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There is no pass by reference in Go
- eliben 9y agoInterestingly, the term "reference type" has been removed from the Go spec in 2013: https://github.com/golang/go/commit/b34f0551387fcf043d65cd7d96a0214956578f94 https://github.com/golang/go/commit/b34f0551387fcf043d65cd7d... Yet some semi-official sources (like The Go Programming Language book) do refer to channels as "reference types". TBH, I find cheney's post not fully useful without explaining what does happen behind the scenes in maps so that passing them by value doesn't incur large copies.
- maxpblum 9y agoI haven't used Go before; is there a common conception that you can pass by reference in Go?
- fiorix 9y agoanswer is in the title
- franciskim 9y agoIt is not
- bsaul 9y agoI guess in every language with pointers, people often confuse passing the pointer as a value with passing the object as a reference.
- jbooth 9y agoI'm still confused and I've been working in various languages for 15 years. If I pass a java object to a function, I'm passing a (probably) 8-byte pointer to some memory with a class tag, fields, whatever. It goes on the stack just like an 8 byte long. In languages that support pointers more directly, I'm doing the same thing, maybe minus the class tag in the pointed-to memory. Address is in an 8 byte type, put it on the stack and access the pointed-to struct in your new frame. Yet I've seen interview questions about whether you're "passing by reference" or "passing a reference by value" like there's some big meaningful difference and one answer is wrong. I don't get it. Is this just one of those nerd arguments where we're debating semantics for the sake of it?
- lvh 9y agoIn all of those cases, you're probably passing a reference by value, semantically. ("Probably", because you didn't specify which languages, so I'm making an educated guess.) The usual litmus test is having two objects (or ints or whatever), write a fn `swap(a, b)` where the two are swapped after the call is over. Can't do that in Java or Go or Python or C; but you can in e.g. C++ and you sorta-can in Lisp. In C++, you'd see int& (or whatever) show up in the arguments of swap().
- jbooth 9y agoOk, yeah, I saw upthread about C++ and Pascal having a more formal reference type. But in C and Go you can pass pointers to pointers, in Java and Python objects with mutable object references.. It seems like the whole article could have been "Go does not have the C++ reference type, so it's not really in the mix when calling functions".
- 9y ago
- nachexnachex 9y agoSome programmers make a liberal use of "passing by reference" (v. "passing by value") a variable when referring to passing a pointer to it (v. its value) to a function. This is common language in a C environment, where there's also no pass by ref (quick example: [0]). I'm guilty of this myself. It's possible that this may create confusion to other people more familiar with, for example, the C++ concept of passing by reference which is passing an actual reference (v. "passing a pointer"). [0] https://www.tutorialspoint.com/cprogramming/c_function_call_by_reference.htm https://www.tutorialspoint.com/cprogramming/c_function_call_...
- kstenerud 9y agoIt's a question of semantics and the poorly chosen word "reference". The roots come from C, where pretty much every book on the subject talked about the difference between passing a huge, expensive value to a function vs passing a cheap pointer "reference" to it. And while the semantic purists will scream and holler, the fact remains that, colloquially, a pointer is a reference to something, not the something itself. And so this annoying debate rears its ugly head a few times a year and makes it into HN or Slashdot or whatever.
- lvh 9y agoNobody is debating what the word "reference" means. The debate is that "pass-by-reference" is already a different thing from passing something __as__ a reference. I appreciate that it's confusing to someone who isn't familiar with the term, but so's most of CS. We had a term and it referred to a thing, and having it refer to two different things is worse.
- kstenerud 9y agoAnd yet this is the reality we live in. No matter how much talk of "should", the reality stares you implacably in the face. This is a UX problem. Practically speaking, the most important concept is that of cost, which the naive answer covers. The distinction only matters in a language that actually differentiates the two. Otherwise it's little more than trivia, and certainly nothing to get worked up over.
- ivanbakel 9y agoThe difference between pass-as-reference and pass-by-reference is more than trivia - if you were ignorant of it in a language, you might find trouble in the future when a function decides to alter its argument values in a way that negatively affects the caller. Part of the reason we should work to clarify the language is to avoid misconceptions about behaviour based on what kind of "reference" is meant.
