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Many of the recent C++ standards have been focused on expanding and cleaning up its powerful compile-time and metaprogramming capabilities, which it initially i

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++.

jesse__ 7mo ago
> 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
- logicchains 7mo ago
  Did you read the article? This is called reflection, and is exactly what C++26 introduces.
  - jesse__ 7mo ago
    Yeah, like 50 years too late.
- Conscat 7mo ago
  > Why, for example, is printing the source-code text of an enum value so goddamn hard? Aside from this being trivial in C++26, imo it isn't actually that tricky. Here's a very quick implementation I made awhile ago: https://github.com/Cons-Cat/libCat/blob/3f54e47f0ed182771fce8d0a80366ce63adb36fc/src/libraries/enum/cat/enum#L37 https://github.com/Cons-Cat/libCat/blob/3f54e47f0ed182771fce...
  - jesse__ 7mo ago
    > Aside from this being trivial in C++26 Great, it took them 51 years to make a trivial operation trivial. Call me next millennium when they start to figure out the nontrivial stuff, I guess.
rienbdj 7mo ago
> 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?
- tialaramex 7mo ago
  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...
- menaerus 7mo ago
  No, they are not.
  - aldanor 7mo ago
    Incorrect.
sidkshatriya 7mo ago
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.
- germandiago 7mo ago
  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.
  - aw1621107 7mo ago
    > 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 7mo ago
      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 7mo ago
        > 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 https://rust.godbolt.org/z/rrE1Wrx36

jesse__ 7mo ago

> 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

logicchains 7mo ago
Did you read the article? This is called reflection, and is exactly what C++26 introduces.
- jesse__ 7mo ago
  Yeah, like 50 years too late.
Conscat 7mo ago
> Why, for example, is printing the source-code text of an enum value so goddamn hard? Aside from this being trivial in C++26, imo it isn't actually that tricky. Here's a very quick implementation I made awhile ago: https://github.com/Cons-Cat/libCat/blob/3f54e47f0ed182771fce8d0a80366ce63adb36fc/src/libraries/enum/cat/enum#L37 https://github.com/Cons-Cat/libCat/blob/3f54e47f0ed182771fce...
- jesse__ 7mo ago
  > Aside from this being trivial in C++26 Great, it took them 51 years to make a trivial operation trivial. Call me next millennium when they start to figure out the nontrivial stuff, I guess.

rienbdj 7mo ago

> 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?

tialaramex 7mo ago
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...
menaerus 7mo ago
No, they are not.
- aldanor 7mo ago
  Incorrect.

sidkshatriya 7mo ago

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.

germandiago 7mo ago
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.
- aw1621107 7mo ago
  > 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 7mo ago
    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 7mo ago
      > 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 https://rust.godbolt.org/z/rrE1Wrx36