[#BloombergLP-bslstl-Variant_ConvertsWithoutNarrowing-0b]
= xref:BloombergLP.adoc[BloombergLP]::xref:BloombergLP/bslstl.adoc[bslstl]::Variant_ConvertsWithoutNarrowing
:relfileprefix: ../../
:mrdocs:

Metafunction that is `true_type` when `t_SOURCE` converts to `t_DEST` without narrowing, and `false_type` otherwise.

== Synopsis

Declared in `<bslstl_variant.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<
    class t_DEST,
    class t_SOURCE,
    class = void>
struct Variant_ConvertsWithoutNarrowing
    : xref:bsl/false_type.adoc[bsl::false_type]
----

== Base Classes

[cols="1,4"]
|===
| Name| Description
| `xref:bsl/false_type.adoc[bsl::false_type]`
| Metafunction representing the boolean constant `false`.
|===

== Specializations

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bslstl/Variant_ConvertsWithoutNarrowing-0a.adoc[`Variant_ConvertsWithoutNarrowing<t_DEST, t_SOURCE, bsl::void_t<decltype(Variant_ArrayHelper<t_DEST>{{std::declval<t_SOURCE>()}})>>`] 
| This component‐private metafunction is derived from `bsl::true_type` if (template parameter) `t_SOURCE` can be converted to (template parameter) `t_DEST` without narrowing, and `bsl::false_type` otherwise. A conversion from pointer or pointer‐to‐member type to cv `bool` is considered narrowing even if the compiler does not implement P1957R2 (which was adopted as a DR); however, on compilers that do not implement P1957R2, we do not have the ability to check whether a user‐defined conversion sequence to cv `bool` would use a narrowing standard conversion, so on those compilers, we permit conversion to a `t_DEST` that is cv `bool` only if `t_SOURCE` is also cv `bool`, and not when `t_SOURCE` is a class type. This behavior is not expected to pose a problem for users migrating from `bdlb::Variant`, because that class does not support implicit conversions from an argument type to an alternative type.
|===



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