[aw1621107]

> In C++, a class like std::lock_guard also provides "Automatic unlock". AFAICT, the article argues that only Rust's API provides that.

The issue isn't automatic unlocking. From the article:

> The problem? Nothing stops you from accessing account without locking the mutex first. The compiler won’t catch this bug.

i.e., a C++ compiler will happily compile code that modifies `account` without taking the lock first. Your lock_guard example suffers from this same issue.

Nothing in the C++ stdlib provides an API that makes it impossible to access `account` without first taking the lock, and while you can write C++ classes that approximate the Rust API you can't quite reach the same level of robustness without external help.

[questioner8216]

That is a different topic from what I wrote about.

The article wrote:

> Automatic unlock: When you lock, you receive a guard. When the guard goes out of scope, it automatically unlocks. No manual cleanup needed.

And presented Rust as being different from C++ regarding that, and the C++ example was not idiomatic, since it did not use something like std::lock_guard.

I have not addressed the rest of your comment, since it is a different topic, sorry.

[aw1621107]

Fair point with respect to the separate topic. My apologies.

As for the automatic cleanup bit, perhaps the article is trying to focus purely on the mutex types themselves? Or maybe they included the "when you lock" bit to emphasize that you can't forget to unlock the mutex (i.e., no reliance on unenforced idioms). Hard to say given the brevity/nature of the section, and in the end I think it's not that much of a problem given the general topic of the blogpost.

[questioner8216]

When comparing languages, posting unidiomatic code, and then making claims based on that unidiomatic code, is generally not fair nor correct.

[aw1621107]

That's true if the claims hinge on said unidiomatic code. As I said, it's not clear to me that that is the case here.

[questioner8216]

It seems completely clear. He first gives unidiomatic C++ code, then next gives idiomatic Rust code, and differentiates the two based on the code snippets. It is a mistake on his part, and I do not see how it could reasonably be viewed otherwise. It is not a huge mistake, but it is still a clear mistake.

[aw1621107]

Perhaps it might help to clarify precisely what claim(s) you think are being made?

From my reading, the section (and the article in general, really) is specifically focusing on mutexes, so the observations the article makes are indeed accurate in that respect (i.e., C++'s std::mutex indeed does not have automatic unlocking; you need to use an external construct for that functionality). Now, if the article were talking about locking patterns more generally, I think your criticism would hold more weight, but I think the article is more narrowly focused than that.

For a bit of a more speculative read, I think it's not unreasonable to take the C++ code as a general demonstration of the mutex API "languages other than Rust" use rather than trying to be a more specific comparison of locking patterns in Rust and C++. Consider the preceding paragraph:

> In languages other than Rust, you typically declare a mutex separately from your data, then manually lock it before entering the critical section and unlock it afterward. Here’s how it looks in C++:

I don't think it's unreasonable to read the "it" in the final sentence as "that pattern"; i.e., "Here's what that pattern looks like when written in C++". The example code would be perfectly correct in that case - it shows a mutex declared separately from the data, and it shows that mutex being manually locked before entering the critical section and unlocked after.