23 General utilities library [utilities]

23.7 Variants [variant]

23.7.3 Class template variant [variant.variant] Value status [variant.status]

constexpr bool valueless_by_exception() const noexcept;

Effects: Returns false if and only if the variant holds a value.

[Note: A variant might not hold a value if an exception is thrown during a type-changing assignment or emplacement. The latter means that even a variant<float, int> can become valueless_­by_­exception(), for instance by

struct S { operator int() { throw 42; }};
variant<float, int> v{12.f};

end note]

constexpr size_t index() const noexcept;

Effects: If valueless_­by_­exception() is true, returns variant_­npos. Otherwise, returns the zero-based index of the alternative of the contained value.