| ▲ | jandrewrogers a day ago |
| Many of the recent C++ standards have been focused on expanding and cleaning up its powerful compile-time and metaprogramming capabilities, which it initially inherited by accident decades ago. It is difficult to overstate just how important these features are for high-performance and high-reliability systems software. These features greatly expand the kinds of safety guarantees that are possible to automate and the performance optimizations that are practical. Without it, software is much more brittle. This isn’t an academic exercise; it greatly reduces the amount of code and greatly increases safety. The performance benefits are nice but that is more on the margin. One of the biggest knocks against Rust as a systems programming language is that it has weak compile-time and metaprogramming capabilities compared to Zig and C++. |
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| ▲ | rienbdj a day ago | parent | next [-] |
| > One of the biggest knocks against Rust as a systems programming language is that it has weak compile-time and metaprogramming capabilities compared to Zig and C++. Aren’t Rust macros more powerful than C++ template metaprogramming in practice? |
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| ▲ | menaerus 16 hours ago | parent | next [-] | | No, they are not. | | |
| ▲ | aw1621107 15 hours ago | parent | next [-] | | They are both; there are things that Rust's macros can do metaprogramming-wise that C++ templates cannot do and vice-versa. Rust's macros work on a syntactic level, so they are more powerful in that they can work with "normally" invalid code and perform token-to-token transformations (and in the case of proc macros effectively function as compiler extensions/plugins) and less powerful in that they don't have access to semantic information. | |
| ▲ | aldanor 12 hours ago | parent | prev [-] | | Incorrect. |
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| ▲ | tialaramex a day ago | parent | prev [-] | | Rust has two separate macro systems. It has declarative "by example" macros which are a nicer way to write the sort of things where you show an intern this function for u8 and ask them to create seven more just like it except for i8, u16, i16, u32, i32, u64, i64. Unlike the C pre-processor these macros understand how loops work (sort of) and what types are, and so on, and they have some hygiene features which make them less likely to cause mayhem. Declarative macros deliberately don't share Rust's syntax because they are macros for Rust so if they shared the same syntax everything you do is escape upon escape sequence as you want the macro to emit a loop but not loop itself etc. But other than the syntax they are pretty friendly, a one day Rust bootstrap course should probably cover these macros at least enough that you don't use copy-paste to make those seven functions by hand. However the powerful feature you're thinking of is procedural or "proc" macros and those are a very different beast. The proc macros are effectively compiler plugins, when the compiler sees we invoked the proc macro, it just runs that code, natively. So in that sense these are certainly more powerful, they can for example install Python, "Oh, you don't have Python, but I'm a proc macro for running Python, I'll just install it...". Mara wrote several "joke" proc macros which show off how dangerous/ powerful it is, you should not use these, but one of them for example switches to the "nightly" Rust compiler and then seamlessly compiles parts of your software which don't work in stable Rust... |
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| ▲ | jesse__ a day ago | parent | prev | next [-] |
| > powerful compile-time and metaprogramming capabilities While I agree that, generally, compile time metaprogramming is a tremendously powerful tool, the C++ template metaprogramming implementation is hilariously bad. Why, for example, is printing the source-code text of an enum value so goddamn hard? Why can I not just loop over the members of a class? How would I generate debug vis or serialization code with a normal-ish looking function call (spoiler, you can't, see cap'n proto, protobuf, flatbuffers, any automated dearimgui generator) These things are incredibly basic and C++ just completely shits all over itself when you try to do them with templates |
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| ▲ | sidkshatriya 21 hours ago | parent | prev [-] |
| One of the biggest knocks against Rust as a systems programming language is that it has weak compile-time and metaprogramming capabilities compared to Zig and C++ In the space of language design, everything "more powerful" is not necessary good. Sometimes less power is better because it leads to more optimisable code, less implementation complexity, less abstraction, better LSP support. TL;DR More flexibility and complexity is not always good. Though I would also challenge the fact that Rust's metaprogramming model is "not powerful enough". I think it can be. |
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| ▲ | germandiago 19 hours ago | parent [-] | | But compile-time processing is certainly useful in a performance-oriented language. And not only for performance but also for thread safety (eliminates initialization races, for example, for non-trivial objects). Rust is just less powerful. For example you cannot design something that comes evwn close to expression templates libraries. | | |
