[#BloombergLP-bslstl-Variant_OverloadHelper-05]
= xref:BloombergLP.adoc[BloombergLP]::xref:BloombergLP/bslstl.adoc[bslstl]::Variant_OverloadHelper
:relfileprefix: ../../
:mrdocs:

This component‐private metafunction provides a member typedef `Index` representing the value that should be computed by `Variant_ConvertIndex`. The primary template is instantiated when the partial specialization below is not viable because no viable alternative exists for the conversion or because the best match is not "unique" (see the documentation of `Variant_ConvertIndex` for an explanation).

== Synopsis

Declared in `<bslstl_variant.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<
    class t_SRC,
    class t_VARIANT,
    class = void>
struct Variant_OverloadHelper;
----

== Type Aliases

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bslstl/Variant_OverloadHelper-05/Index.adoc[`Index`] 
| Index constant equal to `variant_npos` when no unique conversion exists.
|===


== Specializations

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bslstl/Variant_OverloadHelper-0d.adoc[`Variant_OverloadHelper<t_SRC, bsl::variant<t_HEAD, t_TAIL...>, bsl::void_t<decltype(Variant_OverloadSetImp<t_SRC, 0, t_HEAD, t_TAIL...>::
                             candidate(std::declval<t_SRC>()))>>`] 
| This partial specialization is used when a "unique" best match is found for converting `std::declval<t_SRC>()` to one of (template parameters) `t_HEAD, t_TAIL...`.
|===



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