23 General utilities library [utilities]

23.7 Variants [variant]

constexpr bool valueless_by_exception() const noexcept;

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

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};
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.