[#bsl-is_empty]
= xref:bsl.adoc[bsl]::is_empty
:relfileprefix: ../
:mrdocs:

This `struct` is a meta‐function to determine whether the (template parameter) `t_TYPE` is an empty class type. This `struct` derives from `bsl::true_type` if the `t_TYPE` is empty, and from `bsl::false_type` otherwise. This meta‐function has the same syntax as the `is_empty` meta‐function defined in the C++11 standard [meta.unary.prop]; on C++03 platforms, however, this meta‐function defaults to `true_type` if `t_TYPE` is a `class` or `struct` with no non‐static data members other than bit‐fields of length 0, no virtual member functions, no virtual base classes, and no base class `B` for which `is_empty<B>::value` is `false`; otherwise `is_empty` defaults to `false_type`. Note that this meta‐function will fail to compile for a union that is the same size as an empty class in C++03.

== Synopsis

Declared in `<bslmf_isempty.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<class t_TYPE>
struct is_empty
    : xref:bsl/integral_constant-02.adoc[bsl::integral_constant<bool, ::std::is_empty<t_TYPE>::value>]
----

== Base Classes

[cols="1,4"]
|===
| Name| Description
| `xref:bsl/integral_constant-02.adoc[bsl::integral_constant<bool, ::std::is_empty<t_TYPE>::value>]`
| Metafunction representing a compile‐time constant of the specified (template parameter) `t_TYPE` with the specified (template parameter) `t_VALUE`.
|===

== Type Aliases

[cols="1,4"]
|===
| Name| Description
| xref:bsl/integral_constant-02/type.adoc[`type`] 
| Alias for this `integral_constant` specialization.
| xref:bsl/integral_constant-02/value_type.adoc[`value_type`] 
| 
|===


== Member Functions

[cols="1,4"]
|===
| Name| Description
| xref:bsl/integral_constant-02/operator_call.adoc[`operator()`] 
| Return a copy of the template argument `t_VALUE`.
| xref:bsl/integral_constant-02/2conversion.adoc[`operator value_type`] 
| 
|===


== Static Data Members

[cols="1"]
|===
| Name
| xref:bsl/integral_constant-02/value.adoc[`value`] 
|===


== Derived Classes

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bslma/AllocatorTraits_IsAlwaysEqual-0a.adoc[`AllocatorTraits_IsAlwaysEqual<ALLOC, false>`] 
| This `struct` template sets the boolean type for the attribute named `is_always_equal` to `bsl::is_empty<ALLOC>` if the given (template parameter) `ALLOC` does not define such an alias (i.e., if `false == AllocatorTraits_HasIsAlwaysEqual<ALLOCATOR_TYPE>::value`).
|===


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