CODEWITHSUNDEEP
rusterror-handling
manuel.menendez
manuel.menendez

Reputation: 305

How to handle errors for Not found key in Rust

I'm trying to capture errors on Rust, but I don't want the program to exit if I found it. I'm very new to Rust. Basically, I want to find a key from the Windows registry and if it doesn't exist, then create it.

Im using a crate called winreg for that.

This would be a section of my program:

fn main() -> io::Result<()> {
    ...
    ...
    ...

    key.set_value("TestSZ", &"written by Rust")?;

    // here I'm getting a value that exists
    let sz_val: String = key.get_value("TestSZ")?;
    
    // but this key doesn't exist
    let other: String = key.get_value("NOT_EXISTING_KEY")?;

    println!("TestSZ = {}", sz_val);
    println!("TestSZ = {}", other);

    Ok(())
}

If I compile that I receive this in the console:

And now lets write something...
An existing key has been opened
TestSZ = written by Rust
Error: Os { code: 2, kind: NotFound, message: "Couldn't find the pecified file." }
error: process didn't exit successfully: `target\debug\playground.exe` (exit code: 1)

In a pseudocode way, I would like something like:

if other == null {
println!("Nothing found!");
create_key();
}

If I analize get_value it looks like this:

pub fn get_value<T: FromRegValue, N: AsRef<OsStr>>(&self, name: N) -> io::Result<T>

I don't know what that means. I've been reading about errors and everything I do fails.

If I do let other: String = key.get_value("NOT_EXISTING_KEY").expect("Failed to read product name"); then the program exits, showing the error.

But I don't want the program to fail, I want to capture the error and do a different flow if I don't find the key (for example, create it).

Does anyone know how can I deal with this?

Upvotes: 1

Views: 1578

Answers (2)

manuel.menendez
manuel.menendez

Reputation: 305

I got it this way:

match key.get_value("NOT_EXISTING_KEY") {
        Ok(value) => {
            println!("found: {}", value);
            value
        },
        Err(err) => {
            println!("not found: {}", err);
            String::from("")},
    };

It was expecting a String

Upvotes: 0

huh-hulk
huh-hulk

Reputation: 324

In Rust, a function that can fail usually returns a Result<OkType, ErrorType> data type. This type is a structured enum, which means, that it can tell you not only if error has occured, but also what kind of error, so you could act accordingly.

You can process enums with match statements. Or alternatively, Result type has shortcuts like Result::unwrap or Result::expect that basically say: "If there is an error, just tell me what kind and crash the program."

I'm not very familiar with Windows Registry, so I'm not sure how bulletproof the following code snippet is, but it should give you an idea on how you can process errors with a match statement.

fn main() {
  // ...

  let anykey_value = match key.get_value("AnyKey") {
    // If the key is present, initialize `anykey_value` variable
    // with the returned value
    Ok(value) => value,
    // If the key is not found, do the following steps:
    Err(error) => {
      println!("Nothing found!");
      // Try to set an empty string as the value for "AnyKey".
      // If fails: panic with the following message.
      key.set_value("AnyKey", &"").expect("Failed to create key \"AnyKey\"");
      // Initialize `anykey_value` variable with an empty string.
      ""
    }
  }
  
  // Will print the value stored in "AnyKey"
  // or an empty string, if the key was just created.
  println!("AnyKey = {}", anykey_value);
  
  // ...
}

Also you can checkout Error handling chapter from The Rust Programming Language book. It might be helpful.

Upvotes: 1

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