| # Function pointer types |
| |
| > **<sup>Syntax</sup>**\ |
| > _BareFunctionType_ :\ |
| > [_ForLifetimes_]<sup>?</sup> _FunctionTypeQualifiers_ `fn`\ |
| > `(` _FunctionParametersMaybeNamedVariadic_<sup>?</sup> `)` _BareFunctionReturnType_<sup>?</sup> |
| > |
| > _FunctionTypeQualifiers_:\ |
| > `unsafe`<sup>?</sup> (`extern` [_Abi_]<sup>?</sup>)<sup>?</sup> |
| > |
| > _BareFunctionReturnType_:\ |
| > `->` [_TypeNoBounds_] |
| > |
| > _FunctionParametersMaybeNamedVariadic_ :\ |
| > _MaybeNamedFunctionParameters_ | _MaybeNamedFunctionParametersVariadic_ |
| > |
| > _MaybeNamedFunctionParameters_ :\ |
| > _MaybeNamedParam_ ( `,` _MaybeNamedParam_ )<sup>\*</sup> `,`<sup>?</sup> |
| > |
| > _MaybeNamedParam_ :\ |
| > [_OuterAttribute_]<sup>\*</sup> ( ( [IDENTIFIER] | `_` ) `:` )<sup>?</sup> [_Type_] |
| > |
| > _MaybeNamedFunctionParametersVariadic_ :\ |
| > ( _MaybeNamedParam_ `,` )<sup>\*</sup> _MaybeNamedParam_ `,` [_OuterAttribute_]<sup>\*</sup> `...` |
| |
| Function pointer types, written using the `fn` keyword, refer to a function |
| whose identity is not necessarily known at compile-time. They can be created |
| via a coercion from both [function items] and non-capturing [closures]. |
| |
| The `unsafe` qualifier indicates that the type's value is an [unsafe |
| function], and the `extern` qualifier indicates it is an [extern function]. |
| |
| Variadic parameters can only be specified with [`extern`] function types with |
| the `"C"` or `"cdecl"` calling convention. |
| |
| An example where `Binop` is defined as a function pointer type: |
| |
| ```rust |
| fn add(x: i32, y: i32) -> i32 { |
| x + y |
| } |
| |
| let mut x = add(5,7); |
| |
| type Binop = fn(i32, i32) -> i32; |
| let bo: Binop = add; |
| x = bo(5,7); |
| ``` |
| |
| ## Attributes on function pointer parameters |
| |
| Attributes on function pointer parameters follow the same rules and |
| restrictions as [regular function parameters]. |
| |
| [IDENTIFIER]: ../identifiers.md |
| [_Abi_]: ../items/functions.md |
| [_ForLifetimes_]: ../trait-bounds.md#higher-ranked-trait-bounds |
| [_TypeNoBounds_]: ../types.md#type-expressions |
| [_Type_]: ../types.md#type-expressions |
| [_OuterAttribute_]: ../attributes.md |
| [`extern`]: ../items/external-blocks.md |
| [closures]: closure.md |
| [extern function]: ../items/functions.md#extern-function-qualifier |
| [function items]: function-item.md |
| [unsafe function]: ../unsafe-keyword.md |
| [regular function parameters]: ../items/functions.md#attributes-on-function-parameters |
