3 ms·

> > Only (possibly cv-qualified) non-POD class types (or arrays thereof) with automatic storage duration were considered to be default-initialized when no initi

> > Only (possibly cv-qualified) non-POD class types (or arrays thereof) with automatic storage duration were considered to be default-initialized when no initializer is used. Each direct non-variant non-static data member M of T has a default member initializer or, if M is of class type X (or array thereof), X is const-default-constructible, if T is a union with at least one non-static data member, exactly one variant member has a default member initializer, if T is not a union, for each anonymous union member with at least one non-static data member (if any), exactly one non-static data member has a default member initializer, and each potentially constructed base class of T is const-default-constructible. That's just a list of very simple rules for each kind of type. As a C++-phob person, C++ has a lot of footguns, but this isn't one of them.

andrepd 11mo ago
This was a random snippet of complicated rules to illustrate what I mean. C++ has significant footguns and complexity when it comes to initialising an object. Even sticking by "Effective Modern C++" rules is a significant cognitive burden on something that should be simple and straightforward. Rust has (mostly) no such footguns; its complexity follows from the essential complexity of the problems at hand.
- 1718627440 11mo ago
  I see. But including a snippet that actually supports your claim would have been more useful.
- jpc0 11mo ago
  Rust prevents the footgun, but also prevents shooting in the direction where your foot would be even if it isn't there. There are absolutely places where that is required and in Rust those situations become voodoo to write. C++ be default has more complexity but has the same complexity regardless of domain. Rust by default has much less complexity, but in obscure situations outside of the beaten path the complexity dramatically ramps up far above C++. This is not an argument for or against either language, it's a compromise on language design, you can choose to dislike the compromise but that doesn't mean it was the wrong one, it just means you don't like it. A relatively simple but complex example, I want variable X to be loaded into a registerer in thos function and only written to memory at the end of the function. That is complex in C/C++ but you can look at decompilation and attempt to coerce the compiler into that. In rust everything is so abstracted I wouldn't know where to begin looking to coerce the compiler into generating that machine code and might just decide to implement it in ASM, which defeats the point of using a high level language. Granted you might go the FFMPEG route ans just choose to do that regardless but rust makes it much harder. You don't always need that level of control but when you do it seems absurdly complex.
  - andrepd 11mo ago
    > I want variable X to be loaded into a registerer in thos function and only written to memory at the end of the function. > That is complex in C/C++ but you can look at decompilation and attempt to coerce the compiler into that. > In rust everything is so abstracted I wouldn't know where to begin looking I don't know if I fully understand what you want to do, but (1) controlling register allocation is the realm of inline asm, be it in C, C++, or Rust. And (2) if "nudging" the compiler is what you want, then it's literally the same thing in Rust as in C++, it's a matter of inspecting the asm yourself or plonking your function onto godbolt.
    - 1718627440 11mo ago
      I think the issue is that naive translation of C into ASM, which is somewhat simulated by -O0, is usable in C, while it isn't in Rust.
      - jpc0 11mo ago
        This. I agree that you will probably just end up writing ASM but it was a trivial example, there are non-trivial examples involving jump tables and unrolling loops etc. Effectively weird optimisations that rely on the virtual machine the compiler is building for vs reality, there's just more abstractions with rust than with C++ by the virtue of the safety mechanism, it's just plain not possible to have the one without the other. The hardware can do legal things that rust cannot allow or can allow but you need to write extremely convoluted code, C/C++ is closer to the metal in that regard. Don't get me wrong I am all for the right abstractions, it allows insane optimisations that humans couldn't dream of, but there is a flip side.
  - 1718627440 11mo ago
    My high level understanding of the UB concept is, that it means false positives to the question "Is that a valid program?". Given that the philosophy of C is mostly "Do what the programmer wrote, no questions asked.", it leads to designing the language so, that the probability of false negatives goes to zero. This obviously means, that the number of false positives goes up. Rust basically takes the opposite approach of making false positives go to zero, which makes the false negatives go up, which you need to work around with unsafe or type gymnastics. The third approach is to make both false positives and negatives be zero, by restricting the set of programs, which is what non systems languages do.

andrepd 11mo ago

This was a random snippet of complicated rules to illustrate what I mean. C++ has significant footguns and complexity when it comes to initialising an object. Even sticking by "Effective Modern C++" rules is a significant cognitive burden on something that should be simple and straightforward. Rust has (mostly) no such footguns; its complexity follows from the essential complexity of the problems at hand.

1718627440 11mo ago
I see. But including a snippet that actually supports your claim would have been more useful.
jpc0 11mo ago
Rust prevents the footgun, but also prevents shooting in the direction where your foot would be even if it isn't there. There are absolutely places where that is required and in Rust those situations become voodoo to write. C++ be default has more complexity but has the same complexity regardless of domain. Rust by default has much less complexity, but in obscure situations outside of the beaten path the complexity dramatically ramps up far above C++. This is not an argument for or against either language, it's a compromise on language design, you can choose to dislike the compromise but that doesn't mean it was the wrong one, it just means you don't like it. A relatively simple but complex example, I want variable X to be loaded into a registerer in thos function and only written to memory at the end of the function. That is complex in C/C++ but you can look at decompilation and attempt to coerce the compiler into that. In rust everything is so abstracted I wouldn't know where to begin looking to coerce the compiler into generating that machine code and might just decide to implement it in ASM, which defeats the point of using a high level language. Granted you might go the FFMPEG route ans just choose to do that regardless but rust makes it much harder. You don't always need that level of control but when you do it seems absurdly complex.
- andrepd 11mo ago
  > I want variable X to be loaded into a registerer in thos function and only written to memory at the end of the function. > That is complex in C/C++ but you can look at decompilation and attempt to coerce the compiler into that. > In rust everything is so abstracted I wouldn't know where to begin looking I don't know if I fully understand what you want to do, but (1) controlling register allocation is the realm of inline asm, be it in C, C++, or Rust. And (2) if "nudging" the compiler is what you want, then it's literally the same thing in Rust as in C++, it's a matter of inspecting the asm yourself or plonking your function onto godbolt.
  - 1718627440 11mo ago
    I think the issue is that naive translation of C into ASM, which is somewhat simulated by -O0, is usable in C, while it isn't in Rust.
    - jpc0 11mo ago
      This. I agree that you will probably just end up writing ASM but it was a trivial example, there are non-trivial examples involving jump tables and unrolling loops etc. Effectively weird optimisations that rely on the virtual machine the compiler is building for vs reality, there's just more abstractions with rust than with C++ by the virtue of the safety mechanism, it's just plain not possible to have the one without the other. The hardware can do legal things that rust cannot allow or can allow but you need to write extremely convoluted code, C/C++ is closer to the metal in that regard. Don't get me wrong I am all for the right abstractions, it allows insane optimisations that humans couldn't dream of, but there is a flip side.
- 1718627440 11mo ago
  My high level understanding of the UB concept is, that it means false positives to the question "Is that a valid program?". Given that the philosophy of C is mostly "Do what the programmer wrote, no questions asked.", it leads to designing the language so, that the probability of false negatives goes to zero. This obviously means, that the number of false positives goes up. Rust basically takes the opposite approach of making false positives go to zero, which makes the false negatives go up, which you need to work around with unsafe or type gymnastics. The third approach is to make both false positives and negatives be zero, by restricting the set of programs, which is what non systems languages do.