As someone who started in Asm, I think pass-by-value/pass-by-reference are also intuitive there in the true "understood without needing any explanation" sense, since basically everything you'd pass to a procedure is a value of some sort, and it only depends on whether you are to interpret that value as a memory address.
In my intro-to-java-class, indirection was one of the first things we learnt when we learnt about objects: variables hold a reference to an object, not the object itself. I don't understand why pointers are somehow a great insight that is exclusive to lower-level programmers.
And if a java person ended up trying to make a method that swaps two primitive values, they'd have to learn about the distinction between passing a primitive value into a method as opposed to passing a reference to an object.
>I don't understand why pointers are somehow a great insight that is exclusive to lower-level programmers.
They're not. You don't need to be a lower level programmer to understand pointers, but it helps immensely if you know at least a bit of a low level language.
When you're getting into details like call-by-reference vs call-by-value, it really helps to have a mental model what the computer is doing. You don't need to be an assembly programmer, but it does help if you spend a little time studying assembly language.
The example given would be, in Python,
def double(x):
x *= 2
a = [4]
double(a[0])
[1] http://www.cs.sfu.ca/~cameron/Teaching/383/PassByName.html
And C++ shows actual reference passing.
No, it isn't.
In C, a pointer is a reference. It supports arithmetic, but it works like a reference as well, so the name applies.
What might confuse people is that in C, you do have to specifically ask for a reference to something, whereas in Java, everything which isn't a primitive type is already a reference.
It's true your code can't dereference those references such that it gets access to all of the data in a List object, for example. C works the same way, again: It's entirely possible to say something like 'typedef struct foo mfoo;' and then declare all of your functions to take pointers to type mfoo, and client code will never be able to get its hands on a struct foo object. FILE pointers work just like this, for example.
If you pass a pointer to a function, then you are saying, hey allocate a new variable to put on the stack which is a pointer. If you pass a reference, then at the language level, you are saying hey I want that function to use THIS variable that I already have. While the implementation may certainly be free to use a pointer, they are not fundamentally the same concepts.
Or so I think.
(It's ironic, by the way, that in seeking to be more pedantic, you reference stacks. Stacks are not part of C.)
Sure, the C standard uses the English word "reference", as in "refers to" and "referred to", but letting that confuse you without understanding the meaning of the word in context is what lead to this Java mess in the first place. Don't bring C into it too.
In addition, pass-by-reference and pass-by-value are very useful terms for anyone interested in accurately speaking about and understanding what will happen when they run the program they write.
There are people who consider accuracy and understanding to be pedantry. More often than not, in my experience, they are a larger source of bugs than those who strive to truly understand how their tools work.
I am not confused, I disagree.
> In addition, pass-by-reference and pass-by-value are very useful terms for anyone interested in accurately speaking about and understanding what will happen when they run the program they write.
The terms would be useful if people agreed on their meaning (they do not) and if languages actually followed one way or the other consistently (they do not). That is the very reason this thread exists.
If you wish to accurately speak about what will happen, then speak about it. Describe what actually happens, instead of spending your time arguing about which round hole the square peg fits in better.
> There are people who consider accuracy and understanding to be pedantry.
In practice, pass-by-value and pass-by-reference provide neither accuracy nor understanding. Stop using them, and these arguments instantly vanish.
> More often than not, in my experience, they are a larger source of bugs than those who strive to truly understand how their tools work.
That I strive to truly understand how my tools work is the very reason I reject these terms as worthless.
Most people agree on the meaning (read the JSL - it says Java is pass-by-value. Read Wikipedia too. Or really any other real reference that isn't some confused people in a forum.)
> If you wish to accurately speak about what will happen, then speak about it
That's what the article does. That's what the JSL does. People in these forums who still don't get it need something more. That's what I'm giving them.
I assume you mean the JLS, in which case it -- very wisely -- does not use the term "pass-by-value" (or any similar term I've seen) to label its semantics. It simply describes its semantics.
> Read Wikipedia too. Or really any other real reference that isn't some confused people in a forum.)
So is it "most people" or a reference that, if anything, supports my point (JLS), an un-encyclopedia, and unspecified "real references" that aren't from people you enjoy insulting?
> That's what the article does.
That's one thing the article does, yes. Why is it you think you need to point that out?
> People in these forums who still don't get it need something more. That's what I'm giving them.
I agree you are giving people in this forum "something more". I suspect we disagree as to what that "something" is.
Well, maybe not the exact letters "p-a-s-s-b-y-v-a-l-u-e" in a row like that, but sometimes you have to read and comprehend.
The JLS does define exactly what happens when parameters are passed...
http://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html... http://docs.oracle.com/javase/specs/jls/se8/html/jls-8.html#...
And that happens to match the definition of pass-by-value/call-by-value exactly (even Java is listed as an example language for pass-by-value)
http://en.wikipedia.org/wiki/Evaluation_strategy#Call_by_val...
> people you enjoy insulting
I haven't insulted anyone, have I? At least, not intentionally. No name-calling, sticking to the facts... I'm sorry if that somehow offends you. Some of us appreciate correctness and don't attribute the chase of it as ad hominem when it isn't.
