[#BloombergLP-bdlb-Variant]
= xref:BloombergLP.adoc[BloombergLP]::xref:BloombergLP/bdlb.adoc[bdlb]::Variant
:relfileprefix: ../../
:mrdocs:

Discriminated union that holds a value of one of up to 20 alternative types.

== Synopsis

Declared in `<bdlb_variant.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<class... TYPES>
class Variant
    : public xref:BloombergLP/bdlb/VariantImp.adoc[VariantImp<bslmf::TypeList<TYPES...>::ListType>]
----

== Description

This class provides a "variant" type, i.e., a type capable of storing values from a list of template parameter types. Note that if the number `N` of types is smaller than 20, unused parameters default to `bslmf::Nil`, but it is more economical to use `VariantN`, which accepts exactly `N` template arguments, as this leads to shorter symbols and debug string information.

== Base Classes

[cols="1,4"]
|===
| Name| Description
| `xref:BloombergLP/bdlb/VariantImp.adoc[VariantImp<bslmf::TypeList<TYPES...>::ListType>]`
| This class provides the implementation of `Variant` (except for the creators) given a list of template parameter `TYPES`.
|===

== Member Functions

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bdlb/Variant/2constructor-035.adoc[`Variant`]         [.small]#[constructor]#
| Constructors
| xref:BloombergLP/bdlb/Variant/operator_assign-0e.adoc[`operator=`] 
| Assignment operators
| xref:BloombergLP/bdlb/Variant/apply.adoc[`apply`] 
| `apply` overloads
| xref:BloombergLP/bdlb/Variant/applyRaw.adoc[`applyRaw`] 
| `applyRaw` overloads
| xref:BloombergLP/bdlb/Variant/assign.adoc[`assign`] 
| Assign the specified `TYPE` `value` to this object.
| xref:BloombergLP/bdlb/VariantImp/assignTo.adoc[`assignTo`] 
| Assign `value` converted to `TYPE` to this object.
| xref:BloombergLP/bdlb/VariantImp/bdexStreamIn.adoc[`bdexStreamIn`] 
| DEPRECATED: Do not use.
| xref:BloombergLP/bdlb/VariantImp/bdexStreamOut.adoc[`bdexStreamOut`] 
| DEPRECATED: Do not use.
| xref:BloombergLP/bdlb/VariantImp/createInPlace.adoc[`createInPlace`] 
| Emplace a `TYPE` instance constructed from `arguments`.
| xref:BloombergLP/bdlb/VariantImp/is.adoc[`is`] 
| Return whether this variant currently holds a `TYPE` value.
| xref:BloombergLP/bdlb/VariantImp/isUnset.adoc[`isUnset`] 
| Return whether this variant currently holds no value.
| xref:BloombergLP/bdlb/VariantImp/maxSupportedBdexVersion.adoc[`maxSupportedBdexVersion`] 
| DEPRECATED: Do not use.
| xref:BloombergLP/bdlb/VariantImp/print.adoc[`print`] 
| Format this object to the specified output `stream`.
| xref:BloombergLP/bdlb/VariantImp/reset.adoc[`reset`] 
| Destroy the current value held by this variant (if any), and reset this variant to the unset state.
| xref:BloombergLP/bdlb/VariantImp/swap.adoc[`swap`] 
| Swap the value of this object with `other`.
| xref:BloombergLP/bdlb/Variant/the.adoc[`the`] 
| `the` overloads
| xref:BloombergLP/bdlb/VariantImp/typeIndex.adoc[`typeIndex`] 
| Return the 1‐based index of the held type, or 0 if unset.
| xref:BloombergLP/bdlb/VariantImp/typeInfo.adoc[`typeInfo`] 
| Return `typeid(void)`.
| xref:BloombergLP/bdlb/Variant/2conversion-09.adoc[`operator BloombergLP::bslmf::NestedTraitDeclaration<Variant, HasPrintMethod>`] 
| Nested trait declaration for `bdlb::HasPrintMethod`.
| xref:BloombergLP/bdlb/Variant/2conversion-0a.adoc[`operator BloombergLP::bslmf::NestedTraitDeclaration<Variant, IsBitwiseCopyable, Traits::k_VARIANT_IS_BITWISE_COPYABLE>`] 
| Nested trait declaration for `bslmf::IsBitwiseCopyable`.
| xref:BloombergLP/bdlb/Variant/2conversion-058.adoc[`operator BloombergLP::bslmf::NestedTraitDeclaration<Variant, IsBitwiseMoveable, Traits::k_VARIANT_IS_BITWISE_MOVEABLE>`] 
| Nested trait declaration for `bslmf::IsBitwiseMoveable`.
| xref:BloombergLP/bdlb/Variant/2conversion-050.adoc[`operator BloombergLP::bslmf::NestedTraitDeclaration<Variant, UsesBslmaAllocator, Traits::k_VARIANT_USES_BSLMA_ALLOCATOR>`] 
| Nested trait declaration for `bslma::UsesBslmaAllocator`.
| xref:BloombergLP/bdlb/VariantImp/2conversion-0e.adoc[`operator BloombergLP::bslmf::NestedTraitDeclaration<VariantImp, HasPrintMethod>`] 
| Nested trait declaration for `bdlb::HasPrintMethod`.
| xref:BloombergLP/bdlb/VariantImp/2conversion-02.adoc[`operator BloombergLP::bslmf::NestedTraitDeclaration<VariantImp, IsBitwiseCopyable, Traits::k_VARIANT_IS_BITWISE_COPYABLE>`] 
| Nested trait declaration for `bslmf::IsBitwiseCopyable`.
| xref:BloombergLP/bdlb/VariantImp/2conversion-05.adoc[`operator BloombergLP::bslmf::NestedTraitDeclaration<VariantImp, IsBitwiseMoveable, Traits::k_VARIANT_IS_BITWISE_MOVEABLE>`] 
| Nested trait declaration for `bslmf::IsBitwiseMoveable`.
| xref:BloombergLP/bdlb/VariantImp/2conversion-04.adoc[`operator BloombergLP::bslmf::NestedTraitDeclaration<VariantImp, UsesBslmaAllocator, Traits::k_VARIANT_USES_BSLMA_ALLOCATOR>`] 
| Nested trait declaration for `bslma::UsesBslmaAllocator`.
|===


== Non-Member Functions

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bdlb/operator_not_eq-045.adoc[`operator!=`]
| Return whether `lhs` and `rhs` do not have the same value.
| xref:BloombergLP/bdlb/operator_eq-0101.adoc[`operator==`]
| Grant the equality comparison operator access to this class's private members.
| xref:BloombergLP/bdlb/operator_eq-0fc.adoc[`operator==`]
| Return whether `lhs` and `rhs` have the same value.
| xref:BloombergLP/bdlb/swap-04.adoc[`swap`]
| Swap the values of the specified `a` and `b` objects.
|===


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