std::nullopt_t
From cppreference.com
| Defined in header <optional>
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||
| struct nullopt_t; |
(since C++17) | |
std::nullopt_t is an empty class type used to indicate optional type with uninitialized state. In particular, std::optional has a constructor with nullopt_t as a single argument, which creates an optional that does not contain a value.
std::nullopt_t must be a non-aggregate LiteralType and cannot have a default constructor or an initializer-list constructor.
It must have a constexpr constructor that takes some implementation-defined literal type.
[edit] Notes
The constraints on nullopt_t's constructors exist to support both op = {}; and op = nullopt; as the syntax for disengaging an optional object.
A possible implementation of this class is
struct nullopt_t { explicit constexpr nullopt_t(int) {} };
[edit] See also
| (C++17) |
an object of type nullopt_t (constant) |