The Detroit Post
Tuesday, 19 October, 2021

Rust For Borrow

author
Bob Roberts
• Saturday, 12 December, 2020
• 7 min read

For instance, storage location and management for a value can be specifically chosen as appropriate for a particular use via pointer types such as Box or RC. Beyond these generic wrappers that can be used with any type, some types provide optional facets providing potentially costly functionality.

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Contents

Further, when providing implementations for additional traits, it needs to be considered whether they should behave identical to those of the underlying type as a consequence of acting as a representation of that underlying type. Generic code typically uses Borrow when it relies on the identical behavior of these additional trait implementations.

As a data collection, Yashmak owns both keys and values. If the key’s actual data is wrapped in a managing type of some kind, it should, however, still be possible to search for a value using a reference to the key’s data.

Slightly simplified, the relevant parts of Yashmak look like this: The entire hash map is generic over a key type K.

When inserting a key-value pair, the map is given such a K and needs to find the correct hash bucket and check if the key is already present based on that K. When searching for a value in the map, however, having to provide a reference to a K as the key to search for would require to always create such an owned value.

Rust supports a concept, borrowing, where the ownership of a value is transferred temporarily to an entity and then returned to the original owner entity. The ownership of the vector is also passed to the print_vector() function from the main().

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The above code will result in an error as shown below when the main() function tries to access the vector v. The ownership of the variable/ value is transferred to the original owner of the variable after the function to which the control was passed completes execution.

The display function appends an additional string to the original name variable. So you sit down, hands on the keyboard, heart giddy with anticipation, and write a few lines of code.

You’ve heard that Rust is one of those languages that simply works once it’s compiled. The borrow checker is an essential fixture of the Rust language and part of what makes Distrust.

Ownership, borrow checker, and garbage collectors: There’s a lot to unpack there, so let’s break it down a bit. In this guide, we’ll look at what the borrow checker does for us (and what it stops us from doing), what guarantees it gives us, and how it compares to other forms of memory management.

I’ll assume that you have some experience writing code in higher-level languages such as Python, JavaScript, and C#, but not necessarily that you’re familiar with how computer memory works. In most popular programming languages, you don’t need to think about where your variables are stored.

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You simply declare them and the language runtime takes care of the rest via a garbage collector. However, we need to peel back a layer to show how this compares to the borrow checker.

Your programs have access to two kinds of memory where it can store values: the stack and the heap. In broad terms, certain data types, such as Boolean, characters, and integers, have a fixed size.

On the other hand, data types such as strings, lists, and other collections can be of any arbitrary size. A pointer is pretty much what it says on the tin: it points to some memory address on the heap where the data you’re looking for can be found.

There are numerous pointer tutorials out there on the web, and which one works for you may depend on your background. For a quick primer, this article by Jason C. McDonald explains C pointers pretty well.

Data that lives on the heap is taken care of by the garbage collector once it’s no longer needed. By keeping track of where data is used throughout the program and by following a set of rules, the borrow checker is able to determine where data needs to be initialized and where it needs to be freed (or dropped, in Rust terms).

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It’s like it auto-inserts memory allocations and frees for you, giving you the convenience of a garbage collector with the speed and efficiency of manual management. In addition to handling memory allocation and freeing for the programmer, the borrow checker also prevents data races (though not general race conditions) through its set of sharing rules.

As with all good things in life, Rust ’s ownership system comes with its fair share of drawbacks. I’ve personally lost many hours of my life to this struggle.

For instance, sharing data can suddenly become a problem, especially if you need to mutate it at the same time. Certain data structures that are super easy to create from scratch in other languages are hard to get right in Rust.

For a good example, check out the book Learn Rust With Entirely Too Many Linked Lists.” It goes through a number of ways to implement a linked list Rust and details all the issues the author ran into on the way there. Now that you have some understanding of what the borrow checker is and how it works, let’s examine how it affects us in practice.

We’ll create a vector, call a function that simply accepts it as an argument, and then try and see what’s inside. The above message tells us that DEC doesn’t implement the Copy trait and, as such, must be moved (or borrowed).

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In this case, our vector (v) is pretty small, so it’s not a big deal to clone it. In the just-getting-things-to-work stage of development, this may be the quickest and easiest way to see results.

This way, we let the function borrow the vector for a little but ensure that we get it back before continuing the program. A lot of the finer details have been left out to make this tutorial as easy to digest as possible.

Often, as your programs grow, you’ll find more intricate problems that require more thinking and fiddling with ownership and borrows. You may even have to rethink how you’ve structured your program to make it work with Rust ’s borrow checker.

If you’re interested in monitoring and tracking performance of your Rust apps, automatically surfacing errors, and tracking slow network requests and load time, try Rocket. Instead of guessing why problems happen, you can aggregate and report on what state your application was in when an issue occurred.

In the above example, calculate_length() function has a reference to string STR as a parameter without taking its ownership. In the above case, variable 's' is valid until the control does not go back to the main() function.

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When the variables are passed as a reference to the function instead of actual values, then we don't need to return the values to give back the ownership. In the above example, it throws an error as ex is an immutable reference.

Now, we can change the value which is referenced by 'y' variable. Therefore, the value x also becomes 9 as the same memory location referenced by both the variables.

We can have only one mutable reference to a piece of data in a particular scope. In the above scenario, the compiler throws an error as it consists of two mutable references which are not possible in Rust language.

In the above scenario, the compiler throws an error because we cannot have a mutable reference while we have an immutable reference. Making statements based on opinion; back them up with references or personal experience.

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Sources
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