# Pass by reference? **URL:** \ **Category:** Ruby\ **Created:** [January 27, 2007, 12:28am UTC](https://www.ruby-forum.com/t/pass-by-reference/83534 "2007-01-27T00:28:29Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![ddancer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ecb155/32.png) [@ddancer](https://www.ruby-forum.com/u/ddancer)\ **Post date:** [January 27, 2007, 12:28am UTC](https://www.ruby-forum.com/t/pass-by-reference/83534/1 "2007-01-27T00:28:29Z")
Hello, Is there a way to pass variables by reference into function. I have large string to pass and the pass by value seems to be eating up too much memory. Thanks, Andy ---
**Author:** ![ddancer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ecb155/32.png) [@ddancer](https://www.ruby-forum.com/u/ddancer)\ **Post date:** [January 27, 2007, 12:33am UTC](https://www.ruby-forum.com/t/pass-by-reference/83534/2 "2007-01-27T00:33:29Z")
Andy K. wrote: > Hello, > > Is there a way to pass variables by reference into function. > > I have large string to pass and the pass by value seems to be eating up > too much memory. You don’t actually need to do anything: everything in ruby is passed by reference, (excepted integers, symbols, nil, false and true - am I missing one ?) Cheers, Vincent ---
**Author:** ![ddancer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ecb155/32.png) [@ddancer](https://www.ruby-forum.com/u/ddancer)\ **Post date:** [January 27, 2007, 12:35am UTC](https://www.ruby-forum.com/t/pass-by-reference/83534/3 "2007-01-27T00:35:05Z")
Everything is pass-by-reference in Ruby. There is no pass-by-value. So your memory problems must have another cause. Andy K. wrote: > Hello, > > Is there a way to pass variables by reference into function. > > I have large string to pass and the pass by value seems to be eating up > too much memory. > > Thanks, > > Andy ---
**Author:** ![ddancer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ecb155/32.png) [@ddancer](https://www.ruby-forum.com/u/ddancer)\ **Post date:** [January 27, 2007, 1:16am UTC](https://www.ruby-forum.com/t/pass-by-reference/83534/4 "2007-01-27T01:16:33Z")
Hi – On Sat, 27 Jan 2007, Per V. wrote: > Andy K. wrote: > > > Hello, > > > > Is there a way to pass variables by reference into function. > > > > I have large string to pass and the pass by value seems to be eating up > > too much memory. > > Everything is pass-by-reference in Ruby. There is no pass-by-value. So > your memory problems must have another cause. I’ve always looked at it as: pass by value, where the “values” are (mostly) references. For example: a = “abcde” # a now contains a reference to that string do\_something(a) # pass the reference contained in a David ---
**Author:** ![ddancer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ecb155/32.png) [@ddancer](https://www.ruby-forum.com/u/ddancer)\ **Post date:** [January 27, 2007, 1:25am UTC](https://www.ruby-forum.com/t/pass-by-reference/83534/5 "2007-01-27T01:25:33Z")
Phrogz wrote: > If you pass an immutable type by reference, does it make a sound? > Er, I mean… > If you passed an immutable type by reference, how would you know that > it wasn’t passed by value? Is there any way for the function you’re calling to modify the value of the variable in the caller? Pass by reference can do that. - Martin ---
**Author:** ![ddancer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ecb155/32.png) [@ddancer](https://www.ruby-forum.com/u/ddancer)\ **Post date:** [January 27, 2007, 12:55am UTC](https://www.ruby-forum.com/t/pass-by-reference/83534/6 "2007-01-27T00:55:31Z")
Per V. schrieb: > Everything is pass-by-reference in Ruby. There is no pass-by-value. So > your memory problems must have another cause. May be, that you do in fact a copy by some other operations, that create a new String object and copy the original. E.g. s = t + ‘’ creates a new string with the same contents. There are several possibilities to do things like that without recognizing it. Wolfgang Nádasi-Donner ---
**Author:** ![ddancer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ecb155/32.png) [@ddancer](https://www.ruby-forum.com/u/ddancer)\ **Post date:** [January 27, 2007, 1:20am UTC](https://www.ruby-forum.com/t/pass-by-reference/83534/7 "2007-01-27T01:20:33Z")
On Jan 26, 4:32 pm, Vincent F. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > You don’t actually need to do anything: everything in ruby is passed > by reference, (excepted integers, symbols, nil, false and true - am I > missing one ?) How do you know if those are passed by reference or by value? It’s a very existential question: If you pass an immutable type by reference, does it make a sound? Er, I mean… If you passed an immutable type by reference, how would you know that it wasn’t passed by value? I leave you with this: irb(main):001:0\> values = [1, :foo, true, false, nil] =\> [1, :foo, true, false, nil] irb(main):002:0\> puts values.map{ |v| v.object\_id } 3 231138 2 0 4 =\> nil irb(main):003:0\> def foo( \*args ); puts args.map{ |v| v.object\_id }; end =\> nil irb(main):004:0\> foo( \*values ) 3 231138 2 0 4 =\> nil irb(main):005:0\> b = :foo =\> :foo irb(main):006:0\> b.object\_id =\> 231138 It’s almost the reverse of what you said - it’s not that immediate values are passed by value, but rather that every reference to an immediate value refers to the same object. ---
