4 ms·

> complex language which limits you Rust really does not limit you. Anything possible in C or C++ can be done in Rust if you're willing to use unsafe code.

ncmncm 4y ago
This is not the case if you are designing a library. Much of what is routinely used to capture semantics into mainstream C++ libraries is wholly impossible to express in Rust. (C is not even a participant, here.) This is not about template metaprogramming, just ordinary stuff. Most of that difference is in things that happen at compile time, so this is not a question of Turing completeness.
- lelanthran 4y ago
  > Much of what is routinely used to capture semantics into mainstream C++ libraries is wholly impossible to express in Rust. (C is not even a participant, here.) This is not about template metaprogramming, just ordinary stuff. Do you have any examples?
  - bluGill 4y ago
    All above languages are turning complete, so if you can express it in one you can express it in another. The question isn't can you write it, the question is how hard is it to do, and how performant the code will be. The heart of C++ is destructors: a bit of code that you can write and the compiler will ensure runs when code goes out of scope/is deleted. You can do this in C by remembering to manually call the right code when doing clean up, but it is easy to forget and thus error pron. (I think Rust has this too?) C++ gives you the ability to do a virtual base class interface, which - as most people know - just means it writes a vtable behind the scene for you. Sometimes people write a vtable by hand in C: it is just a struct of function pointers, but the syntax to do it in C++ is a lot nicer. If you need an interface of some sort the win goes to C++ because the syntax is a lot nicer. (I'm not sure what Rust does about interfaces, I think it has something) C++ gives you control over copying structs. In C structs are only copied member wise, if the struct has a pointer you need to keep track of both copies so you don't free it early. In C++ you write a copy function that will make a copy of the pointer. You can do this in C by remembering to call the right function when copying a struct, but the default is the wrong thing. (I'm not sure what rust has here, but at the very least the borrow checker will stop you from making a mistake) C++ gives you move objects - a way to express that a struct is going out of scope, but only after a different one is taking over the contents. This is a variation of the previous, except that you know the original doesn't need valid data anymore and so you just copy pointers and null them in the original. C doesn't have this concept, you can get around it with use of pointers and manual copying of structs in the right places, but the code is ugly. (again, I'm not sure what rust does, if nothing else the borrow checker should allow the compiler to make some optimizations on this lines) There are a lot more areas where C++ gives you syntax to write correct code that C does not. Rust intentionally doesn't have some of them (class inheritance has been abused often, but I still find it useful enough in a few cases that I think rust is wrong for throwing it out), and in other cases has come up with a better syntax. Overall I don't know enough about Rust to judge it, but I'll take C over C++ anyday. Note, the above is about the advantages of C++ over C. C++ has a lot of warts that are out of scope for that discussion. I am not claiming C++ is perfect. If you are starting a new project you should seriously consider your language options - including some not mentioned here)
    - lelanthran 4y ago
      Thanks, that is a very detailed answer[1]. I'm somewhat confused about why it was downvoted - there is nothing there controversial at all. [1] In my own toy language, I'm kinda leaning towards objects with destructors. Definitely without the footguns in C++, but destructors nonetheless.
      - ncmncm 4y ago
        Possibly because it does not mention almost all of the C++ features that Rust still lacks.
    - Santosh83 4y ago
      > but I'll take C over C++ anyday. Did you mean the opposite?
      - bluGill 4y ago
        opps, yes I did.
    - adjav 4y ago
      I will note that Rust has all of those features you mentioned. 1) Traits 2) You have to be explicit, by implementing the Clone trait, otherwise arguments are passed by reference or moved. 3) Rust has both RAII and linear typing, which is effectively a proven-correct form of moves - you can't access something that has been moved, unlike C++. Rust doesn't have class inheritance, no, but often combining traits and delegating to a "parent" field works. Although sometimes that can be considerably more cumbersome, yeah.
      - ncmncm 4y ago
        I agree that the GP comment does not capture what Rust is unable to express.
    - jstimpfle 4y ago
      You can also see it this way: The philosophy behind C++ is that the existance of a pointer implies ownership. In C, the existance of a pointer only implies liveness (normally; but really the code can decide on its own whether its safe to dereference the pointer). With the C++ approach I've found myself overthinking the problem many times. But with the insight that pointers are just data, and memory management can be independent, it turns out that programming with raw pointers need not be hard. There is no problem passing a pointer without "move semantics" shenanigans.
anonymoushn 4y ago
I'm not sure this is true. The aliasing model of unsafe Rust seems much more difficult to write correct code for than these other aliasing models.
- notriddle 4y ago
  Use UnsafeCell if that’s your problem.
jb1991 4y ago
You could also say the same thing about Assembly.

