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                合規國際互聯網加速 OSASE為企業客戶提供高速穩定SD-WAN國際加速解決方案。 廣告
                通過`associated_consts`功能,你像這樣可以定義常量: ~~~ trait Foo { const ID: i32; } impl Foo for i32 { const ID: i32 = 1; } fn main() { assert_eq!(1, i32::ID); } ~~~ 任何`Foo`的定義都必須定義`ID`,不定義的話: ~~~ #![feature(associated_consts)] trait Foo { const ID: i32; } impl Foo for i32 { } ~~~ 會給出 ~~~ error: not all trait items implemented, missing: `ID` [E0046] impl Foo for i32 { } ~~~ 實現也可以定義一個默認值: ~~~ #![feature(associated_consts)] trait Foo { const ID: i32 = 1; } impl Foo for i32 { } impl Foo for i64 { const ID: i32 = 5; } fn main() { assert_eq!(1, i32::ID); assert_eq!(5, i64::ID); } ~~~ 如你所見,當實現`Foo`時,你可以不實現它(關聯常量),當作為`i32`時。接著它將會使用默認值。不過,作為`i64`時,我們可以增加我們自己的定義。 關聯常量并不一定要關聯在一個特性上。一個`struct`的`impl`也行: ~~~ #![feature(associated_consts)] struct Foo; impl Foo { pub const FOO: u32 = 3; } ~~~
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