**Author:** ![ddancer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ecb155/32.png) [@ddancer](https://www.ruby-forum.com/u/ddancer)\ **Post date:** [January 27, 2007, 1:44am UTC](https://www.ruby-forum.com/t/pass-by-reference/83534/8 "2007-01-27T01:44:35Z")
Hi – On Sat, 27 Jan 2007, Martin C. Martin wrote: > Phrogz wrote: > > > If you pass an immutable type by reference, does it make a sound? > > Er, I mean… > > If you passed an immutable type by reference, how would you know that > > it wasn’t passed by value? > > Is there any way for the function you’re calling to modify the value of the > variable in the caller? Pass by reference can do that. You can modify the object to which the variable refers: def change\_me(obj) obj \<\< “hi” end arr = [1,2,3] change\_me(arr) p arr # [1, 2, 3, “hi”] In this example, arr contains a reference to an array. In change\_me, obj contains another copy of the same reference, so you can use it to manipulate and change the original array. I still wouldn’t call this pass by reference (see my earlier post in this thread). David ---
**Author:** ![ddancer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ecb155/32.png) [@ddancer](https://www.ruby-forum.com/u/ddancer)\ **Post date:** [January 27, 2007, 8:52am UTC](https://www.ruby-forum.com/t/pass-by-reference/83534/9 "2007-01-27T08:52:02Z")
On 1/27/07, removed\_email\_address@domain.invalid [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > > If you passed an immutable type by reference, how would you know that > > > end > > > this thread). > > > Just to demonstrate your point def modifying a\_param a\_param = “hi” end a\_value = “low” modifying a\_value case a\_value when “hi” puts “Passed by reference” when “low” puts “Passed by value” else puts “Passed away” end or to put it different x = a.object\_id method a a.object\_id == x # for sure Robert David ---
**Author:** ![ddancer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ecb155/32.png) [@ddancer](https://www.ruby-forum.com/u/ddancer)\ **Post date:** [January 27, 2007, 8:51am UTC](https://www.ruby-forum.com/t/pass-by-reference/83534/10 "2007-01-27T08:51:01Z")
On 27.01.2007 01:43, removed\_email\_address@domain.invalid wrote: > > Is there any way for the function you’re calling to modify the value > > p arr # [1, 2, 3, “hi”] > > In this example, arr contains a reference to an array. In change\_me, > obj contains another copy of the same reference, so you can use it to > manipulate and change the original array. > > I still wouldn’t call this pass by reference (see my earlier post in > this thread). Absolutely right: Ruby uses pass by value - with references. irb(main):004:0\> def foo(x) x = 10 end =\> nil irb(main):005:0\> def bar; x = 20; foo(x); x end =\> nil irb(main):006:0\> bar =\> 20 irb(main):007:0\> There is no standard way (i.e. other than involving eval and metaprogramming magic) to make a variable in a calling scope point to another object. And, btw, this is independent of the object that the variable refers to. Immediate objects in Ruby seamlessly integrate with the rest (different like POD’s in Java for example) and from a Ruby language perspective you don’t see any difference (other than performance maybe). This is one of the reasons why Ruby is so elegant. Kind regards robert ---
**Author:** ![ddancer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ecb155/32.png) [@ddancer](https://www.ruby-forum.com/u/ddancer)\ **Post date:** [January 27, 2007, 12:05pm UTC](https://www.ruby-forum.com/t/pass-by-reference/83534/11 "2007-01-27T12:05:23Z")
Hi – On Sat, 27 Jan 2007, Robert D. wrote: > > > > Er, I mean… > > > > obj \<\< “hi” > > > > I still wouldn’t call this pass by reference (see my earlier post in > > modifying a\_value > case a\_value > when “hi” > puts “Passed by reference” > when “low” > puts “Passed by value” > else > puts “Passed away” > end That’s a somewhat different point, though, having to do with variable scope and (re)assignment semantics. What you’re doing here essentially is: a = “low” b = a b = “hi” puts a # low That’s going to happen whether a method call is involved or not. David ---
