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

This component‐private class template implements the functionality of `bsl::optional`. The primary template is instantiated when `TYPE` is allocator‐aware, and holds the allocator that is used to create the stored object.

== Synopsis

Declared in `<bslstl_optional.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<
    class t_TYPE,
    bool t_USES_BSLMA_ALLOC = BloombergLP::bslma::UsesBslmaAllocator<t_TYPE>::value>
class Optional_Base;
----

== Type Aliases

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bslstl/Optional_Base-05/allocator_type.adoc[`allocator_type`] 
| This `typedef` is an alias to `bsl::allocator<char>`.
| xref:BloombergLP/bslstl/Optional_Base-05/value_type.adoc[`value_type`] 
| `value_type` is an alias for the underlying `TYPE` upon which this template class is instantiated, and represents the type of the managed object. The name is chosen so it is compatible with the `std::optional` implementation.
|===


== Member Functions

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bslstl/Optional_Base-05/operator_assign-09.adoc[`operator=`] 
| Assignment operators
| xref:BloombergLP/bslstl/Optional_Base-05/emplace-02.adoc[`emplace`] 
| `emplace` overloads
| xref:BloombergLP/bslstl/Optional_Base-05/get_allocator.adoc[`get_allocator`] 
| Return allocator used for construction of `value_type`.
| xref:BloombergLP/bslstl/Optional_Base-05/has_value.adoc[`has_value`] 
| Return `false` if this object is disengaged, and `true` otherwise.
| xref:BloombergLP/bslstl/Optional_Base-05/operator_star-0b.adoc[`operator*`] 
| Dereference operators
| xref:BloombergLP/bslstl/Optional_Base-05/operator_ptr-0a.adoc[`operator‐>`] 
| Member access operators
| xref:BloombergLP/bslstl/Optional_Base-05/reset.adoc[`reset`] 
| Reset this object to the default constructed state (i.e., to a disengaged state).
| xref:BloombergLP/bslstl/Optional_Base-05/swap.adoc[`swap`] 
| Efficiently exchange the value of this object with the value of the specified `other` object. This method provides the no‐throw exception‐safety guarantee if the template parameter `t_TYPE` provides that guarantee and the result of the `hasValue` method for the two objects being swapped is the same. The behavior is undefined unless this object was created with the same allocator as `other`.
| xref:BloombergLP/bslstl/Optional_Base-05/value-07a.adoc[`value`] 
| `value` overloads
| xref:BloombergLP/bslstl/Optional_Base-05/value_or-07.adoc[`value_or`] 
| `value_or` overloads
| xref:BloombergLP/bslstl/Optional_Base-05/2conversion-076.adoc[`operator BloombergLP::bslmf::NestedTraitDeclaration<Optional_Base, IsBitwiseCopyable, BloombergLP::bslmf::IsBitwiseCopyable<t_TYPE>::value>`] 
| Nested trait declaration for `bslmf::IsBitwiseCopyable`.
| xref:BloombergLP/bslstl/Optional_Base-05/2conversion-04.adoc[`operator BloombergLP::bslmf::NestedTraitDeclaration<Optional_Base, IsBitwiseMoveable, BloombergLP::bslmf::IsBitwiseMoveable<t_TYPE>::value>`] 
| Nested trait declaration for `bslmf::IsBitwiseMoveable`.
| xref:BloombergLP/bslstl/Optional_Base-05/2conversion-0a.adoc[`operator BloombergLP::bslmf::NestedTraitDeclaration<Optional_Base, UsesAllocatorArgT>`] 
| Nested trait declaration for `bslmf::UsesAllocatorArgT`.
| xref:BloombergLP/bslstl/Optional_Base-05/2conversion-0b.adoc[`operator BloombergLP::bslmf::NestedTraitDeclaration<Optional_Base, UsesBslmaAllocator>`] 
| Nested trait declaration for `bslma::UsesBslmaAllocator`.
| xref:BloombergLP/bslstl/Optional_Base-05/2conversion-071.adoc[`operator bool`] 
| Return `false` if this object is disengaged, and `true` otherwise.
|===


== Protected Type Aliases

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bslstl/Optional_Base-05/AllocType.adoc[`AllocType`] 
| This `typedef` is an alias to `allocator_type`.
|===


== Protected Member Functions

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bslstl/Optional_Base-05/2constructor-0f.adoc[`Optional_Base`]         [.small]#[constructor]#
| Constructors
| xref:BloombergLP/bslstl/Optional_Base-05/assignOrEmplace.adoc[`assignOrEmplace`] 
| If `*this` holds an object, assign to that object the value of the specified `rhs`, converted to `t_TYPE`. Otherwise, construct a held object from `rhs`, converted to `t_TYPE`. The allocators of `*this` and `rhs` remain unchanged.
| xref:BloombergLP/bslstl/Optional_Base-05/dereferenceRaw-049.adoc[`dereferenceRaw`] 
| `dereferenceRaw` overloads
|===


== Specializations

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bslstl/Optional_Base-08.adoc[`Optional_Base<t_TYPE, false>`] 
| Specialization of `Optional_Base` for `value_type` that is not allocator‐aware when `std::optional` is available.
|===


== Derived Classes

[cols="1,4"]
|===
| Name| Description
| xref:bsl/optional-072.adoc[`optional`] 
| 
|===


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