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martinmoene merged 1 commit into from
Oct 15, 2026
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Update the type comparison for C++23 expected #88

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18 changes: 11 additions & 7 deletions README.md
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Expand Up @@ -291,29 +291,33 @@ You can use the R3 revision of P2505, which lacks `error_or`, and uses `remove_c
<a id="comparison"></a>
## Comparison with like types

The standard-library columns describe C++23; availability in earlier language versions is listed separately.
The `nonstd::expected` column describes the implementation supplied by this library when `nsel_CONFIG_SELECT_EXPECTED=nsel_EXPECTED_NONSTD`; selecting the standard implementation instead gives the `std::expected` behavior.

|Feature |<br>std::pair|std:: optional |std:: expected |nonstd:: expected |Boost. Expected |Nonco expected |Andrei Expected |Hagan required |
|----------------------|-------------|---------------|---------------|------------------|----------------|---------------|----------------|---------------|
|More information | see [14] | see [5] | see [1] | this work | see [4] | see [7] | see [8] | see [13] |
|More information | see [14] | see [5] | [C++23 expected](https://timsong-cpp.github.io/cppwp/n4950/expected) | this work | see [4] | see [7] | see [8] | see [13] |
| | | | | | | | | |
| C++03 | yes | no | no | no/not yet | no (union) | no | no | yes |
| C++11 | yes | no | no | yes | yes | yes | yes | yes |
| C++14 | yes | no | no | yes | yes | yes | yes | yes |
| C++17 | yes | yes | no | yes | yes | yes | yes | yes |
| C++23 | yes | yes | yes | yes | not assessed | not assessed | not assessed | not assessed |
| | | | | | | | | |
|DefaultConstructible | T param | yes | yes | yes | yes | no | no | no |
|In-place construction | no | yes | yes | yes | yes | yes | no | no |
|DefaultConstructible | T param | yes | if T is void or default-constructible | yes | yes | no | no | no |
|In-place construction | yes, piecewise | yes | yes | yes | yes | yes | no | no |
|Literal type | yes | yes | yes | yes | yes | no | no | no |
| | | | | | | | | |
|Disengaged information| possible | no | yes | yes | yes | yes | yes | no |
|Vary disengaged type | yes | no | yes | yes | yes | no | no | no |
|Engaged nonuse throws | no | no | no | no | error_traits | no | no | yes |
|Disengaged use throws | no | yes, value() | yes, value() | yes, value() | yes,<br>value()| yes,<br>get() | yes,<br>get() | n/a |
| | | | | | | | | |
|Proxy (rel.ops) | no | yes | yes | yes | yes | no | no | no |
|References | no | yes | no/not yet | no/not yet | no/not yet | yes | no | no |
|Chained visitor(s) | no | no | yes | yes | yes | no | no | no |
|Comparison operators | yes | yes | equality only | equality; ordering for nsel_P0323R <= 2 | yes | no | no | no |
|References | yes | no | no | no | no/not yet | yes | no | no |
|Chained visitor(s) | no | yes (C++23) | yes (C++23) | yes | yes | no | no | no |

Note 1: std:&#58;*experimental*:&#58;expected
Note 1: `std::expected` is provided by `<expected>` in C++23, in namespace `std` rather than `std::experimental`. Its monadic operations are `and_then`, `or_else`, `transform`, and `transform_error`. The older third-party implementations have not been reassessed for C++23 compatibility here.

Note 2: sources for [Nonco expected](https://github.com/martinmoene/spike-expected/tree/master/nonco), [Andrei Expected](https://github.com/martinmoene/spike-expected/tree/master/alexandrescu) and [Hagan required](https://github.com/martinmoene/spike-expected/tree/master/hagan) can befound in the [spike-expected](https://github.com/martinmoene/spike-expected) repository.

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