[#RPCMethod-MaybeArg]
= xref:RPCMethod.adoc[RPCMethod]::MaybeArg
:relfileprefix: ../
:mrdocs:

Helper to get an optional request argument.

== Synopsis

Declared in `<rpc/util.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<typename R>
auto
MaybeArg(std::string_view key) const;
----

== Description

Use this function when the argument is optional and does not have a default value. If the argument is required or has a default value, use Arg instead.

This function only works during m_fun(), i.e., it should only be used in RPC method implementations. It internally checks whether the user‐passed argument isNull() and parses (from JSON) and returns the user‐passed argument, or a falsy value if no argument was passed.

The instantiation of this helper for type R must match the corresponding RPCArg::Type.

== Return Value

For trivially copyable types, a std::optional<R> is returned. For other types, a R* pointer to the argument is returned. If the argument is not provided, std::nullopt or a null pointer is returned.

== Parameters

[cols="1,4"]
|===
| Name| Description
| *key*
| The name of the argument to fetch.
|===

== See Also

Arg for handling arguments that are required or have a default value.


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