std::variant::valueless_by_exception (3) Linux Manual Page

std::variant<Types…>::valueless_by_exception – std::variant<Types…>::valueless_by_exception

Synopsis

constexpr bool valueless_by_exception() const noexcept; (since C++17)
Returns false if and only if the variant holds a value.

Notes

A variant may become valueless in the following situations:
* (guaranteed) an exception is thrown during the move initialization of the contained value from the temporary in copy_assignment
* (guaranteed) an exception is thrown during the move initialization of the contained value during move_assignment
* (optionally) an exception is thrown when initializing the contained value during a type-changing assignment
* (optionally) an exception is thrown when initializing the contained value during a type-changing emplace
Since variant is never permitted to allocate dynamic memory, previous value cannot be retained in these situations.
This applies even to variants of non-class types:
A variant that is valueless by exception is treated as being in an invalid state: index returns variant_npos, get and visit throw bad_variant_access.