19 General utilities library [utilities]

19.7 Variants [variant]

19.7.3 Class template variant [variant.variant]

19.7.3.5 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};
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.