[#BloombergLP-bslstl-Function_InvokerUtilNullCheck-03]
= xref:BloombergLP.adoc[BloombergLP]::xref:BloombergLP/bslstl.adoc[bslstl]::Function&lowbar;InvokerUtilNullCheck
:relfileprefix: ../../
:mrdocs:

Customization point that detects whether a `FUNC` callable is null&period;

== Synopsis

Declared in `&lt;bslstl&lowbar;function&lowbar;invokerutil&period;h&gt;`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template&lt;class FUNC&gt;
struct Function&lowbar;InvokerUtilNullCheck;
----

== Description

Provides an `isNull` static method that that returns whether or not its argument is &quot;null&quot;, i&period;e&period;, it cannot be invoked&period; For must `FUNC` types `isNull` always returns `false` as every instance of `FUNC` is invocable&period; However, specializations of this class, especially for pointer types, have `isNull` functions that sometimes return `true`&period; This class is a customization point&colon; types outside of this component can (but rarely should) specialize this template&period; In particular, `bslstl&lowbar;function` contains a specialization for `bsl&colon;&colon;function`&period;

== Static Member Functions

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bslstl/Function_InvokerUtilNullCheck-03/isNull.adoc[`isNull`] 
| Return `false`&period;
|===


== Specializations

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bslstl/Function_InvokerUtilNullCheck-09.adoc[`Function&lowbar;InvokerUtilNullCheck&lt;bsl::function&lt;PROTO&gt;&gt;`] 
| Specialization of null checker for instantiations of `bsl&colon;&colon;function`&period; This specialization treats an empty `bsl&colon;&colon;function` as a null object&period;
| xref:BloombergLP/bslstl/Function_InvokerUtilNullCheck-0a.adoc[`Function&lowbar;InvokerUtilNullCheck&lt;FUNC*&gt;`] 
| Specialization of dispatcher for pointer objects&period;
| xref:BloombergLP/bslstl/Function_InvokerUtilNullCheck-07.adoc[`Function&lowbar;InvokerUtilNullCheck&lt;MEMTYPECLASS::*&gt;`] 
| Specialization of dispatcher for pointer&hyphen;to&hyphen;member objects&period;
|===



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