**Author:** ![ddancer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ecb155/32.png) [@ddancer](https://www.ruby-forum.com/u/ddancer)\ **Post date:** [January 27, 2007, 12:11pm UTC](https://www.ruby-forum.com/t/pass-by-reference/83534/12 "2007-01-27T12:11:25Z")
Hi – On Sat, 27 Jan 2007, Robert K. wrote: > > > > > > > change\_me(arr) > > irb(main):004:0\> def foo(x) x = 10 end > =\> nil > irb(main):005:0\> def bar; x = 20; foo(x); x end > =\> nil > irb(main):006:0\> bar > =\> 20 > irb(main):007:0\> I’m not sure that demonstrates the “values that are references” thing, though. You’re reassigning to x in foo, which creates a new local x; but I think that’s just part of the assignment semantics. Or are you assuming that if pass by reference were involved, then assignment would work differently? David ---
**Author:** ![ddancer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ecb155/32.png) [@ddancer](https://www.ruby-forum.com/u/ddancer)\ **Post date:** [January 27, 2007, 12:13pm UTC](https://www.ruby-forum.com/t/pass-by-reference/83534/13 "2007-01-27T12:13:25Z")
On 1/27/07, removed\_email\_address@domain.invalid [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > puts a # low > > That’s going to happen whether a method call is involved or not. David, it is not my fault that parameter passing by value has the same semantics as reference assignment 😉 (it probably is what is happening behind the scenes, a new reference is allocated and the formal parameter is copied to it). Do you see what I wanted to show? Cheers Robert David ---
**Author:** ![ddancer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ecb155/32.png) [@ddancer](https://www.ruby-forum.com/u/ddancer)\ **Post date:** [January 27, 2007, 12:35pm UTC](https://www.ruby-forum.com/t/pass-by-reference/83534/14 "2007-01-27T12:35:26Z")
> > > > > Code \>\>\>\>\> def modifying(a\_param) a\_param = “hi” end def modifying2(a\_param=a\_value) a\_param = “hi” end def modifying3(a\_param) a\_param[0…-1]= “hi” end a\_value = “low” modifying a\_value case a\_value when “hi” puts “Passed by reference” when “low” puts “Passed by value” else puts “Passed away” end a\_value = “low” modifying2 a\_value case a\_value when “hi” puts “Passed by reference” when “low” puts “Passed by value” else puts “Passed away” end a\_value = “low” modifying3 a\_value case a\_value when “hi” puts “Passed by reference” when “low” puts “Passed by value” else puts “Passed away” end > > > > > Output \>\>\>\>\> Passed by value Passed by value Passed by reference > > > > > EoE \>\>\>\>\> I would name it somehow “call by assignment of value to local variable”, because the value of the actual parameter is assigned to the local variable used as a formal parameter. except for “Fixnum” etc. this is a pointer to an object. I one changes the object itself, it will work like “by reference”, otherwise only the local variable is affected, without having any effect to the outside world. Wolfgang Nádasi-Donner ---
**Author:** ![ddancer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ecb155/32.png) [@ddancer](https://www.ruby-forum.com/u/ddancer)\ **Post date:** [January 27, 2007, 12:47pm UTC](https://www.ruby-forum.com/t/pass-by-reference/83534/15 "2007-01-27T12:47:28Z")
Hi – On Sat, 27 Jan 2007, Wolfgang Nádasi-Donner wrote: > def modifying3(a\_param) > else > else > else > puts “Passed away” > end > > > > > > > Output \>\>\>\>\> > > Passed by value > Passed by value But you’ve broken the bindings. It’s a whole new ballgame, as we say here 🙂 > Passed by reference I would say: a reference, passed by value. > > > > > > EoE \>\>\>\>\> > > I would name it somehow “call by assignment of value to local variable”, > because the value of the actual parameter is assigned to the local variable > used as a formal parameter. > > except for “Fixnum” etc. this is a pointer to an object. I one changes the > object itself, it will work like “by reference”, otherwise only the local > variable is affected, without having any effect to the outside world. I consider the variable assignment semantics to be a separate matter. For example: def x(a) a = “hello” end Here, Ruby’s rule is that the second a is a completely new binding, and destroys the original binding. I don’t think that has anything to do with whether the first a is a reference or a value. In other words, even if Ruby had pass by reference, I assume that a = “hello” would still destroy the binding and create a new local variable. (Of course, it’s purely speculative, since it would be a completely different language.) David ---
