std:: nullopt

From cppreference.com

Defined in header `<optional>`
inline constexpr nullopt_t nullopt { /unspecified/ } ;		(since C++17)

std::nullopt is a constant of type std::nullopt_t . Used to indicate that an std::optional does not contain a value.

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Feature test macros (C++20)
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Concepts library (C++20)
Metaprogramming library (C++11)
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Member functions
optional::optional
optional::~optional
optional::operator=
Observers
optional::operator-> optional::operator*
optional::operator bool optional::has_value
optional::value
optional::value_or
Iterators
optional::begin (C++26)
optional::end (C++26)
Monadic operations
optional::and_then (C++23)
optional::transform (C++23)
optional::or_else (C++23)
Modifiers
optional::emplace
optional::swap
optional::reset
Non-member functions
operator== operator!= operator< operator<= operator> operator>= operator<=> (C++20)
make_optional
swap (std::optional)
Deduction guides
Helper classes
hash <std::optional>
nullopt_t
in_place_t
bad_optional_access
Helper objects
nullopt
in_place