Скачать или смотреть Understanding the Error: cannot return value referencing temporary value in Rust

Dive into Rust's ownership semantics to understand why returning references to temporary values causes compilation errors. Learn about stack vs static memory and how to fix these common mistakes in your Rust code.
---
This video is based on the question https://stackoverflow.com/q/72532855/ asked by the user 'linuxfish' ( https://stackoverflow.com/u/1543462/ ) and on the answer https://stackoverflow.com/a/72533305/ provided by the user 'Peter Hall' ( https://stackoverflow.com/u/493729/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: cannot return value referencing temporary value

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding the Error: cannot return value referencing temporary value in Rust

As a Rust newcomer, you might face several puzzling compiler errors, one of which is the message: cannot return value referencing temporary value. This error can be particularly confusing for those who have just started learning about Rust's ownership and borrowing system. In this guide, we will unpack this error, explore the mechanisms behind it, and ultimately guide you on fixing similar issues in your Rust code.

The Problem Explained

The Code Snippet

Let’s discuss the problematic code example that generates this error. Here’s the snippet of code that doesn't compile due to the aforementioned issue:

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

Understanding the Error

When this code is compiled, Rust raises the error: cannot return value referencing temporary value. The core of this issue revolves around the fact that in the above example, each match arm returns an array, which is a temporary value. But why does this matter?

Temporary Values and Stack Memory: In Rust, if you return a reference to something that only lives within the scope of the function (like local variables), the compiler will reject this. When the function ends, the temporary value (the array in this case) gets dropped, making any references to it invalid.

Why does the Altered Code Compile Successfully?

When we modify the original code to use references to the arrays by adding a & symbol, it compiles correctly:

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

What changed?

Use of References: When using the &, we are providing a reference to the array stored in static memory. This means that the references are valid for the entire duration of the program, avoiding the problem of dangling pointers.

Clarifying the Mechanism

To further clarify, let’s consider these two snippets where we explicitly show what's happening under the hood using an additional variable:

Array Assignment

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

Here, the array is local to the function, and trying to take references would cause problems because the array data doesn't have a static lifespan.

Using References Properly

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

Conclusion

Understanding Rust's ownership and borrowing rules is crucial for writing safe and efficient code. Avoiding the common error message cannot return value referencing temporary value translates to knowing when to use references versus owned data. When you strive to grasp how Rust manages memory, you'll become more adept at crafting effective programs that take full advantage of these powerful features.

By utilizing references correctly, as explained in this guide, you can avoid compilation errors and improve the reliability of your Rust applications.

This topic will serve you well as you continue your journey in mastering Rust programming!