**Author:** ![ddancer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ecb155/32.png) [@ddancer](https://www.ruby-forum.com/u/ddancer)\ **Post date:** [January 27, 2007, 1:00pm UTC](https://www.ruby-forum.com/t/pass-by-reference/83534/16 "2007-01-27T13:00:30Z")
On 27.01.2007 12:10, removed\_email\_address@domain.invalid wrote: > > > > > If you pass an immutable type by reference, does it make a sound? > > > > > obj \<\< “hi” > > > > > I still wouldn’t call this pass by reference (see my earlier post in > > > > > irb(main):007:0\> > > I’m not sure that demonstrates the “values that are references” thing, > though. You’re reassigning to x in foo, which creates a new local x; > but I think that’s just part of the assignment semantics. Or are you > assuming that if pass by reference were involved, then assignment > would work differently? I probably should have chosen different variable names. I was trying to make the point, that you cannot change a variable in bar by assigning in foo. Kind regards robert ---
**Author:** ![ddancer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ecb155/32.png) [@ddancer](https://www.ruby-forum.com/u/ddancer)\ **Post date:** [January 27, 2007, 1:15pm UTC](https://www.ruby-forum.com/t/pass-by-reference/83534/17 "2007-01-27T13:15:34Z")
May be it will be clearer to name an assignment “a = someobject” by what it really means: “Give a reference to object ‘someobject’ a name ‘a’ by definition”. It’s close to the “:=” used for definition in mathematics. If one uses this “a” as a “reference to ‘someobject’” as an actual parameter in a method call, it means, that the “reference to ‘someobject’” is actually used in the method. Clearly you cannot change “a”, because you don’t have access to this name “a” inside the method, but you can change the object “someobject” usually without any problems. If you change “someobject” this will have consequences for ‘a’, because the object ist changed, the reference is still the same. Wolfgang Nádasi-Donner ---
**Author:** ![ddancer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ecb155/32.png) [@ddancer](https://www.ruby-forum.com/u/ddancer)\ **Post date:** [January 27, 2007, 5:46pm UTC](https://www.ruby-forum.com/t/pass-by-reference/83534/18 "2007-01-27T17:46:10Z")
removed\_email\_address@domain.invalid schrieb: > Right – I take that as the starting point – but what I mean is that > the way the assignment/rebinding semantics work doesn’t necessarily > pertain to the reference/value question. I agree. reference/value came from the world before talking about objects. I remember in Algol60 somehow the wording “after ‘integer i;’ the variable 'i IS an integer”. We would never say that after writing “s = ‘A Ruby String object’” the variable ‘s’ IS a String object - it gives a name for a special String object, or references a special String object (see 1). Wolfgang Nádasi-Donner (1): This wording is is used from a German, who tries to find out the best English word for what in German will be called “benennt”, or “bezeichnet”. I don’t know what wording is really good in English. ---
**Author:** ![ddancer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ecb155/32.png) [@ddancer](https://www.ruby-forum.com/u/ddancer)\ **Post date:** [January 27, 2007, 12:40pm UTC](https://www.ruby-forum.com/t/pass-by-reference/83534/19 "2007-01-27T12:40:27Z")
> > > > > Code \>\>\>\>\> def modifying(a\_param) a\_param = “hi” end def modifying2(a\_param=a\_value) a\_param = “hi” end def modifying3(a\_param) a\_param[0…-1]= “hi” end a\_value = “low” modifying a\_value case a\_value when “hi” puts “Passed by reference” when “low” puts “Passed by value” else puts “Passed away” end a\_value = “low” modifying2 a\_value case a\_value when “hi” puts “Passed by reference” when “low” puts “Passed by value” else puts “Passed away” end a\_value = “low” modifying3 a\_value case a\_value when “hi” puts “Passed by reference” when “low” puts “Passed by value” else puts “Passed away” end > > > > > Output \>\>\>\>\> Passed by value Passed by value Passed by reference > > > > > EoE \>\>\>\>\> I would name it somehow “call by assignment of value to local variable”, because the value of the actual parameter is assigned to the local variable used as a formal parameter. except for “Fixnum” etc. this is a pointer to an object. If one changes the object itself, it will work like “by reference”, otherwise only the local variable is affected, without having any effect to the outside world. Wolfgang Nádasi-Donner ---
**Author:** ![ddancer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/ecb155/32.png) [@ddancer](https://www.ruby-forum.com/u/ddancer)\ **Post date:** [January 27, 2007, 1:38pm UTC](https://www.ruby-forum.com/t/pass-by-reference/83534/20 "2007-01-27T13:38:35Z")
Hi – On Sat, 27 Jan 2007, Wolfgang Nádasi-Donner wrote: > any problems. If you change “someobject” this will have consequences for ‘a’, > because the object ist changed, the reference is still the same. Right – I take that as the starting point – but what I mean is that the way the assignment/rebinding semantics work doesn’t necessarily pertain to the reference/value question. David [Next page](https://www.ruby-forum.com/t/pass-by-reference/83534.md?page=2)