There is no pass by reference in Go
dave.cheney.net
dave.cheney.net
- "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 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".)
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?
Call by name is actually a closure based mechanism under the hood. It's like lazy evaluation, but with side effects allowed:
(let ((0 42))
(a #(1 2 3))
(foo (aref a (incf i))))
(foo (lambda () (aref a (incf i)))
Call by need, described below that, seems exactly like the insertion of delay and force. The call-by-need argument expression is basically wrapped in an implicit delay, and callee refers to the argument value using implicit force. delay and force are just additional wrapping around a closure to add a flag to squash multiple evaluations.
I just remembered I implemented some macros to simulate call by name in the TXR solution to the "Man or boy test" task on Rosetta Code:
https://rosettacode.org/wiki/Man_or_boy_test#TXR
With these macros, Knuth's test is expressed exactly in the original form (modulo the Algol being cast into S-exp syntax).
Call by name isn't just lambda with funcall (as might be wrongly inferred from my comments in the other reply), because it supports assignment. If we pass a variable A with call by name, the caller can assign to it. This all works with the given macros, and it probably more or less corresponds to the Algol mechanisms.
(The task description is "Imitate Knuth's example in Algol 60 in another language, as far as possible." so the solutions which don't actually implement a call-by-name syntax and semantics don't go as far as this solution.)
In languages with macros, you get to cheat a little, and write something that looks like the above without _really_ having those semantics. However, in languages like Lisps where your macro language is extremely powerful, you might not really miss it so much.
There is a lot of convenience in _not_ having pass-by-ref! Sometimes I feel like the ardent opposition just doesn't want to admit their favorite language is missing a feature ;-) If I call a function f(a, b), I know that a, b are still pointing at the same object in e.g. Python. That is not the case in C++ (pass-by-ref) or Lisp (arbitrary macros). Predictable language semantics make code easier to reason about. That's a good thing, and it's quite plausible that the added complexity and confusion of supporting pass-by-ref isn't worth the expressiveness.
I appreciate that the term is confusing, but it does mean something. I submit that making it mean two things would be even more confusing. If it's confusing, just say "passing (an argument) _as a reference_" which is unambiguously about what you're passing, and not how you're passing it.
What has happened here is that the double meaning of 'reference' has caused some confusion over the term pass-by-reference, which is purely a term of art. So long as a person understands what gets passed and, especially, its implications, I am not particularly bothered by the mistaken usage.
def f(x):
global a
a = "No, you don't"
a = "I know"
f(a)
print a
But yes, call by reference can be confusing. It also can be very useful, though. That's why many modern languages have call by reference, but also require callers to specify it at each call site. I think that gives you the best of both worlds.
In general, you can use pointers in Object Pascal, but the language offers a lot of options for avoiding them, if you want to, and you get type-checked references in the process. Records (structs) are stack-based and pass-by-value, by default, and class references/class instances (instances are always heap-allocated) are pass-by-reference, by default. So, for the majority of applications, you can use parameters in a myriad number of ways in order to improve performance or provide the proper calling semantics, without any of the hassle of address-of or de-referencing operators.
PBV and PBR seems to be a tiny distinction and yet, we have blog posts and interview questions and online debates about whether a language falls into the first or second category.
For PR reasons, they didn't want to have "pointers" (arithmetic, etc. ew). So Java has "references".
But what happens when you call a method on a null reference? Not NullReferenceException, but NullPointerException.
Well, it's just a rename for PR reasons. They actually don't have pointers.
>But what happens when you call a method on a null reference? Not NullReferenceException, but NullPointerException.
That's because it's the underlying pointer access that causes the exception. Doesn't mean the reference is itself a pointer -- it just abstracts over one.
They do. Everywhere.
Every Java variable of Object type is a pointer. (Also, they're all nullable....ugh.)
If Java had real references (see original comment by jasode) you'd be able to write something like C#'s int.TryParse ( https://msdn.microsoft.com/en-us/library/f02979c7(v=vs.110).... ).
No, it's a reference. It just has a pointer underneath.
>If Java had real references (see original comment by jasode) you'd be able to write something like C#'s int.TryParse
If Java had one specific type of references. The common use of the term though is still what Java has.
The way you are defining "reference" makes it immaterially different from "pointer".
Can a pointer be null? Yes, in C.
Can a "reference" be null? Yes, in Java.
Can a pointer be used in arithmetic operations? Yes, in C.
Can a pointer not be used in arithmetic operations? Yes, in Go.
If this is really the common definition, it isn't a useful one.
The main difference in this definition is on whether you need to dereference to access the value pointed at (like you do in C) or not.
function noop($a, $b) {
$tmp = $a;
$a = $b;
$b = $tmp;
}
function swap(&$c, &$d) {
$tmp = $c;
$c = $d;
$d = $tmp;
}
$first = [1, 2];
$second = [3, 4];
noop($first, $second);
swap($first, $second);
I would call the first two pointers (to [1,2] and [3,4]), and the latter two references (to $first and $second). What would you call them?
(Note: the latter function is impossible to write in Java.)
In the first case, $a and $b pass the arrays by copying their values. The second example uses references.
Java can't do the change visible externally because it passes its references by value -- but that's not a necessity for all languages with references.
As the PHP manual itself says: "References in PHP are a means to access the same variable content by different names. They are not like C pointers; for instance, you cannot perform pointer arithmetic using them, they are not actual memory addresses, and so on".
To put it another way, the inconsistency in Java isn't that primitives and objects are passed differently, it's that they're stored differently. Given how they're stored, they're passed the same way.
If your cognitive model prefers to think of everything as pass pointer by value, you think that.
In Java, Python, Javascript and many other languages, objects pointers are passed by value, but the contents of which are shared between the caller and the callee, so the modification of which is visible from both sides. If we take the definition of pass-by-reference or call-by-reference semantics to mean that, then these languages are pass by reference by default. Apparently, that's not the definition, which in my opinion is rather strange that we are still stuck in a definition that predates OOP, when people mostly concerned with simple primitive values as opposed to composites.
It appears that Barbara Liskov has recognised this and coined call-by-sharing in 1974. 43 years later, practitioners are still arguing in the C++ frame of mind and failed to disseminate the more modern and suitable term of call-by-sharing. This is rather disappointing to me.
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.
Knowing the difference can have important ramifications; particularly in the case of a slice which has its header copied when passed to a function.
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.
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.
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.
There isn't much practical difference between using
// void c_func_alters_deref_pfoo(Foo * pfoo)
Foo foo = { ... };
c_func_alters_deref_pfoo(&foo);
// void cpp_func_alters_foo(Foo & foo)
Foo foo = { ... };
cpp_func_alters_foo(foo);
What's the difference between a logical and syntactical argument?
> 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
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.
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.
I'm saying the linked blog post can be improved significantly by spending a paragraph on explaining this. Otherwise it just sounds like magic.
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;
}
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])
}
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++.
func fn(m &map[int]int) {
m = make(map[int]int)
}
...
fn(m)
func fn(m *map[int]int) {
*m = make(map[int]int)
}
...
fn(&m)
E.x. this works
int square(const int & num) {
return num * num;
}
int main() {
square(5);
}
fn takes_ref(a: &i32) -> i32 {
*a
}
fn main() {
takes_ref(64);
}
let x = returns_a_ref();
takes_ref(x);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.
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?
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().
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".
Also you say "two objects", but if the objects were compound, e.g. arrays, then I could write swap in C.
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_...