[#BloombergLP-bslstl-Variant_ConstructsFromType]
= xref:BloombergLP.adoc[BloombergLP]::xref:BloombergLP/bslstl.adoc[bslstl]::Variant&lowbar;ConstructsFromType
:relfileprefix: ../../
:mrdocs:

Determine whether `t&lowbar;TYPE` constructs a unique alternative of `t&lowbar;VARIANT`&period;

== Synopsis

Declared in `&lt;bslstl&lowbar;variant&period;h&gt;`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template&lt;
    class t&lowbar;VARIANT,
    class t&lowbar;TYPE&gt;
struct Variant&lowbar;ConstructsFromType
    : xref:bsl/integral_constant-02.adoc[bsl::integral&lowbar;constant&lt;bool, !bsl&colon;&colon;is&lowbar;same&lt;typename bsl&colon;&colon;remove&lowbar;cvref&lt;t&lowbar;TYPE&gt;&colon;&colon;type, t&lowbar;VARIANT&gt;&colon;&colon;value &amp;&amp; !Variant&lowbar;IsTag&lt;typename bsl&colon;&colon;remove&lowbar;cvref&lt;t&lowbar;TYPE&gt;&colon;&colon;type&gt;&colon;&colon;value &amp;&amp; !bsl&colon;&colon;is&lowbar;same&lt;typename Variant&lowbar;CorrespondingStdVariant&lt;t&lowbar;VARIANT&gt;&colon;&colon;type, typename bsl&colon;&colon;remove&lowbar;cvref&lt;t&lowbar;TYPE&gt;&colon;&colon;type&gt;&colon;&colon;value &amp;&amp; Variant&lowbar;IsAlternativeConstructibleFrom&lt;t&lowbar;VARIANT, t&lowbar;TYPE&gt;&colon;&colon;value&gt;]
----

== Description

This component&hyphen;private metafunction is derived from `bsl&colon;&colon;true&lowbar;type` if (template parameter) `t&lowbar;TYPE` is neither a tag type, nor the type (template parameter) `t&lowbar;VARIANT` (modulo cv&hyphen;qualification), nor the corresponding (possibly cv&hyphen;qualified) `std&colon;&colon;variant` type, there is a unique best match alternative in `t&lowbar;VARIANT` for `std&colon;&colon;declval&lt;t&lowbar;TYPE&gt;()`, and that alternative is constructible from `std&colon;&colon;declval&lt;t&lowbar;TYPE&gt;()`&semi; otherwise, this metafunction is derived from `bsl&colon;&colon;false&lowbar;type`&period;

This metafunction is used to constrain the converting constructor and converting assignment operator of `bsl&colon;&colon;variant`&period;

== Base Classes

[cols="1,4"]
|===
| Name| Description
| `xref:bsl/integral_constant-02.adoc[bsl::integral&lowbar;constant&lt;bool, !bsl&colon;&colon;is&lowbar;same&lt;typename bsl&colon;&colon;remove&lowbar;cvref&lt;t&lowbar;TYPE&gt;&colon;&colon;type, t&lowbar;VARIANT&gt;&colon;&colon;value &amp;&amp; !Variant&lowbar;IsTag&lt;typename bsl&colon;&colon;remove&lowbar;cvref&lt;t&lowbar;TYPE&gt;&colon;&colon;type&gt;&colon;&colon;value &amp;&amp; !bsl&colon;&colon;is&lowbar;same&lt;typename Variant&lowbar;CorrespondingStdVariant&lt;t&lowbar;VARIANT&gt;&colon;&colon;type, typename bsl&colon;&colon;remove&lowbar;cvref&lt;t&lowbar;TYPE&gt;&colon;&colon;type&gt;&colon;&colon;value &amp;&amp; Variant&lowbar;IsAlternativeConstructibleFrom&lt;t&lowbar;VARIANT, t&lowbar;TYPE&gt;&colon;&colon;value&gt;]`
| Metafunction representing a compile&hyphen;time constant of the specified (template parameter) `t&lowbar;TYPE` with the specified (template parameter) `t&lowbar;VALUE`&period;
|===

== Type Aliases

[cols="1,4"]
|===
| Name| Description
| xref:bsl/integral_constant-02/type.adoc[`type`] 
| Alias for this `integral&lowbar;constant` specialization&period;
| xref:bsl/integral_constant-02/value_type.adoc[`value&lowbar;type`] 
| Alias for the (template parameter) `t&lowbar;TYPE`&period;
|===


== Member Functions

[cols="1,4"]
|===
| Name| Description
| xref:bsl/integral_constant-02/operator_call.adoc[`operator()`] 
| Return a copy of the template argument `t&lowbar;VALUE`&period;
| xref:bsl/integral_constant-02/2conversion.adoc[`operator t&lowbar;TYPE`] 
| Return a copy of the template argument `t&lowbar;VALUE`&period;
|===


== Static Data Members

[cols="1,4"]
|===
| Name| Description
| xref:bsl/integral_constant-02/value.adoc[`value`] 
| The compile&hyphen;time constant value of this metafunction&period;
|===



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