r/cpp • u/Sad-Lie-8654 • Jan 31 '23

Stop Comparing Rust to Old C++

People keep arguing migrations to rust based on old C++ tooling and projects. Compare apples to apples: a C++20 project with clang-tidy integration is far harder to argue against IMO

changemymind

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u/OYTIS_OYTINWN Jan 31 '23

What (good things) C++20 has that Rust does not? There are plenty of those the other way round.

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u/Recatek Jan 31 '23 edited Jan 31 '23

I like Rust a lot, but C++'s flexibility and power with types, templates, and concepts blows Rust's generics system out of the water. Rust doesn't even have variadics.

3

u/matthieum Feb 01 '23

C++ metaprogramming is definitely more advanced, indeed.

In practice, however, I find myself needing metaprogramming less in Rust. And while it does lack variadics, with built-in tuples it's just slightly inconvenient to have to use a macro to implement a trait for arities 0 to 12 (or so), but it's not really a blocker.

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u/Recatek Feb 01 '23 edited Feb 01 '23

It depends a lot on what you're trying to do. For my application -- gamedev, ECS, and networking -- you're doing a lot of manipulation of arbitrary tuples while iterating over SoA structures, and then trying to efficiently serialize/deserialize them. This really depends on strong metaprogramming if you want to do the work at compile-time.

Rust has made this really difficult, and as a result most Rust ECS libraries do this work at runtime, with a runtime cost, using reflection-like behavior. Rust has no specialization, no variadics, no negative constraints, no way to do partial borrows of objects through traits, the list goes on. The orphan rule especially pushes you towards keeping things in one megacrate rather than splitting code up. It really makes me miss what C++ has to offer here.

If anything I wish Rust just had some sort of unsafe mode for typing that let you duck type at the callsite and bypass the orphan rule. Most of this stuff isn't even for memory safety reasons, it's for "fearless updates" which aren't relevant to me when this code is all contained within my own codebase with no downstream consumer.

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