Propagating errors with ? in Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the ? operator do in Rust?

It checks if a function returned an error. If there is an error, it returns it immediately from the current function. Otherwise, it continues with the successful value.

beginner

How does ? help with error propagation?

It lets you write less code by automatically returning errors up the call stack without needing explicit match or if let statements.

intermediate

Can ? be used in functions that do not return a Result or Option?

No. The function must return a compatible type like Result or Option for ? to work.

beginner

Example: What does this code do?<br><pre>let file = std::fs::File::open("foo.txt")?;</pre>

It tries to open the file "foo.txt". If it fails, the error is returned from the current function immediately. If it succeeds, the file handle is stored in file.

intermediate

Why is using ? considered more readable than manual error handling?

Because it reduces nested code and boilerplate, making the main logic clearer and easier to follow.

What happens when ? encounters an error in a Result?

A It returns the error from the current function immediately.

B It ignores the error and continues.

C It panics and stops the program.

D It converts the error to a string.

The ? operator returns the error immediately from the current function, propagating it up.

Which return types are compatible with using ??

A Only <code>Result</code> and <code>Option</code>.

B Any type.

C Only <code>String</code>.

D Only <code>bool</code>.

? works with Result and Option types to propagate errors or None values.

What is the main benefit of using ? in Rust?

A Converts errors to warnings.

B Makes the program run faster.

C Automatically fixes errors.

D Simplifies error handling by reducing boilerplate.

? reduces the need for explicit error checks, making code cleaner and easier to read.

Can you use ? inside a function that returns void (unit type)?

A Only if you import a special crate.

B Yes, always.

C No, the function must return <code>Result</code> or <code>Option</code>.

D Only in async functions.

? requires the function to return a compatible type like Result or Option.

What does this code do?<br>

let content = std::fs::read_to_string("file.txt")?;

A Prints the file content to the screen.

B Reads the file content or returns an error immediately.

C Ignores any errors and returns empty string.

D Creates a new file named "file.txt".

It tries to read the file content. If it fails, the error is returned immediately using ?.

Explain how the ? operator helps with error propagation in Rust.

Think about how errors move up the call stack without extra code.

Describe a simple example where using ? makes your Rust code easier to read.

Imagine you want to open a file and return an error if it fails.