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Go structs are copied on assignment (and other things about Go I'd missed)

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Re: Go structs are copied on assignment (and other things about Go I'd missed)

#41

Post by neonsunset »

rickstanley wrote: post #32
eterm wrote: post #28

This sometimes catches out people C#/.Net too, it's a big difference between Class and Struct, Class is reference type and Struct is value type. (see fiddle below), but in practice people very rarely reach for structs, so people don't tend to build up the muscle memory of using them, even if they intuitively understand the difference between reference types and value types from general use of other types. (Fiddle dem…

Non-C# developer question: what use-case/situation would a `struct` make sense to use instead of a `class`? Just out of curiosity. [Edit] Well, there's a nice, special article for this very question: https://learn.microsoft.com/en-us/dotnet/standard/design-gui...

Please note that the article is quite old and does not encompass the wide variety of scenarios C# is effective at.

Structs are used for but not limited to: all kinds of "transient" data containers, pass by value semantics, precise control over layout of data in memory, low-level programming and interoperating with C/C++, zero-cost abstractions via struct generics (ala Rust or templates in C++), lightweight wrappers over existing types, etc.

Most C# codebases use them without even noticing in `foreach` loops - enumerators are quite often structs, and so are Span and Memory (which are .NET slice types for arrays, strings and pretty much every other type of contiguous memory, including unmanaged). Tuple syntax uses structs too - `(int x, int y)` is `ValueTuple` behind the scenes.

.NET has gotten quite good at optimizing structs, so the more general response is "you use them for the same things you use structs in C, C++". Or the same reason you would pick plain T struct in Rust over Box/Arc.

Intro to structs: https://learn.microsoft.com/en-us/dotnet/csharp/language-ref...

Re: Go structs are copied on assignment (and other things about Go I'd missed)

#42

Post by metaltyphoon »

jerf wrote: post #15
zuzuleinen wrote: post #4

To generalize the title into a rule is good to remember that in Go everything is passed by value(copy).

That is the case for almost every modern language. C++ is one of the few languages that has "references" and at least last I looked that's a language accommodation over what are pointers being passed by value in the assembly, at least until compiler optimizations take over (and that's not limited to references either). If you're in 2024 and you're in some programming class making a big deal about pass-by-value versus…

Both C# and Swift makes a distinct difference by having both struct and classes.

Re: Go structs are copied on assignment (and other things about Go I'd missed)

#43

Post by jerf »

azundo wrote: post #30
jerf wrote: post #15

Earlier quoted context omitted.

That is the case for almost every modern language. C++ is one of the few languages that has "references" and at least last I looked that's a language accommodation over what are pointers being passed by value in the assembly, at least until compiler optimizations take over (and that's not limited to references either). If you're in 2024 and you're in some programming class making a big deal about pass-by-value versus…

Is python no longer a modern language? Objects are certainly not copied when passed to a function.

Python copies references by value.

    $ python3
    Python 3.12.3 (main, Jul 31 2024, 17:43:48) [GCC 13.2.0] on linux
    Type "help", "copyright", "credits" or "license" for more information.
    >>> def x():
    ...     v = 1
    ...     y(v)
    ...     print(v)
    ... 
    >>> def y(val):
    ...     val += 1
    ... 
    >>> x()
    1
A pass-by-reference language would print 2.

Everything in a modern language is passed by copy. Exactly what is copied varies and can easily be pointers/references. But there were languages once upon a time that didn't work that way. It's a dead distinction now, though, unless you go dig one of them up.

If you want a specific one to look at, look at Forth. Note how when you call a function ("invoke a word", closest equivalent concept), the function/word doesn't get a copy of anything. It directly gets the actual value. There is no new copy, no new memory location, it gets the actual same memory as the caller was using, and not as a "pointer"... directly. Nothing works like that any more.

Re: Go structs are copied on assignment (and other things about Go I'd missed)

#44

Post by jerf »

metaltyphoon wrote: post #42
jerf wrote: post #15

Earlier quoted context omitted.

That is the case for almost every modern language. C++ is one of the few languages that has "references" and at least last I looked that's a language accommodation over what are pointers being passed by value in the assembly, at least until compiler optimizations take over (and that's not limited to references either). If you're in 2024 and you're in some programming class making a big deal about pass-by-value versus…

Both C# and Swift makes a distinct difference by having both struct and classes.

That is not what the pass-by-copy vs. pass-by-reference distinction is. Both are passing by value, one is just a pointer (under the hood) and the other is not. But the incoming parameter is a copy.

