2 title = "`basic_outcome<T, EC, EP, NoValuePolicy>`"
3 description = "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."
6 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 {{% api "is_basic_outcome<T>" %}}.
8 *Requires*: Concept requirements if C++ 20, else static asserted:
10 - That trait {{% api "type_can_be_used_in_basic_result<R>" %}} is true for `T`, `EC` and `EP`.
11 - That either `EC` is `void` or `DefaultConstructible`.
12 - That either `EP` is `void` or `DefaultConstructible`.
14 *Namespace*: `BOOST_OUTCOME_V2_NAMESPACE`
16 *Header*: `<boost/outcome/basic_outcome.hpp>`
18 *Inclusions*: The very lightest weight of C and C++ header files:
21 2. `<initializer_list>`
25 6. If {{% api "BOOST_OUTCOME_USE_STD_IN_PLACE_TYPE" %}} is `1`, `<utility>` (defaults to `1` for C++ 17 or later only)
26 7. If C++ exceptions disabled and `BOOST_OUTCOME_DISABLE_EXECINFO` undefined only (used to print stack backtraces on "exception throw"):
27 1. `<sal.h>` (Windows only)
28 2. `<stddef.h>` (Windows only)
29 3. `<string.h>` (Windows only)
30 4. `<execinfo.h>` (POSIX only)
35 This very light weight set of inclusion dependencies makes basic outcome suitable for use in global header files of very large C++ codebases.
39 `basic_outcome` extends {{% api "basic_result<T, E, NoValuePolicy>" %}} with a third state to transport,
40 conventionally (but not necessarily) some sort of "abort" or "exceptional" state which a function can
41 return to indicate that not only did the operation fail, but it did so *catastrophically* i.e. please
42 abort any attempt to retry the operation.
44 A perfect alternative is to throw a C++ exception for the abort code path, and indeed most programs
45 ought to do exactly that instead of using `basic_outcome`. However there are a number of use cases
46 where choosing `basic_outcome` shines:
48 1. Where C++ exceptions or RTTI is not available, but the ability to fail catastrophically without
49 terminating the program is important.
50 2. Where deterministic behaviour is required even in the catastrophic failure situation.
51 3. In unit test suites of code using Outcome it is extremely convenient to accumulate test failures
52 into a `basic_outcome` for later reporting. A similar convenience applies to RPC situations, where
53 C++ exception throws need to be accumulated for reporting back to the initiating endpoint.
54 4. Where a function is "dual use deterministic" i.e. it can be used deterministically, in which case
55 one switches control flow based on `.error()`, or it can be used non-deterministically by throwing
56 an exception perhaps carrying a custom payload.
58 ### Public member type aliases
60 - `value_type` is `T`.
61 - `error_type` is `EC`.
62 - `exception_type` is `EP`.
63 - `value_type_if_enabled` is `T` if construction from `T` is available, else it is a usefully named unusable internal type.
64 - `error_type_if_enabled` is `EC` if construction from `EC` is available, else it is a usefully named unusable internal type.
65 - `exception_type_if_enabled` is `EP` if construction from `EP` is available, else it is a usefully named unusable internal type.
66 - `rebind<A, B = EC, C = EP, D = NoValuePolicy>` is `basic_outcome<A, B, C, D>`.
68 ### Protected member predicate booleans
70 - `predicate::constructors_enabled` is constexpr boolean true if:
71 1. Decayed `value_type` and decayed `error_type` are not the same type, or both are `void`.
72 2. Decayed `value_type` and decayed `exception_type` are not the same type, or both are `void`.
73 3. Decayed `error_type` and decayed `exception_type` are not the same type, or both are `void`.
75 - `predicate::implicit_constructors_enabled` is constexpr boolean true if:
76 1. `predicate::constructors_enabled` is true.
77 2. Trait {{% api "is_error_type<E>" %}} is not true for both decayed `value_type` and decayed `error_type` at the same time.
78 3. `value_type` is not implicitly constructible from `error_type` and `error_type` is not implicitly constructible from `value_type`.<br>OR<br>trait {{% api "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.
79 4. `value_type` is not implicitly constructible from `exception_type`.
