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Discover how to properly access struct fields within Rust's `new` associated function, ensuring effective management of ownership and borrowing.
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This video is based on the question https://stackoverflow.com/q/77794918/ asked by the user 'zkharit' ( https://stackoverflow.com/u/11042390/ ) and on the answer https://stackoverflow.com/a/77795067/ provided by the user 'Mouhong Lin' ( https://stackoverflow.com/u/290617/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Accessing Struct Fields in Rust's new Function: A Comprehensive Guide

When working with Rust, you may find yourself needing to create constructors for your structures, particularly using an associated function named new. A common issue arises when you want to access one field of the struct in order to derive another field within the same function.

In this post, I’ll walk you through how to accomplish this, focusing on ensuring proper ownership and borrowing practices in Rust. Let's dive into the details!

Understanding the Problem

Suppose you have a struct called Wallet that contains two fields:

private_key: A secret key you want to generate independently.

public_key: A public key derived from the private key.

You might want to create a constructor method, new, but encounter an error when trying to access self inside this function, as there is no instance available before the struct has been fully instantiated.

Example Code Breakdown

Here's a basic example of what you might start with:

[[See Video to Reveal this Text or Code Snippet]]

The attempted call to self.private_key.public_key() fails because self refers to an instance rather than the struct itself before it is created.

The Solution: Use Scoped Variables

You may be wondering how you can generate both keys inside a single method. Fortunately, we can resolve this by using temporary variables within the new function.

Step-by-Step Approach

Generate the Private Key: First, create the private_key and store it in a local variable.

Derived Public Key: Then, access this private key to derive the public_key.

Here's how to implement that:

[[See Video to Reveal this Text or Code Snippet]]

Handling Ownership

It is crucial to manage ownership correctly when working with methods in Rust. The public_key() function can have different signatures that affect ownership:

If defined as:

[[See Video to Reveal this Text or Code Snippet]]

It moves the private_key, leaving the Wallet's private_key field unusable after the call.

However, if defined as:

[[See Video to Reveal this Text or Code Snippet]]

It borrows the private_key, allowing you to keep using it afterward.

Conclusion

By employing scoped variables instead of attempting to access self, you can easily create constructors that derive one field from another in Rust. Just ensure that your method signatures respect Rust's ownership rules to prevent any compilation issues.

Implementing this technique not only provides a cleaner solution but also helps maintain the robustness that Rust is known for.

Happy coding!