std::variant<Types...>::valueless_by_exception (3) - Linux Manuals

std::variant<Types...>::valueless_by_exception: std::variant<Types...>::valueless_by_exception

NAME

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:

  struct S {
      operator int() { throw 42; }
  };
  variant<float, int> v{12.f}; // OK
  v.emplace<1>(S()); // v may be valueless

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.