &

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

  19. const error_type &error() const &

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

  20. error_type &&error() &&

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

  21. const error_type &&error() const &&

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

  22. exception_type &assume_exception() & noexcept

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

  23. const exception_type &assume_exception() const & noexcept

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

  24. const exception_type &&assume_exception() const && noexcept

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

  25. exception_type &&assume_exception() && noexcept

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

  26. exception_type &exception() &

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

  27. const exception_type &exception() const &

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

  28. exception_type &&exception() &&

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

  29. const exception_type &&exception() const &&

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

  30. exception_type failure() const noexcept

    Synthesising observer of the stored exception or error. Available if the traits is_error_code_available<T> and is_exception_ptr_available<T> are both true. Never throws.

  31. failure_type<error_type, exception_type> as_failure() const &

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

Modifiers

  1. void swap(basic_outcome &)

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

  2. failure_type<error_type, exception_type> as_failure() &&

    Return the output from free function failure() containing a move of any errored and/or excepted state.

Comparisons

See above for why LessThanComparable is not implemented.

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

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

  2. bool operator==(const basic_outcome<A, B, C, D> &) const

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

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

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

  4. bool operator==(const failure_type<A, B> &) const

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

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

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

  6. bool operator!=(const basic_outcome<A, B, C, D> &) const

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

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

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

  8. bool operator!=(const failure_type<A, B> &) const

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