# String copy-on-write question

**URL:** <https://www.ruby-forum.com/t/string-copy-on-write-question/136699>\
**Category:** Ruby\
**Created:** [May 5, 2008, 5:40pm UTC](https://www.ruby-forum.com/t/string-copy-on-write-question/136699 "2008-05-05T17:40:25Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![larsch](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/ad7895/32.png) [@larsch](https://www.ruby-forum.com/u/larsch)\
**Post date:** [May 5, 2008, 5:40pm UTC](https://www.ruby-forum.com/t/string-copy-on-write-question/136699/1 "2008-05-05T17:40:25Z")

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Hello group,

Ruby implements copy-on-write for strings, so you can do stuff like  
this very cheaply:

str = 0.chr \* (2\*\*24) # 16MiB allocated  
str[100…-1] # this costs only a small amount of memory

How come this optimization does not apply in this case?:

str[100…-2] # this costs around 16MiB bytes of memory

As a side effect, if using regexps on a large string, the pre-match  
and post-match variables behave differently:

s = 0.chr \* (2**23) + “Hello” + 0.chr \* (2**23) # About 16MiB  
allocated (after GC)  
s.scan(/Hello/) { |m| p m } # This is free  
p $’.size # This is free  
p $`.size # This costs another 8MiB.

Any insights?

Lars

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**Author:** ![larsch](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/ad7895/32.png) [@larsch](https://www.ruby-forum.com/u/larsch)\
**Post date:** [May 5, 2008, 6:21pm UTC](https://www.ruby-forum.com/t/string-copy-on-write-question/136699/2 "2008-05-05T18:21:16Z")

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On 05.05.2008 18:07, ts wrote:

> > p $`.size # This costs another 8MiB.
> 
> same reason here.

Interesting. Do you also happen to know why not an additional field is  
used that stores the length? Is the reason maybe usage of C library  
string functions that work on zero terminated strings?

Cheers

robert

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**Author:** ![larsch](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/ad7895/32.png) [@larsch](https://www.ruby-forum.com/u/larsch)\
**Post date:** [May 5, 2008, 6:08pm UTC](https://www.ruby-forum.com/t/string-copy-on-write-question/136699/3 "2008-05-05T18:08:08Z")

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Lars C. wrote:

Well, it’s best if you look at rb\_str\_substr() in string.c

> str[100…-1] # this costs only a small amount of memory

ruby just need to adjust the pointer and the length in the new  
object

> str[100…-2] # this costs around 16MiB bytes of memory

one character is missing from the previous string, if it do the  
same thing than previously then it must

- adjust the pointer
- adjust the length
- add \0 at the end

This mean that fatally it has modified the string, this is why it  
duplicate.

> p $’.size # This is free  
> p $`.size # This costs another 8MiB.

same reason here.

Guy Decoux

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**Author:** ![larsch](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/ad7895/32.png) [@larsch](https://www.ruby-forum.com/u/larsch)\
**Post date:** [May 5, 2008, 6:50pm UTC](https://www.ruby-forum.com/t/string-copy-on-write-question/136699/4 "2008-05-05T18:50:43Z")

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On 05.05.2008 18:33, ts wrote:

> Robert K. wrote:
> 
> > Interesting. Do you also happen to know why not an additional field is  
> > used that stores the length?
> 
> I’ve not understood : it has a field which give it the length of  
> the string, for example with

Ah, ok. This happens when one is too lazy to look into the source. 🙂  
Somehow I had assumed that the length was not stored because you made  
the point that the \0 could not be inserted without altering the  
original. I concluded, there is no length. 🙂

> str = ‘0’ \* 200  
> str[100 … -1]
> 
> the first object (in str) will have 200 for its length  
> the field length in the new object will have the value 100
> 
> > ```
> > Is the reason maybe usage of C library 
> > 
> > ```
> > 
> > string functions that work on zero terminated strings?
> 
> only matz know this 🙂

Well, maybe he’ll stop by and enlighten us.

Kind regards

robert

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<div class="post-metadata">

**Author:** ![larsch](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/ad7895/32.png) [@larsch](https://www.ruby-forum.com/u/larsch)\
**Post date:** [May 5, 2008, 6:35pm UTC](https://www.ruby-forum.com/t/string-copy-on-write-question/136699/5 "2008-05-05T18:35:09Z")

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Robert K. wrote:

> Interesting. Do you also happen to know why not an additional field is  
> used that stores the length?

I’ve not understood : it has a field which give it the length of  
the string, for example with

str = ‘0’ \* 200  
str[100 … -1]

the first object (in str) will have 200 for its length  
the field length in the new object will have the value 100

> ```
> Is the reason maybe usage of C library 
> 
> ```
> 
> string functions that work on zero terminated strings?

only matz know this 🙂

Guy Decoux