tmp/tmpfdli91lr/{from.md → to.md}
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| 1 |
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### `variant` helper classes <a id="variant.helper">[[variant.helper]]</a>
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``` cpp
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template <class T> struct variant_size;
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```
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*Remarks:* All specializations of `variant_size` shall meet the
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`UnaryTypeTrait` requirements ([[meta.rqmts]]) with a base
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characteristic of `integral_constant<size_t, N>` for some `N`.
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``` cpp
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template <class T> class variant_size<const T>;
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template <class T> class variant_size<volatile T>;
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template <class T> class variant_size<const volatile T>;
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```
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Let `VS` denote `variant_size<T>` of the cv-unqualified type `T`. Then
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each of the three templates shall meet the `UnaryTypeTrait`
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requirements ([[meta.rqmts]]) with a base characteristic of
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`integral_constant<size_t, VS::value>`.
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``` cpp
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template <class... Types>
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struct variant_size<variant<Types...>> : integral_constant<size_t, sizeof...(Types)> { };
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```
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``` cpp
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template <size_t I, class T> class variant_alternative<I, const T>;
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template <size_t I, class T> class variant_alternative<I, volatile T>;
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template <size_t I, class T> class variant_alternative<I, const volatile T>;
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```
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Let `VA` denote `variant_alternative<I, T>` of the cv-unqualified type
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`T`. Then each of the three templates shall meet the
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`TransformationTrait` requirements ([[meta.rqmts]]) with a member
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typedef `type` that names the following type:
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- for the first specialization, `add_const_t<VA::type>`,
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- for the second specialization, `add_volatile_t<VA::type>`, and
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- for the third specialization, `add_cv_t<VA::type>`.
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``` cpp
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variant_alternative<I, variant<Types...>>::type
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```
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*Requires:* `I < sizeof...(Types)`.
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*Value:* The type `T_I`.
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