[#FOLLY_CREATE_FREE_INVOKER]
= FOLLY_CREATE_FREE_INVOKER
:mrdocs:

* FOLLY_CREATE_FREE_INVOKER

== Synopsis

Declared in `<folly/functional/Invoke.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
#define FOLLY_CREATE_FREE_INVOKER(classname, funcname, ...)

----

== Description

Used to create an invoker type bound to a specific free‐invocable name.

Example:

FOLLY_CREATE_FREE_INVOKER(foo_invoker, foo);

The type `foo_invoker` is generated in the current namespace and may be used as follows:

namespace Deep { struct CanFoo {}; int foo(CanFoo const&, Bar&) { return 1; } int foo(CanFoo&&, Car&&) noexcept { return 2; } }

using traits = folly::invoke_traits<foo_invoker>;

traits::invoke(Deep::CanFoo{}, Car{}) // 2

traits::invoke_result<Deep::CanFoo, Bar&> // has member traits::invoke_result_t<Deep::CanFoo, Bar&> // int traits::invoke_result<Deep::CanFoo, Bar&&> // empty traits::invoke_result_t<Deep::CanFoo, Bar&&> // error

traits::is_invocable_v<CanFoo, Bar&> // true traits::is_invocable_v<CanFoo, Bar&&> // false

traits::is_invocable_r_v<int, CanFoo, Bar&> // true traits::is_invocable_r_v<char*, CanFoo, Bar&> // false

traits::is_nothrow_invocable_v<CanFoo, Bar&> // false traits::is_nothrow_invocable_v<CanFoo, Car&&> // true

traits::is_nothrow_invocable_v<int, CanFoo, Bar&> // false traits::is_nothrow_invocable_v<char*, CanFoo, Bar&> // false traits::is_nothrow_invocable_v<int, CanFoo, Car&&> // true traits::is_nothrow_invocable_v<char*, CanFoo, Car&&> // false

When a name has one or more primary definition in a fixed set of namespaces and alternate definitions in the namespaces of its arguments, the primary definitions may automatically be found as follows:

FOLLY_CREATE_FREE_INVOKER(swap_invoker, swap, std);

In this case, `swap_invoke_traits::invoke(int&, int&)` will use the primary definition found in `namespace std` relative to the current namespace, which may be equivalent to `namespace ::std`. In contrast:

namespace Deep { struct HasData {}; void swap(HasData&, HasData&) { throw 7; } }

using traits = invoke_traits<swap_invoker>;

HasData a, b; traits::invoke(a, b); // throw 7

== Parameters

[cols="1,4"]
|===
| Name| Description
| *classname*
| the name of the generated invoker type
| *funcname*
| the free‐invocable name to bind
|===


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