[英]error[E0308]: mismatched types , Default generic type is invalid
我嘗試在兩個不同的地方指定類型,但編譯器仍然拋出錯誤。
use serde::Serialize;
// 1: try to set default type.
struct Student<T = Option<String>> {
age: i32,
data: T
}
impl<T> Student<T> where T: Serialize {
fn new(age: i32) -> Self {
Student {
age,
data: <Option<String>>::None // 2. try to set default type
}
}
// fn new_with_data(age: i32, data: T) -> Self {
// Student {
// age,
// data
// }
// }
}
fn main() {
let stu1 = Student::new(123);
//let stu2 = Student::new_with_data(123, <Option<()>>::None);
}
得到錯誤:
Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
--> src/main.rs:13:19
|
8 | impl<T> Student<T> where T: Serialize {
| - this type parameter
...
13 | data: <Option<String>>::None
| ^^^^^^^^^^^^^^^^^^^^^^ expected type parameter `T`, found enum `std::option::Option`
|
= note: expected type parameter `T`
found enum `std::option::Option<std::string::String>`
For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` due to previous error
問題是當您設置默認類型時, impl
塊不要求T
是默認類型。 <Option<String>>::None
僅在類型為Option<String>
時才是有效值,但這並不要求必須如此。
/// `T` is unconstrained and may or may not be `Option<T>`
impl<T> Student<T> {}
/// This implements student for only one specific generic type (`Option<String>`). This
/// functionality will not be available for other generic types.
impl Student<Option<String>> {}
/// Here we don't include the generic so it implements for only the default generic. This is
/// equivalent to the previous version, but the generic is assumed to be the default value.
impl Student {}
我懷疑您想要做的是使數據通用。 我們沒有將Option
設為通用,而是將其保存的數據設為通用。 我們仍然可以設置默認值而不會遇到這個問題。
struct Student<T = String> {
age: i32,
/// Some optional generic data
data: Option<T>
}
impl<T> Student<T> where T: Serialize {
fn new(age: i32) -> Self {
Student {
age,
data: None,
}
}
fn new_with_data(age: i32, data: T) -> Self {
Student {
age,
data: Some(data),
}
}
}
另一種選擇是保持類型相同,但將impl
分成兩部分。 這樣,如果數據不是Option
,我們可以要求使用new_with_data
,但在某些情況下仍然允許使用new
。
struct Student<T = Option<String>> {
age: i32,
data: T
}
impl<T> Student<Option<T>> where T: Serialize {
fn new(age: i32) -> Self {
Student {
age,
data: None,
}
}
}
impl<T> Student<T> where T: Serialize {
fn new_with_data(age: i32, data: T) -> Self {
Student { age, data }
}
}
根據你的方法,我做了一點修改,就實現了我想要的。
use serde::Serialize;
struct Student<T = Option<String>> {
age: i32,
/// Some optional generic data
data: T
}
impl Student<Option<String>> {
fn new(age: i32) -> Self {
Student {
age,
data: None,
}
}
}
impl<T> Student<T> where T: Serialize {
fn new_with_data(age: i32, data: T) -> Self {
Student {
age, data
}
}
}
fn main() {
let stu1 = Student::new(123);
let stu2 = Student::new_with_data(123, 2222);
}
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