| ▲ | ux266478 15 hours ago | parent | next [-] | | > And not only for performance but also for thread safety This is already built-in to the language as a facet of the affine type system. I'm curious as to how familiar you actually are with Rust? > Rust is just less powerful. On the contrary. Zig and C++ have nothing even remotely close to proc macros. And both languages have to defer things like thread safety into haphazard metaprogramming instead of baking them into the language as a basic semantic guarantee. That's not a good thing. | | |
| ▲ | germandiago 9 hours ago | parent [-] | | Writing general generic code without repetition for Rust without specialization is ome thing where it fails. It does not have variadics or so powerful compile metaprogramming. It does not come even remotely close. Proc macros is basically plugins. I do not think thos is even part of the "language" as such. It is just plugging new stuff into the compiler. |
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| ▲ | aw1621107 17 hours ago | parent | prev [-] | | > For example you cannot design something that comes evwn close to expression templates libraries. You keep saying this and it's still wrong. Rust is quite capable of expression templates, as its iterator adapters prove. What it isn't capable of (yet) is specialization, which is an orthogonal feature. | | |
| ▲ | Conscat 16 hours ago | parent | next [-] | | Rust cannot take a const function and evaluate that into the argument of a const generic or a proc macro. As far as I can tell, the reasons are deeply fundamental to the architecture of rustc. It's difficult to express HOW FUNDAMENTAL this is to strongly typed zero overhead abstractions, and we see where Rust is lacking here in cases like `Option` and bitset implementations. | | |
| ▲ | aw1621107 16 hours ago | parent [-] | | > Rust cannot take a const function and evaluate that into the argument of a const generic Assuming I'm interpreting what you're saying here correctly, this seems wrong? For example, this compiles [0]: const fn foo(n: usize) -> usize {
n + 1
}
fn bar<const N: usize>() -> usize {
N + 1
}
pub fn baz() -> usize {
bar::<{foo(0)}>()
}
In any case, I'm a little confused how this is relevant to what I said?[0]: https://rust.godbolt.org/z/rrE1Wrx36 |
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| ▲ | menaerus 16 hours ago | parent | prev [-] | | > Rust is quite capable of expression templates, as its iterator adapters prove. AFAIU iterator adapters are not quite what expression templates are because they rely on the compiler optimizations rather than the built-in feature of the language, which enable you to do this without relying on the compiler pipeline. | | |
| ▲ | aw1621107 15 hours ago | parent [-] | | I had always thought expression templates at the very least needed the optimizer to inline/flatten the tree of function calls that are built up. For instance, for something like x + y * z I'd expect an expression template type like sum<vector, product<vector, vector>> where sum would effectively have: vector l;
product& r;
auto operator[](size_t i) {
return l[i] + r[i];
}
And then product<vector, vector> would effectively have: vector l;
vector r;
auto operator[](size_t i) {
return l[i] * r[i];
}
That would require the optimizer to inline the latter into the former to end up with a single expression, though. Is there a different way to express this that doesn't rely on the optimizer for inlining? | | |
| ▲ | menaerus 14 hours ago | parent [-] | | Expression templates do not rely on optimizer since you're not dealing with the computations directly but rather expressions (nodes) through which you are deferring the computation part until the very last moment (when you have a fully built an expression of expressions, basically almost an AST). This guarantees that you get zero cost when you really need it. What you're describing is something keen of copy elision and function folding though inlining which is pretty much basics in any c++ compiler and happens automatically without special care. | | |
| ▲ | aw1621107 14 hours ago | parent [-] | | > since you're not dealing with the computations directly but rather expressions (nodes) through which you are deferring the computation part until the very last moment (when you have a fully built an expression of expressions, basically almost an AST). Right, I understand that. What is not exactly clear to me is how you get from the tree of deferred expressions to the "flat" optimized expression without involving the optimizer. Take something like the above example for instance - w = x + y * z for vectors w/x/y/z. How do you get from that to effectively for (size_t i = 0; i < w.size(); ++i) {
w[i] = x[i] + y[i] * z[i];
}
without involving the optimizer at all? | | |
| ▲ | menaerus 2 hours ago | parent [-] | | The example is false because that's not how you would write an expression template for given computation so the question being how is it that the optimizer is not involved is also not quite set in the correct context so I can't give you an answer for that. Of course that the optimizer is generally going to be involved, as it is for all the code and not the expression templates, but expression templates do not require the optimizer in the way you're trying to suggest. Expression templates do not rely on O1, O2 or O3 levels being set - they work the same way in O0 too and that may be the hint you were looking for. |
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