80 5. `error_type` is not implicitly constructible from `exception_type`.
81 6. `exception_type` is not implicitly constructible from `value_type`.
82 7. `exception_type` is not implicitly constructible from `error_type`.
84 - `predicate::enable_value_converting_constructor<A>` is constexpr boolean true if:
85 1. `predicate::constructors_enabled` is true.
86 2. Decayed `A` is not this `basic_outcome` type.
87 3. `predicate::implicit_constructors_enabled` is true.
88 4. Decayed `A` is not an `in_place_type_t`.
89 5. Trait {{% api "is_error_type_enum<E, Enum>" %}} is false for `error_type` and decayed `A`.
90 6. `value_type` is implicitly constructible from `A` and `error_type` is not implicitly constructible from `A`.<br>OR<br>`value_type` is the exact same type as decayed `A` and `value_type` is implicitly constructible from `A`.
91 7. `exception_type` is not implicitly constructible from `A`.
93 - `predicate::enable_error_converting_constructor<A>` is constexpr boolean true if:
94 1. `predicate::constructors_enabled` is true.
95 2. Decayed `A` is not this `basic_outcome` type.
96 3. `predicate::implicit_constructors_enabled` is true.
97 4. Decayed `A` is not an `in_place_type_t`.
98 5. Trait {{% api "is_error_type_enum<E, Enum>" %}} is false for `error_type` and decayed `A`.
99 6. `value_type` is not implicitly constructible from `A` and `error_type` is implicitly constructible from `A`.<br>OR<br>`error_type` is the exact same type as decayed `A` and `error_type` is implicitly constructible from `A`.
100 7. `exception_type` is not implicitly constructible from `A`.
102 - `predicate::enable_error_condition_converting_constructor<ErrorCondEnum>` is constexpr boolean true if:
103 1. `predicate::constructors_enabled` is true.
104 2. Decayed `ErrorCondEnum` is not this `basic_outcome` type.
105 3. Decayed `ErrorCondEnum` is not an `in_place_type_t`.
106 4. Trait {{% api "is_error_type_enum<E, Enum>" %}} is true for `error_type` and decayed `ErrorCondEnum`.
107 5. `exception_type` is not implicitly constructible from `ErrorCondEnum`.
109 - `predicate::enable_exception_converting_constructor<A>` is constexpr boolean true if:
110 1. `predicate::constructors_enabled` is true.
111 2. Decayed `A` is not this `basic_outcome` type.
112 3. `predicate::implicit_constructors_enabled` is true.
113 4. Decayed `A` is not an `in_place_type_t`.
114 5. `value_type` is not implicitly constructible from `A`.
115 6. `error_type` is not implicitly constructible from `A`.
116 7. `exception_type` is implicitly constructible from `A`.
118 - `predicate::enable_error_exception_converting_constructor<A, B>` is constexpr boolean true if:
119 1. `predicate::constructors_enabled` is true.
120 2. Decayed `A` is not this `basic_outcome` type.
121 3. `predicate::implicit_constructors_enabled` is true.
122 4. Decayed `A` is not an `in_place_type_t`.
123 5. `value_type` is not implicitly constructible from `A`.
124 6. `error_type` is implicitly constructible from `A`.
125 7. `value_type` is not implicitly constructible from `B`.
126 8. `exception_type` is implicitly constructible from `B`.
128 - `predicate::enable_compatible_conversion<A, B, C, D>` is constexpr boolean true if:
129 1. `predicate::constructors_enabled` is true.
130 2. `basic_outcome<A, B, C, D>` is not this `basic_outcome` type.
131 3. `A` is `void` OR `value_type` is explicitly constructible from `A`.
132 4. `B` is `void` OR `error_type` is explicitly constructible from `B`.
133 5. `C` is `void` OR `exception_type` is explicitly constructible from `C`.
135 - `predicate::enable_make_error_code_compatible_conversion<A, B, C, D>` is constexpr boolean true if:
136 1. `predicate::constructors_enabled` is true.
137 2. `basic_outcome<A, B, C, D>` is not this `basic_outcome` type.
138 3. Trait {{% api "is_error_code_available<E>" %}} is true for decayed `error_type`.