- tetromino_ 9y ago
- jasode 9y agoAn extended explanation missing from Dave Cheney's answer is that the "reference" etymology has at least 2 meanings. This is why his explanation is virtually the same as the one 20 years ago in the C Language FAQ[1] compiled by Steve Summit. - "reference" definition 1: an alias that cannot be null which is what a "reference type" is in Pascal and C++. This might be thought of as a stricter "computer-science" definition. The C and Go languages don't have this type of "reference". - "reference" definition 2: a pointer variable that lets programmers change the "thing pointed to" instead of the pointer variable itself to avoid copying unnecessary bytes. This is the colloquial usage. It also doesn't help that we call the star operator the "de-REFERENCING" operator[2] instead of the "de-POINTERIZING" operator. (What are we "derefencing" if it's not a "reference"?!?) Cheney is talking about definition 1. [1] http://www.di-srv.unisa.it/~vitsca/LAB/C-faq.html http://www.di-srv.unisa.it/~vitsca/LAB/C-faq.html excerpt: 4.11: Does C even have "pass by reference"? A: Not really. Strictly speaking, C always uses pass by value. You can simulate pass by reference yourself, by defining functions which accept pointers and then using the & operator when calling, and the compiler will essentially simulate it for you when you pass an array to a function (by passing a pointer instead, see question 6.4 et al.). However, C has nothing truly equivalent to formal pass by reference or C++ reference parameters. (On the other hand, function-like preprocessor macros can provide a form of "pass by name".) [2] https://en.wikipedia.org/wiki/Dereference_operator https://en.wikipedia.org/wiki/Dereference_operator
- dtzur 9y agoYap, your comment is better than the entire article. Thanks!
- mannykannot 9y agoIndeed. Definition 2 is broadly in line with what "reference" means in general usage, while definition 1 is a term of art in the vocabulary of certain languages. Before languages had reference types or pointers, the distinction between pass-by-reference and pass-by-value (or also pass-by-name in Lisp and Algol) told you important things about the semantics of the language, such as whether a function you called could modify the objects you gave as arguments, or incorporate them by reference into some other object, and how arguments that are expressions are handled. Some of those distinctions become blurred when pointers or references are passed by value; the distinction on how you pass them now becomes a distinction on what you pass. In practice, is there any language that passes pointers or references by reference?
- htns 9y agoDoesn't the post stumble over right in the second section? "It is not possible to create a Go program where two variables share the same storage location in memory". So what? The C example didn't show that either.
- lvh 9y agoYou are correct that C also does not have pass-by-reference. However, the second listing is a C++ program, not a C program, and C++ very much has pass-by-reference. The C++ program demonstrates true aliases, as you can tell from the output of the printf (see comment on same line).
- htns 9y agoI have always found that is conflating the semantics of using with the method of passing. Even with/out aliases you are still passing a reference.
- lvh 9y agoI don't think that's accurate. I can pass something locally declared as int to a fn that takes int& according to it's signature. It seems that's precisely what the confusion is about :)
- kstenerud 9y agoOften, I'll deliberately give the naive "pointers are references" answer in a job interview, just to gauge the interviewer's response. It's actually a good proxy to determining company culture. If he starts to patiently explain the difference, I'll stop him and talk about the practical implications of the concept, and we'll have a good laugh. If he gets annoyed or stiffens and things become tense, I'll have a good proxy for what to expect should I make the mistake of working there.
- mannykannot 9y agoThen there are those interviewers who silently cross you off the list and move on. They are probably type II people, however, so no harm done from either point of view.