ncmncm 4y ago

This is not the case if you are designing a library. Much of what is routinely used to capture semantics into mainstream C++ libraries is wholly impossible to express in Rust. (C is not even a participant, here.) This is not about template metaprogramming, just ordinary stuff. Most of that difference is in things that happen at compile time, so this is not a question of Turing completeness.

lelanthran 4y ago
> Much of what is routinely used to capture semantics into mainstream C++ libraries is wholly impossible to express in Rust. (C is not even a participant, here.) This is not about template metaprogramming, just ordinary stuff. Do you have any examples?
- bluGill 4y ago
  All above languages are turning complete, so if you can express it in one you can express it in another. The question isn't can you write it, the question is how hard is it to do, and how performant the code will be. The heart of C++ is destructors: a bit of code that you can write and the compiler will ensure runs when code goes out of scope/is deleted. You can do this in C by remembering to manually call the right code when doing clean up, but it is easy to forget and thus error pron. (I think Rust has this too?) C++ gives you the ability to do a virtual base class interface, which - as most people know - just means it writes a vtable behind the scene for you. Sometimes people write a vtable by hand in C: it is just a struct of function pointers, but the syntax to do it in C++ is a lot nicer. If you need an interface of some sort the win goes to C++ because the syntax is a lot nicer. (I'm not sure what Rust does about interfaces, I think it has something) C++ gives you control over copying structs. In C structs are only copied member wise, if the struct has a pointer you need to keep track of both copies so you don't free it early. In C++ you write a copy function that will make a copy of the pointer. You can do this in C by remembering to call the right function when copying a struct, but the default is the wrong thing. (I'm not sure what rust has here, but at the very least the borrow checker will stop you from making a mistake) C++ gives you move objects - a way to express that a struct is going out of scope, but only after a different one is taking over the contents. This is a variation of the previous, except that you know the original doesn't need valid data anymore and so you just copy pointers and null them in the original. C doesn't have this concept, you can get around it with use of pointers and manual copying of structs in the right places, but the code is ugly. (again, I'm not sure what rust does, if nothing else the borrow checker should allow the compiler to make some optimizations on this lines) There are a lot more areas where C++ gives you syntax to write correct code that C does not. Rust intentionally doesn't have some of them (class inheritance has been abused often, but I still find it useful enough in a few cases that I think rust is wrong for throwing it out), and in other cases has come up with a better syntax. Overall I don't know enough about Rust to judge it, but I'll take C over C++ anyday. Note, the above is about the advantages of C++ over C. C++ has a lot of warts that are out of scope for that discussion. I am not claiming C++ is perfect. If you are starting a new project you should seriously consider your language options - including some not mentioned here)
  - lelanthran 4y ago
    Thanks, that is a very detailed answer[1]. I'm somewhat confused about why it was downvoted - there is nothing there controversial at all. [1] In my own toy language, I'm kinda leaning towards objects with destructors. Definitely without the footguns in C++, but destructors nonetheless.
    - ncmncm 4y ago
      Possibly because it does not mention almost all of the C++ features that Rust still lacks.
  - Santosh83 4y ago
    > but I'll take C over C++ anyday. Did you mean the opposite?
    - bluGill 4y ago
      opps, yes I did.
  - adjav 4y ago
    I will note that Rust has all of those features you mentioned. 1) Traits 2) You have to be explicit, by implementing the Clone trait, otherwise arguments are passed by reference or moved. 3) Rust has both RAII and linear typing, which is effectively a proven-correct form of moves - you can't access something that has been moved, unlike C++. Rust doesn't have class inheritance, no, but often combining traits and delegating to a "parent" field works. Although sometimes that can be considerably more cumbersome, yeah.
    - ncmncm 4y ago
      I agree that the GP comment does not capture what Rust is unable to express.
  - jstimpfle 4y ago
    You can also see it this way: The philosophy behind C++ is that the existance of a pointer implies ownership. In C, the existance of a pointer only implies liveness (normally; but really the code can decide on its own whether its safe to dereference the pointer). With the C++ approach I've found myself overthinking the problem many times. But with the insight that pointers are just data, and memory management can be independent, it turns out that programming with raw pointers need not be hard. There is no problem passing a pointer without "move semantics" shenanigans.

anonymoushn 4y ago

I'm not sure this is true. The aliasing model of unsafe Rust seems much more difficult to write correct code for than these other aliasing models.

notriddle 4y ago
Use UnsafeCell if that’s your problem.

jb1991 4y ago

You could also say the same thing about Assembly.