[#BloombergLP-bslmf-ResultType_Fallback-0b]
= xref:BloombergLP.adoc[BloombergLP]::xref:BloombergLP/bslmf.adoc[bslmf]::ResultType_Fallback
:relfileprefix: ../../
:mrdocs:

Metafunction that defines `type` to be the specified `t_FALLBACK` type unless `t_FALLBACK` is `bslmf::Nil`.

== Synopsis

Declared in `<bslmf_resulttype.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<class t_FALLBACK>
struct ResultType_Fallback;
----

== Type Aliases

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bslmf/ResultType_Fallback-0b/type.adoc[`type`] 
| The fallback type, unless `t_FALLBACK` is `bslmf::Nil`.
|===


== Specializations

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bslmf/ResultType_Fallback-07.adoc[`ResultType_Fallback<bslmf::Nil>`] 
| Specialization of `ResultType_Fallback` that does not define `type` if `bslmf::Nil` is specified as the fallback parameter.
|===


== Derived Classes

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bslmf/ResultType_BdeIdiom-0d.adoc[`ResultType_BdeIdiom`] 
| Metafunction to detect the BDE `ResultType` idiom as part of the implementation of `bslmf::ResultType`. This `struct` is instantiated when `t_FUNC::result_type` doesn't exist. This primary template is matched when `t_FUNC::ResultType` also does not exist. The `t_FALLBACK` type, if any, is produced.
|===


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