std::unexpect_t, std::unexpect
From cppreference.com
C++
Feature test macros (C++20)
Concepts library (C++20)
Metaprogramming library (C++11)
Ranges library (C++20)
Filesystem library (C++17)
Concurrency support library (C++11)
Execution control library (C++26)
Utilities library
Type support (basic types, RTTI)
Library feature-test macros (C++20)
(C++11)
(C++20)
(C++26)
(C++20)
Coroutine support (C++20)
Contract support (C++26)
(C++20)(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)(C++20)(C++20)
(C++20)(C++20)(C++20)
General utilities
Relational operators (deprecated in C++20)
(C++20)(C++20)(C++20)
(C++20)(C++20)(C++20)
(C++20)
Swap and type operations
Common vocabulary types
std::expected
unexpect_tunexpect
Defined in header
<expected>
struct unexpect_t { explicit unexpect_t() = default; };
(1)
(since C++23)
inline constexpr std::unexpect_t unexpect{};
(2)
(since C++23)
1) A tag type for in-place construction of an unexpected value in an
std::expected object.2) A constant of type const std::unexpect_t which is usually directly passed to a constructor of
std::expected to construct an unexpected value.[edit] Notes
Like other construction tag types, unexpect_t is a trivial, empty class with an explicit default constructor.