- deleted 9y ago[deleted]
- pebers 9y agoMinor nit... > It is not possible to create a Go program where two variables share the same storage location in memory. It is possible to do so, because the empty struct occupies no space. For example: s := []struct{}{ struct{}{}, struct{}{}, } fmt.Printf("%p %p\n", &s[0], &s[1]) > 0x176f44 0x176f44 But that's kind of a fringe case, and I suppose one might argue that the two variables occupy no space so in a sense they don't share that space. Either way it doesn't detract from the original point of the post :)
- pokstad 9y ago"Maps and channels are not references." But then refer to the Go team's article on maps: Map types are reference types, like pointers or slices, and so the value of m above is nil; it doesn't point to an initialized map. A nil map behaves like an empty map when reading, but attempts to write to a nil map will cause a runtime panic; don't do that. To initialize a map, use the built in make function: m = make(map[string]int) The make function allocates and initializes a hash map data structure and returns a map value that points to it. The specifics of that data structure are an implementation detail of the runtime and are not specified by the language itself. In this article we will focus on the use of maps, not their implementation. Saying maps are not passed by reference is a bit deceiving. The map's pointer location is not passed by reference. Source: https://blog.golang.org/go-maps-in-action https://blog.golang.org/go-maps-in-action
- Rapzid 9y agoIMHO saying they are is misleading as this article correctly points out nothing is, and Rob Pike himself has pointed out the same on many occasions. They are referred to as "reference types" because they contain pointers to the underlying data structure. However the thing stored in the variable is more than just a pointer, and it is passed by value. Knowing the difference can have important ramifications; particularly in the case of a slice which has its header copied when passed to a function.
- brianolson 9y agoBy this obtuse definition of 'pass by reference', Nothing with a C-like function call stack will 'pass by reference'. Programming languages that approximate 'pass by reference' will actually pass a pointer to the called function that points to a value in the callee's stack frame. The pointer in the called function is a local variable, so changing the value of that has no effect in the callee's scope. Changing the contents of the thing pointed to is the only _meaningful_ 'pass by reference', and practically most languages have that. (including Go)
- zackmorris 9y agoHowever, Go has pointers (just not pointer arithmetic): https://tour.golang.org/moretypes/1 https://tour.golang.org/moretypes/1 Does anyone know of an imperative language doesn't have references, but also doesn't have pointers? Perhaps using something like copy-on-write to pass variables to functions.
- deleted 9y ago[deleted]
- moomin 9y agoC# has a name for Go's object semantics: Marshal By Reference. This means that when an object features in a call, the GC pointer aka reference is passed (by value) rather than the object value itself. This reference is fundamentally different from the reference concept in C++. So the term reference is semantically overloaded and means different things in different programming languages. However, the phrase "pass by reference" is language independent and the author is correct, it doesn't have those semantics.
- Myrmornis 9y agoWhen teaching, rather than using a special name like "pointer", wouldn't it be better to simply explain that it's a variable that holds a memory address? Once a student (e.g. who only knows a high-level language) understands that they now have to think about memory explicitly, then the behavior of such a variable will become obvious. AFAIU there are only two special features of pointers: the dereference operation and the additive arithmetic which increments in strides according to their type.
- Animats 9y agoGo lacks the C++ reference operator. You can't write: func fn(m &map[int]int) { m = make(map[int]int) } ... fn(m) You have to write: func fn(m *map[int]int) { *m = make(map[int]int) } ... fn(&m) It's very C. The caller is forced to use different syntax to pass something as a pointer/reference. In C++ and Rust, you can't see from the call whether you're passing a pointer or a reference.
- mastax 9y agoThe difference is in C++ rvalues coerce to const&, while in Rust they do not. E.x. this works int square(const int & num) { return num * num; } int main() { square(5); } this does not fn takes_ref(a: &i32) -> i32 { *a } fn main() { takes_ref(64); } There are still cases where you won't be able to tell if you're using a ref or not, though: let x = returns_a_ref(); takes_ref(x);
- YZF 9y agoC++: void f(map<int, int> m) { m[0] = 0; } int main() { map<int, int> t; t[0] = 1; f(t); cout << t[0] << endl; return 0; } prints 1. Go: func f(m map[int]int) { m[0] = 0 } func main(){ var t = make(map[int]int) t[0] = 1 f(t) fmt.Println(t[0]) } Prints 0. So in C++ maps are passed by value. In Go they are passed by, hmm, "not value". Go could have made maps similar to C++ maps and then people could use a pointer to a map when they wanted that. As it stands maps are "different" which is not a big deal once you're used to it but still makes things a little weird if you're coming from C++.