[#bsl-Variant_VariantAlternativeImpl-06e]
= xref:bsl.adoc[bsl]::Variant_VariantAlternativeImpl
:relfileprefix: ../
:mrdocs:

This component‐private metafunction provides the implementation of `bsl::variant_alternative`.

== Synopsis

Declared in `<bslstl_variant.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<
    std::size_t t_INDEX,
    class t_HEAD = BSLSTL_VARIANT_NOT_A_TYPE,
    class... t_TAIL>
struct Variant_VariantAlternativeImpl
    : Variant_VariantAlternativeImpl<t_INDEX ‐ 1, t_TAIL...>
----

== Description

It recursively strips one type from the front of (template parameters) `t_HEAD, t_TAIL...` for each decrement of (template parameter) `t_INDEX` until `t_INDEX` reaches `0`, at which point the resulting `t_HEAD` is the alternative type.

== Base Classes

[cols="1,4"]
|===
| Name| Description
| `Variant_VariantAlternativeImpl<t_INDEX ‐ 1, t_TAIL...>`
| This component‐private metafunction provides the implementation of `bsl::variant_alternative`.
|===

== Specializations

[cols="1,4"]
|===
| Name| Description
| xref:bsl/Variant_VariantAlternativeImpl-00.adoc[`Variant_VariantAlternativeImpl<0, BSLSTL_VARIANT_NOT_A_TYPE>`] 
| This explicit specialization handles the case of an empty alternative list queried at index `0`. No member typedef is provided.
| xref:bsl/Variant_VariantAlternativeImpl-04.adoc[`Variant_VariantAlternativeImpl<t_INDEX, BSLSTL_VARIANT_NOT_A_TYPE>`] 
| This partial specialization is used when the list of alternatives is exhausted before `t_INDEX` reaches `0`, i.e., `t_INDEX` is out of range.
| xref:bsl/Variant_VariantAlternativeImpl-069.adoc[`Variant_VariantAlternativeImpl<0, t_HEAD, t_TAIL...>`] 
| This partial specialization is the base case of the recursion, reached when `t_INDEX` is `0`; it provides the member typedef `type` equal to `t_HEAD`.
|===


== Derived Classes

[cols="1,4"]
|===
| Name| Description
| `Variant_VariantAlternativeImpl` 
| This component‐private metafunction provides the implementation of `bsl::variant_alternative`.
| xref:bsl/variant_alternative-06.adoc[`variant_alternative<t_INDEX, variant<t_HEAD, t_TAIL...>>`] 
| Compute the alternative type at index `t_INDEX` of a `bsl::variant`.
|===


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