If you don't understand how you're being insulting here, I can't help you.
> The JLS does define exactly what happens when parameters are passed...
Yes, it's very nice in that respect.
> And that happens to match the definition of pass-by-value/call-by-value exactly (even Java is listed as an example language for pass-by-value)
It happens to match the definition of "call-by-value" used by whoever wrote that blurb on Wikipedia you seem to love so much. The irony here, of course, is that people who like to use these terms don't even agree on how they're spelled...
None of this makes the terms useful.
If you like, though, feel free to provide your plethora of references and documentation that supports your claim that pass-by-reference is meaningless, and is also what Java does.
> If you don't understand how you're being insulting here, I can't help you.
There's no help needed. Asking someone to read and comprehend is no more insulting than pointing out where they are wrong. If being wrong insults you, then I suggest you refrain from posting misguided opinions in technical forums and standing by your incorrect position even in the face of overwhelming opposition from numerous sources.
If I was being insulting, I'd link you to the proggit thread and say "look, even proggit got it after a short discussion, why are the hacker news folks taking so long to get this simple concept"... but I haven't. Instead I would like to try to correct those who are mistaken. I feel like anyone who comes and reads the incorrect statements in threads like this should be able to then read the corrections so they too don't fall into the same traps you have.
What conclusion would that be, exactly?
> your claim that pass-by-reference is meaningless
I didn't make that claim.
> and is also what Java does
I didn't make that claim, either. Nor would I ever. Stop putting words in my mouth.
But if you study, you find out they they have very specific meanings.
Calling C pointers "references" is just the tip of the misunderstanding iceberg.
But if you prefer to wander around in ignorance, by all means. Just don't be surprised when your incorrect comments are constantly corrected.
Just to be sure I didn't comment in my sleep, I went back and looked, and I see the word "meaningless" nowhere in my comments.
> But if you study, you find out they they have very specific meanings.
Some people have defined them in very specific ways. The description in the originally linked article is actually one of the ones I hate the least. That doesn't make it good, and doesn't make the terms useful.
> Calling C pointers "references" is just the tip of the misunderstanding iceberg.
What, precisely, will it take to convince you that I understand your position and simply disagree with it?
> But if you prefer to wander around in ignorance
Please stop being insulting.
> The terms would be useful if people agreed on their meaning (they do not) and if languages actually followed one way or the other consistently (they do not).
Go read some actual literature about programming languages so you can get these ignorant notions out if your head.
The terms pass-by-value and pass-by-reference are well defined and have specific meanings both in the context of specific programming languages and independent of any specific programming language.
Pass by value means the value of the caller expression is logically copied in to the parameters, and direct mutations to the parameter itself are not reflected back to the caller.
Pass by reference means the opposite... The parameter is an alias for the caller's expression, and direct mutations are reflected back to the caller.
C pointers are not called references, except in an informal description of how they relate to what they refer to, by any authority on the topic.
I don't know if you are doing it intentionally or not but I'm done feeding your trolling. If you change your mind and want to learn anything else, my door is always open. But if you want to keep up this charade, whatever it is, go troll someone else. No one is going to read this thread this far down, so I feel no more obligation to correct your unfortunate misguided claims in the spirit of helping someone else who is actually looking for the right answer.
I can't imagine what could lead you to think I'd be interested in "learning" from someone who clearly doesn't even understand what my claims/opinions are, but insists they're "ignorant" and "trolling".
What you've accomplished here is not to teach, but to harden my existing biases.
But it's no skin of my back. My motivation was less about helping you (it was clear from the start that was probably futile) but to instead leave the right answers where you left wrong ones so others who are willing to learn can find what they need to succeed.
This far down, as I said, that no longer applies.
Good day.
Again, I think the issue is with terminology as someone else pointed out. Certainly a C pointer can be considered a reference when using the normal English definition of a reference, but "pass by reference" has a specific meaning, and in C you can't "pass by reference"
In C to be able to simulate some aspects of pas-by-reference you need a double pointer.
Because that particular argument used to be a thing.
Good that it's kept crisp.
In my experience, imprecise semantics and the misunderstandings that arise from them are a/the major cause of error, in both the requirements and implementation domains.
C has the address-of operator (&). So you can pass the address of a pointer, change what that points to in a function, and then the variable outside the function has indeed changed to point to a new chunk of memory. Java only has primitives and pointers to objects, and it only allows you to pass those by value to functions. You can't take the address of a primitive or object and pass that.
So basically in C you can pass an object (struct) or even primitive by reference... or at least you can fake it with trivial syntax. In Java you just cannot do that, and I think Java's use of the term "reference" to mean "pointer" makes some people (understandably) confused.
Not correct.
In Java, if you have an object you take the address and pass it. If you have a primitive, you pass a copy of the value. Both things are called "pass-by-value" (something that confuses a lot of people).
And it's all pass by value (for all values you could have - numbers, addresses, booleans, whatever).
Which is exactly how C does it. I don't see how this is a difficult enough concept to grasp that we need an article explicitly pointing out how Java is just like C in this respect.