How to make Rust BTreeMap::get return a String?

Question

use std::collections::btree_map::BTreeMap;
fn main() {
    let mut map: BTreeMap<String, String>;
    map.insert("name".to_string(), "aho".to_string());
    let name: String = map.get("name");
    println!("welcom, {}", name);
}

cargo build :

src/main.rs:5:24: 5:39 error: mismatched types:
 expected `collections::string::String`,
    found `core::option::Option<&collections::string::String>`
(expected struct `collections::string::String`,
    found enum `core::option::Option`) [E0308]
src/main.rs:5     let name: String = map.get("name");
                                     ^~~~~~~~~~~~~~~
src/main.rs:5:24: 5:39 help: run `rustc --explain E0308` to see a detailed explanation
error: aborting due to previous error
Could not compile `hello_world`.

Answer 1

The get() function returns an Option type with a reference to the value corresponding to the key. So you have stored a String on the heap and this function returns a reference to that String based on the key.

I made your example a little shorter. You had an uninitialized BTreeMap , so I initialized it first - let mut map: BTreeMap<String, String> = BTreeMap::new(); . In your case you can easily use an if let statement.

use std::collections::BTreeMap;

fn main() {
    let mut map: BTreeMap<String, String> = BTreeMap::new();
    map.insert("name".into(), "aho".into());

    if let Some(name) = map.get("name") {
        println!("welcome, {}", name);
    } else {
        println!("welcome, stranger");
    }
}

But if you really want to return a String value, you must first extract the Some(name) part of the Option type and then clone it using a clone() function. Because BTreeMap owns this String value, you can't move it out without cloning it. clone() makes a copy of the original value.

use std::collections::BTreeMap;

fn main() {
    let mut map: BTreeMap<String, String> = BTreeMap::new();
    map.insert("name".into(), "aho".into());

    if let Some(name) = map.get("name") {
        let name: String = name.clone();
        println!("welcome, {}", name);
    } else {
        println!("welcome, stranger");
    }
}