projects / rustc.git / blame
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1 | # Function pointer types |
| 2 | ||
| 3 | > **<sup>Syntax</sup>**\ | |
| 4 | > _BareFunctionType_ :\ | |
| 5869c6ff | 5 | > [_ForLifetimes_]<sup>?</sup> _FunctionTypeQualifiers_ `fn`\ |
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6 | > `(` _FunctionParametersMaybeNamedVariadic_<sup>?</sup> `)` _BareFunctionReturnType_<sup>?</sup> |
| 7 | > | |
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8 | > _FunctionTypeQualifiers_:\ |
| 9 | > `unsafe`<sup>?</sup> (`extern` [_Abi_]<sup>?</sup>)<sup>?</sup> | |
| 10 | > | |
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11 | > _BareFunctionReturnType_:\ |
| 12 | > `->` [_TypeNoBounds_] | |
| 13 | > | |
| 14 | > _FunctionParametersMaybeNamedVariadic_ :\ | |
| 15 | > _MaybeNamedFunctionParameters_ | _MaybeNamedFunctionParametersVariadic_ | |
| 16 | > | |
| 17 | > _MaybeNamedFunctionParameters_ :\ | |
| 18 | > _MaybeNamedParam_ ( `,` _MaybeNamedParam_ )<sup>\*</sup> `,`<sup>?</sup> | |
| 19 | > | |
| 20 | > _MaybeNamedParam_ :\ | |
| e74abb32 | 21 | > [_OuterAttribute_]<sup>\*</sup> ( ( [IDENTIFIER] | `_` ) `:` )<sup>?</sup> [_Type_] |
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22 | > |
| 23 | > _MaybeNamedFunctionParametersVariadic_ :\ | |
| e74abb32 | 24 | > ( _MaybeNamedParam_ `,` )<sup>\*</sup> _MaybeNamedParam_ `,` [_OuterAttribute_]<sup>\*</sup> `...` |
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25 | |
| 26 | Function pointer types, written using the `fn` keyword, refer to a function | |
| 27 | whose identity is not necessarily known at compile-time. They can be created | |
| 28 | via a coercion from both [function items] and non-capturing [closures]. | |
| 29 | ||
| 30 | The `unsafe` qualifier indicates that the type's value is an [unsafe | |
| 31 | function], and the `extern` qualifier indicates it is an [extern function]. | |
| 32 | ||
| 33 | Variadic parameters can only be specified with [`extern`] function types with | |
| 34 | the `"C"` or `"cdecl"` calling convention. | |
| 35 | ||
| 36 | An example where `Binop` is defined as a function pointer type: | |
| 37 | ||
| 38 | ```rust | |
| 39 | fn add(x: i32, y: i32) -> i32 { | |
| 40 | x + y | |
| 41 | } | |
| 42 | ||
| 43 | let mut x = add(5,7); | |
| 44 | ||
| 45 | type Binop = fn(i32, i32) -> i32; | |
| 46 | let bo: Binop = add; | |
| 47 | x = bo(5,7); | |
| 48 | ``` | |
| 49 | ||
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50 | ## Attributes on function pointer parameters |
| 51 | ||
| 52 | Attributes on function pointer parameters follow the same rules and | |
| 53 | restrictions as [regular function parameters]. | |
| 54 | ||
| 416331ca | 55 | [IDENTIFIER]: ../identifiers.md |
| 5869c6ff | 56 | [_Abi_]: ../items/functions.md |
| 17df50a5 | 57 | [_ForLifetimes_]: ../trait-bounds.md#higher-ranked-trait-bounds |
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58 | [_TypeNoBounds_]: ../types.md#type-expressions |
| 59 | [_Type_]: ../types.md#type-expressions | |
| e74abb32 | 60 | [_OuterAttribute_]: ../attributes.md |
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61 | [`extern`]: ../items/external-blocks.md |
| 62 | [closures]: closure.md | |
| e1599b0c | 63 | [extern function]: ../items/functions.md#extern-function-qualifier |
| 416331ca | 64 | [function items]: function-item.md |
| 2b03887a | 65 | [unsafe function]: ../unsafe-keyword.md |
| 29967ef6 | 66 | [regular function parameters]: ../items/functions.md#attributes-on-function-parameters |