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. 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:

Namespace: OUTCOME_V2_NAMESPACE

Header: <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 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 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 NOT used, as it is assumed that sizeof(E) will be small for failure handling. This 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

Protected member predicate booleans

Summary of standard requirements provided

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(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.

  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.