我有以下几点:
let mut my_number = 32.90;
如何打印my_number的类型?
使用type和type_of不起作用。有其他方法可以打印数字的类型吗?
我有以下几点:
let mut my_number = 32.90;
如何打印my_number的类型?
使用type和type_of不起作用。有其他方法可以打印数字的类型吗?
当前回答
如果你只是想找出一个变量的类型,并愿意在编译时执行,你可能会导致一个错误,并让编译器拾取它。
例如,将变量设置为一个无效的类型:
let mut my_number: () = 32.90;
// let () = x; would work too
error[E0308]: mismatched types
--> src/main.rs:2:29
|
2 | let mut my_number: () = 32.90;
| ^^^^^ expected (), found floating-point number
|
= note: expected type `()`
found type `{float}`
或者调用无效的方法:
let mut my_number = 32.90;
my_number.what_is_this();
error[E0599]: no method named `what_is_this` found for type `{float}` in the current scope
--> src/main.rs:3:15
|
3 | my_number.what_is_this();
| ^^^^^^^^^^^^
或访问无效字段:
let mut my_number = 32.90;
my_number.what_is_this
error[E0610]: `{float}` is a primitive type and therefore doesn't have fields
--> src/main.rs:3:15
|
3 | my_number.what_is_this
| ^^^^^^^^^^^^
These reveal the type, which in this case is actually not fully resolved. It’s called “floating-point variable” in the first example, and “{float}” in all three examples; this is a partially resolved type which could end up f32 or f64, depending on how you use it. “{float}” is not a legal type name, it’s a placeholder meaning “I’m not completely sure what this is”, but it is a floating-point number. In the case of floating-point variables, if you don't constrain it, it will default to f64¹. (An unqualified integer literal will default to i32.)
参见:
编译器错误消息中的{integer}或{float}是什么?
¹可能仍然有一些让编译器困惑的方法,使它无法在f32和f64之间做出决定;我不确定。它曾经像32.90.eq(&32.90)一样简单,但现在两者都被视为f64,并且可以愉快地进行,所以我不知道。
其他回答
**更新**最近没有被验证工作。
我把一个小板条箱一起做这个基于vbo的答案。它提供了一个宏来返回或打印类型。
把这个放在你的货物里。toml文件:
[dependencies]
t_bang = "0.1.2"
然后你可以这样使用它:
#[macro_use] extern crate t_bang;
use t_bang::*;
fn main() {
let x = 5;
let x_type = t!(x);
println!("{:?}", x_type); // prints out: "i32"
pt!(x); // prints out: "i32"
pt!(5); // prints out: "i32"
}
如果你只是想找出一个变量的类型,并愿意在编译时执行,你可能会导致一个错误,并让编译器拾取它。
例如,将变量设置为一个无效的类型:
let mut my_number: () = 32.90;
// let () = x; would work too
error[E0308]: mismatched types
--> src/main.rs:2:29
|
2 | let mut my_number: () = 32.90;
| ^^^^^ expected (), found floating-point number
|
= note: expected type `()`
found type `{float}`
或者调用无效的方法:
let mut my_number = 32.90;
my_number.what_is_this();
error[E0599]: no method named `what_is_this` found for type `{float}` in the current scope
--> src/main.rs:3:15
|
3 | my_number.what_is_this();
| ^^^^^^^^^^^^
或访问无效字段:
let mut my_number = 32.90;
my_number.what_is_this
error[E0610]: `{float}` is a primitive type and therefore doesn't have fields
--> src/main.rs:3:15
|
3 | my_number.what_is_this
| ^^^^^^^^^^^^
These reveal the type, which in this case is actually not fully resolved. It’s called “floating-point variable” in the first example, and “{float}” in all three examples; this is a partially resolved type which could end up f32 or f64, depending on how you use it. “{float}” is not a legal type name, it’s a placeholder meaning “I’m not completely sure what this is”, but it is a floating-point number. In the case of floating-point variables, if you don't constrain it, it will default to f64¹. (An unqualified integer literal will default to i32.)
参见:
编译器错误消息中的{integer}或{float}是什么?
¹可能仍然有一些让编译器困惑的方法,使它无法在f32和f64之间做出决定;我不确定。它曾经像32.90.eq(&32.90)一样简单,但现在两者都被视为f64,并且可以愉快地进行,所以我不知道。
最好使用这个:
fn print_type_of<T>(_: &T) -> String {
format!("{}", std::any::type_name::<T>())
}
fn main() {
let s = &"hello world".to_string();
let cloned_s = s.clone();
println!("{:?}", print_type_of(&s));
println!("{:?}", print_type_of(&cloned_s));
}
来自https://stackoverflow.com/a/29168659/6774636的推论
在稳定rust中有一个@ChrisMorgan答案可以获得近似类型(“float”),在夜间rust中有一个@ShubhamJain答案可以通过不稳定函数获得精确类型(“f64”)。
现在有一种方法可以得到精确的类型(即在f32和f64之间决定)在稳定的rust:
fn main() {
let a = 5.;
let _: () = unsafe { std::mem::transmute(a) };
}
结果
error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
--> main.rs:3:27
|
3 | let _: () = unsafe { std::mem::transmute(a) };
| ^^^^^^^^^^^^^^^^^^^
|
= note: source type: `f64` (64 bits)
= note: target type: `()` (0 bits)
更新
涡轮鱼的变异
fn main() {
let a = 5.;
unsafe { std::mem::transmute::<_, ()>(a) }
}
略短,但可读性稍差。
我非常喜欢@Coautose之前的回答,但如果有人只想要没有名称空间的类型名称,例如C而不是a::b::C,这里是一个修改后的宏版本,看起来像预期的那样工作:
macro_rules! ty {
($type:ty) => {{
let result = std::any::type_name::<$type>();
match result.rsplit_once(':') {
Some((_, s)) => s,
None => result,
}
}};
}
用法:
debug!("Testing type name: {}", ty!(A));