22 General utilities library [utilities]

22.6 Variants [variant]

22.6.3 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 1: 
It is possible for a variant to hold no 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}; v.emplace<1>(S());
— 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.