139 4. `predicate::enable_compatible_conversion<A, B, C, D>` is not true.
140 5. `A` is `void` OR `value_type` is explicitly constructible from `A`.
141 6. `error_type` is explicitly constructible from `make_error_code(B)`.
142 7. `C` is `void` OR `exception_type` is explicitly constructible from `C`.
144 - `predicate::enable_inplace_value_constructor<Args...>` is constexpr boolean true if:
145 1. `predicate::constructors_enabled` is true.
146 2. `value_type` is `void` OR `value_type` is explicitly constructible from `Args...`.
148 - `predicate::enable_inplace_error_constructor<Args...>` is constexpr boolean true if:
149 1. `predicate::constructors_enabled` is true.
150 2. `error_type` is `void` OR `error_type` is explicitly constructible from `Args...`.
152 - `predicate::enable_inplace_exception_constructor<Args...>` is constexpr boolean true if:
153 1. `predicate::constructors_enabled` is true.
154 2. `exception_type` is `void` OR `exception_type` is explicitly constructible from `Args...`.
156 - `predicate::enable_inplace_value_error_exception_constructor<Args...>` is constexpr boolean true if:
157 1. `predicate::constructors_enabled` is true.
158 2. `predicate::implicit_constructors_enabled` is true.
159 3. Exactly one of `value_type` is explicitly constructible from `Args...`, or `error_type` is explicitly constructible from `Args...`, or `exception_type` is explicitly constructible
162 #### Summary of [standard requirements provided](https://en.cppreference.com/w/cpp/named_req)
164 - ~~`DefaultConstructible`~~, always deleted to force user to choose valued or errored or excepted for every outcome instanced.
165 - `MoveConstructible`, if all of `value_type`, `error_type` and `exception_type` implement move constructors.
166 - `CopyConstructible`, if all of `value_type`, `error_type` and `exception_type` implement copy constructors.
167 - `MoveAssignable`, if all of `value_type`, `error_type` and `exception_type` implement move constructors and move assignment.
168 - `CopyAssignable`, if all of `value_type`, `error_type` and `exception_type` implement copy constructors and copy assignment.
170 - `TriviallyCopyable`, if all of `value_type`, `error_type` and `exception_type` are trivially copyable.
171 - `TrivialType`, if all of `value_type`, `error_type` and `exception_type` are trivial types.
172 - `LiteralType`, if all of `value_type`, `error_type` and `exception_type` are literal types.
173 - ~~`StandardLayoutType`~~. It is implementation defined if `basic_outcome` can be used by C.
174 However all of the three major compilers MSVC, GCC and clang implement C layout of `basic_outcome` as follows:
177 struct outcome_layout {
178 struct result_layout {
183 exception_type exception;
186 If you choose standard layout `value_type`, `error_type` and `exception_type`, `basic_outcome`
187 works fine from C on MSVC, GCC and clang.
188 - `EqualityComparable`, if all of `value_type`, `error_type` and `exception_type` implement equality comparisons with one another.
189 - ~~`LessThanComparable`~~, not implemented due to availability of implicit conversions from `value_type`, `error_type` and `exception_type`, this can cause major surprise (i.e. hard to diagnose bugs), so we don't implement these at all.
191 - ~~`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.
193 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 {{% api "std::optional<T>" %}} will suffice.
195 ### Public member functions
197 #### Disabling constructors
199 {{% children description="true" depth="2" categories="disabling-constructors" %}}
201 #### Copy and move constructors and assignment, and destructor
203 {{% children description="true" depth="2" categories="default-constructors,copy-constructors,move-constructors,copy-assignment,move-assignment,destructors" %}}
205 #### Converting constructors
207 {{% children description="true" depth="2" categories="converting-constructors" %}}
209 #### Inplace constructors
211 {{% children description="true" depth="2" categories="inplace-constructors" %}}
213 #### Tagged constructors
215 {{% children description="true" depth="2" categories="tagged-constructors" %}}
219 {{% children description="true" depth="2" categories="observers" %}}
223 {{% children description="true" depth="2" categories="modifiers" %}}
227 See above for why `LessThanComparable` is not implemented.
229 {{% children description="true" depth="2" categories="comparisons" %}}