See my cousin post: https://news.ycombinator.com/item?id=41220384

This is a distinction so dead that people basically assume that this must be talking about whether things are passed by pointer, because in modern languages, what else would it be? But that is not what pass-by-reference versus pass-by-copy means. This is part of why it's a good idea to let the terminology just die.

Re: Go structs are copied on assignment (and other things about Go I'd missed)

#45

Post by jerf »

klyrs wrote: post #38
jerf wrote: post #15

Earlier quoted context omitted.

That is the case for almost every modern language. C++ is one of the few languages that has "references" and at least last I looked that's a language accommodation over what are pointers being passed by value in the assembly, at least until compiler optimizations take over (and that's not limited to references either). If you're in 2024 and you're in some programming class making a big deal about pass-by-value versus…

Is copying huge blocks of data free in 2024? My benchmarks suggest otherwise, and the world still needs assembly programmers.

See cousin posts. That's not what the terms mean.

Re: Go structs are copied on assignment (and other things about Go I'd missed)

#46

Post by magicalhippo »

jerf wrote: post #44
metaltyphoon wrote: post #42

Earlier quoted context omitted.

Both C# and Swift makes a distinct difference by having both struct and classes.

That is not what the pass-by-copy vs. pass-by-reference distinction is. Both are passing by value, one is just a pointer (under the hood) and the other is not. But the incoming parameter is a copy. See my cousin post: https://news.ycombinator.com/item?id=41220384 This is a distinction so dead that people basically assume that this must be talking about whether things are passed by pointer, because in modern languages…

So what should we call "foo(x)" and "foo(ref x)" in C# to distinguish them if not pass-by-value and pass-by-reference?

Re: Go structs are copied on assignment (and other things about Go I'd missed)

#47

Post by jerf »

magicalhippo wrote: post #46
jerf wrote: post #44

Earlier quoted context omitted.

That is not what the pass-by-copy vs. pass-by-reference distinction is. Both are passing by value, one is just a pointer (under the hood) and the other is not. But the incoming parameter is a copy. See my cousin post: https://news.ycombinator.com/item?id=41220384 This is a distinction so dead that people basically assume that this must be talking about whether things are passed by pointer, because in modern languages…

So what should we call "foo(x)" and "foo(ref x)" in C# to distinguish them if not pass-by-value and pass-by-reference?

C# can call it that specifically if it likes, because the general computer science term is dead, but under the hood you're passing a reference by value. Look to the generated assembler in a non-inlined function. You'll find a copy of a pointer. You did not in true pass-by-refernce langauges.

The fact that is a sensible thing to say in a modern language is another sign the terminology is dead.

Re: Go structs are copied on assignment (and other things about Go I'd missed)

#48

Post by maerF0x0 » 

    func findThing(things []Thing, name string) *Thing {
      for i := range things {
        if things[i].Name == name {
          return &things[i]
        }
      }
      return nil
    }
Also you could just return i or -1, and the consuming code would be clear about what it was doing. Find the index. Update the item at the index.

    if location := findThing(things, name); location != -1 {
         things[location].Name = "updated"
    }

Re: Go structs are copied on assignment (and other things about Go I'd missed)

#49

Post by harrisi »

simonw wrote: post #10

One of the many things I find inspiring about Julia is how quick she is to admit to mistakes she has made or things that she hasn't understood. If she didn't understand it, I can 100% guarantee that there are large numbers of people out there who also didn't understand it - many of whom were probably too embarrassed to ever admit it. I think this is a useful trait for senior software engineers generally. If you're a…

I recently included a big goof in a blog post. I accidentally consumed all my errors and didn't log them anywhere, so I was just flying mostly blind.

It was also in a language I don't have much experience in (Elixir), doing 3D rendering in OpenGL (which I also don't have much experience in), on a Mac (so no tools to debug OpenGL stuff). Definitely wasted more time than I'd like to admit, but I won't be making that mistake again. :)

Re: Go structs are copied on assignment (and other things about Go I'd missed)

#50

Post by trentnix »

MathMonkeyMan wrote: post #2

Donovan and Kernighan's "The Go Programming Language" is one of the best pieces of technical writing I've ever read. Buy it and read it cover to cover. Then read the [Go Language Specification][1] cover to cover. It's dry but refreshingly not legalese. [1]: https://go.dev/ref/spec

Learning Go by Jon Bodner, particularly the newest edition, is also excellent.

An aside: are there any other Go books, particularly ones the explore more specific topics, that are recommended? I've read a few that I didn't find very impressive.

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