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0 pointsjbooth9y ago0 comments

I'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?

0 pointsjbooth9y ago0 comments

I'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.

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?

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5 comments · 1 top-level

lvh9y ago· 4 in thread

In 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().

jboothOP9y ago

Ok, 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".

lvh9y ago

Yep. "Reference type" is a confused and overloaded term, so I prefer to say "pass-by-reference" when I meant int& and "pass-as-reference" (or the, in my opinion confused and confusing, pass-by-object, if that's helpful to the listener).

Myrmornis9y ago

Just to be clear, the rules of your swap challenge are that the arguments of the function have to be ints, not allowed to be int *, right?

Also you say "two objects", but if the objects were compound, e.g. arrays, then I could write swap in C.

lvh9y ago

Yes. It's about swapping the arguments. With a struct or an array or a pointer or whatever, you're not swapping the arguments themselves; but the fact that there are so many things that do something slightly similar probably illustrates the limited utility of pass-by-reference.

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