C++ to Rust Phrasebook

[jpc0]

There are so many different flavours of C++ put there that this guide doesn’t exactly do itself the credit it deserves.

There are easy ways to implement stuff like enums with members in C++, just put an anonymous enum inside a class/struct, its possible but marked as not possible.

Likewise when discussing modules in rust while completely ignoring the existence of modules in C++ that is actually support by modern tooling.

There are other places where there are similar issues I have with the text (you can just add a compiler flag and get the same behaviour. Don’t even try and argue “but rust just does it out of the box”, I would need to rewrite my entire codebase vs just adding a few bits of text to my build system, these things are not equivalent)

They didn’t discuss much about FFI at all, generally sayings “theres a crate for that and if there isn’t let us know”, in my experience the crates are not amazing for esoteric things (anything graphics’s related, ffmpeg is another one) and are actually significantly more painful to use that just writing in a restricted version of C++.

Rust has a happy path, and they are broadening it incrementally, but theres is a lifetime of history before rust even existed in C++ that isn’t that easy to sweep under the rug.

[pjmlp]

As someone that has been around C and C++ communities since 1990's, I also expect that in the long term Rust won't be able to escape this phenomenon, even with editions.

Like any other programming language that has made it into the top 10 over several decades of production code.

The happy path will get fuzzier, as more humans have their own opinion on what means to write code on the ecosystem, with various kinds of backgrounds and their own agendas, companies whose IT refuses to upgrade, the amount of implementations in the industry increases, teachers not bothering to keep up to date,...

[m-schuetz]

Are C++ modules actually production ready now? Last I checked, they still weren't properly supported accross all major compilers.

[Toritori12]

C++ modules still not supported on VSCode, always pissed when I get notifications from this 5 y.o. thread [0].

0: https://github.com/microsoft/vscode-cpptools/issues/6302

[dwattttt]

> There are easy ways to implement stuff like enums with members in C++, just put an anonymous enum inside a class/struct, its possible but marked as not possible.

This guide does exactly that in C++ for methods on enums.

For members, won't this result in all instances of an "enum" having all fields? That's not really comparable to Rust enums, if that's what you mean.

[EliRivers]

One of the common pitfalls I've seen in my time is someone writing a language they are familiar with in a language that just doesn't fit; trying to apply idioms that flow well with one language to another language where that's just not a good way to achieve the same ends.

An example I've seen a lot is a C thinker writing C++ classes with an init() function; sure, it works, but the C++ way is to do that in constructors. (All those about to start listing exceptions to that C++ idiom, please save it to the end, thanks!) The C thinker is still thinking about objects as "allocate memory, then set values" rather than the C++ way where allocation and initialisation are wrapped together into a single action (from the programmer's point of view).

So what are these pitfalls for a C++ thinker when writing Rust? This "phrasebook" is embracing the idea of taking a C++ way of thinking and applying it to Rust, which I'm sure will be fine for many situations, but what are the C++ phrases that are just the wrong way to do things in Rust?

[atq2119]

To be fair, there's a reason for the pattern with init methods you're describing.

C++ constructors can't return values. If construction is fallible, the only way to communicate the error is via C++ exceptions. If you're in a code base that embraces exceptions, that's fine. But (C++) exceptions kind of suck, so many code bases don't, and then you have to find some alternatives.

Over the years, I've increasingly adopted a pattern where the constructor is private in this case and the object construction can be done with static methods - which is a bit more like Rust, actually.

[hardwaregeek]

The rationale is probably that it’s better for C++ devs to write non idiomatic Rust than to keep writing unsafe C++. Like unless they use unsafe and completely circumvent the borrow checker, it’s still gonna be safer. Not letting perfect be the enemy of good and all.

Plus idiomatic rust isn’t that strict a definition. Clippy will guide you for most of the simple stuff and the rest isn’t always worth following. Like people who try to do stuff “correctly” with traits often end up with way more complexity than it’s worth.

[pjmlp]

I hate with passion two phase initialisation, C++ libraries that are bare bones C libraries wrapped in an extern "C" { }, malloc()/free(), C style coding and such.

[acje]

A resource like this is a good place to discuss where the two languages are near and far. Of course there are going to be styles within each language that differ as much as the languages themselves.

[pornel]

The worst pitfall is Rust references == pointers.

They are implemented as pointers, but their role is to give temporary (often exclusive) access that is restricted to a statically know scope, which is pretty specific and fits only some uses of some pointers/C++ references. In C++ pointers typically mean avoiding copying, but Rust references avoid storing/keeping the data. When these goals don't overlap, people get stuck with a dreadful "does not live long enough" whack-a-mole.

[amelius]

I noticed that for most of the examples the Rust version is more verbose.

[pornel]

C++ syntax and semantics are optimized for C++ idioms, and Rust's aren't.

Rust is more related to ML-family languages than C-family and OOP, so it needs to "emulate" some C++ idioms.

This goes both ways, e.g. equivalent of Rust's pattern matching on enums with data translates to verbose and clunky C++.

[modulus1]

And they even made the C++ version more verbose than it should have been. Most people would write:

class Person { int age = 0; };

I wish rust would make default struct field values this easy to write.

[Animats]

The discussion of traits vs. classes glosses over a major difference - Rust traits have no associated data, and you cannot access the data of a parent trait. Trying to do object-oriented programming in Rust quickly leads to a mess. This is a huge problem if you try to write C++ in Rust.

There's no mention of ownership at all.

> Many libraries in Rust will offer two versions of an API, one which returns a Result or Option type and one of which panics, so that the interpretation of the error (expected exceptional case or programmer bug) can be chosen by the caller.

Huh? Usually, in Rust you just put .unwrap() or .expect() on a function call that returns a result if you want a panic.

More generally, most of the differences between Rust and C++ relate not on how to write imperative code, but how to lay out connections between data structures. Most hard problems in Rust relate to ownership design. That's true in C++, too, but you don't discover them until the program crashes at run time.

[npalli]

Inverting the original intent, this is great, learn how Rust improves certain things and use that to write better C++ if possible.

[tialaramex]

Plenty of C++ practitioners who still intend to mostly write C++ after learning some Rust said it actually gave them useful insights they'll be applying in future C++

We can also see in the committee proposal papers "Rust does X" has for years now been a good comeback when you need to show that X is a realistic choice not just for languages like Python which may be less concerned about performance and incur a heavy runtime, a garbage collector, etc., but also a "real" language like C++. The paper which landed code.contains("FOO") in the C++ string handling code is an example, there's a long list of languages which do this but they made sure to mention Rust.

[90s_dev]

I will be using this to learn both Rust and C++ and see which one I like better.

[Tyr42]

I've taken a look, and it's definitely expecting you to know some C++. Or at least, it spends equal time on both, which means it can't warn you about the foot guns in c++.

[Mond_]

This isn't much of a tutorial or learning guide, it's a lookup table that roughly pattern matches C++ to Rust.

Probably not the best place to start learning.

[leoh]

Epic. This is really good.

[gtani]

been awhile sincei looked but i thought Drysdale's Effective Rust was a truly excellent book and aimed at simlar audience

https://www.lurklurk.org/effective-rust/

[unvalley]

Nice, it also helps me like a Rust developer that has no C++ experience.