How to match a String against string literals?
I'm trying to figure out how to match a String
in Rust.
I initially tried matching like this, but I figured out Rust cannot implicitly cast from std::string::String
to &str
.
fn main() {
let stringthing = String::from("c");
match stringthing {
"a" => println!("0"),
"b" => println!("1"),
"c" => println!("2"),
}
}
This has the error:
error[E0308]: mismatched types
--> src/main.rs:4:9
|
4 | "a" => println!("0"),
| ^^^ expected struct `std::string::String`, found reference
|
= note: expected type `std::string::String`
found type `&'static str`
I then tried to construct new String
objects, as I could not find a function to cast a String
to a &str
.
fn main() {
let stringthing = String::from("c");
match stringthing {
String::from("a") => println!("0"),
String::from("b") => println!("1"),
String::from("c") => println!("2"),
}
}
This gave me the following error 3 times:
error[E0164]: `String::from` does not name a tuple variant or a tuple struct
--> src/main.rs:4:9
|
4 | String::from("a") => return 0,
| ^^^^^^^^^^^^^^^^^ not a tuple variant or struct
How to actually match String
s in Rust?
UPDATE
Use .as_str()
Reason
.as_str()
is more concise and enforces stricter type checking. The trait as_ref
is implemented for multiple types and its behaviour could be changed for type String
, leading to unexpected results. Similarly, if the input argument changes type, the compiler will not signal a problem when that type implements the trait as_ref
.
The docs suggest to use as_str
as well https://doc.rust-lang.org/std/string/struct.String.html, https://doc.rust-lang.org/std/primitive.str.html
Old answer:
as_slice
is deprecated, you should now use the trait std::convert::AsRef
instead:
match stringthing.as_ref() {
"a" => println!("0"),
"b" => println!("1"),
"c" => println!("2"),
_ => println!("something else!"),
}
Note that you also have to explicitly handle the catch-all case.
You can do something like this:
match &stringthing[..] {
"a" => println!("0"),
"b" => println!("1"),
"c" => println!("2"),
_ => println!("something else!"),
}
There's also an as_str
method as of Rust 1.7.0:
match stringthing.as_str() {
"a" => println!("0"),
"b" => println!("1"),
"c" => println!("2"),
_ => println!("something else!"),
}
You could also do
match &stringthing as &str {
"a" => println!("0"),
"b" => println!("1"),
"c" => println!("2"),
_ => println!("something else!"),
}
See:
std::string::String
std::ops::Deref
- Deref coercions