20 General utilities library [utilities]

20.7 Variants [variant]

20.7.3 Class template variant [variant.variant]

20.7.3.6 Value status [variant.status]

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constexpr bool valueless_by_exception() const noexcept;
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Effects: Returns false if and only if the variant holds a value.
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[Note 1:
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]
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constexpr size_t index() const noexcept;
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Effects: If valueless_­by_­exception() is true, returns variant_­npos.
Otherwise, returns the zero-based index of the alternative of the contained value.