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Why would str2=str create a new string? How are RStrings modified when they are referenced by other shared RStrings? Is there any way to create a shared RStri

Why would str2=str create a new string? How are RStrings modified when they are referenced by other shared RStrings? Is there any way to create a shared RString other than calling .dup?

pat_shaughnessy 15y ago
Great questions! To be honest I don't have enough knowledge of the MRI internals yet to be able to answer these 100% correctly. Maybe I'll write a follow up post or an update to this one explaining these issues when I have time for more research. But for today: 1. str2 = str doesn't actually create a new string. RString represents a string value, not a string object. So str2=str creates a new RString because that essentially defines what str2's value refers to… Think of RString as an internal pointer to the value that str or str2 is referring to. Sorry - not a good explanation :( 2. How are shared RStrings modified? Not sure yet, but I'm curious to find out. Will let you know on my site somehow. 3. Yes, simply calling str2 = str or in a variety of other ways will do it.
- ben0x539 15y ago
  I'm 99% sure you're wrong about 1). The internal pointer is the VALUE that is pointing to the struct RString. `str2 = str` will make another VALUE point to the same struct RString. Consider str = "foo" str2 = str str.object_id == str2.object_id #=> true I'm fairly sure that implies that str and str2 share everything including the RString object, so it couldn't really have one of str/str2 have a field that points to the other. I am not sure about 2) and 3) but I'd expect some copy-on-write mechanism based on a flag field in the RBasic struct, and taking substrings might be O(1) thanks to sharing too.
- charliesome 15y ago
  Here's a better explanation for #1: In Ruby, most classes (including String) are reference types. This is why `str2 = str` doesn't create a new string. It doesn't create a new RString either - str and str2 are both pointers to the same RString struct.
- flgr 15y ago
  AFAIK str2 = str will just share the RVALUE-Pointer. I think to actually see shared strings behavior, you would have to do str2 = str.clone or str2 = str[0..-1].

Why would str2=str create a new string? How are RStrings modified when they are referenced by other shared RStrings? Is there any way to create a shared RString other than calling .dup?

pat_shaughnessy 15y ago

Great questions! To be honest I don't have enough knowledge of the MRI internals yet to be able to answer these 100% correctly. Maybe I'll write a follow up post or an update to this one explaining these issues when I have time for more research. But for today: 1. str2 = str doesn't actually create a new string. RString represents a string value, not a string object. So str2=str creates a new RString because that essentially defines what str2's value refers to… Think of RString as an internal pointer to the value that str or str2 is referring to. Sorry - not a good explanation :( 2. How are shared RStrings modified? Not sure yet, but I'm curious to find out. Will let you know on my site somehow. 3. Yes, simply calling str2 = str or in a variety of other ways will do it.

ben0x539 15y ago
I'm 99% sure you're wrong about 1). The internal pointer is the VALUE that is pointing to the struct RString. `str2 = str` will make another VALUE point to the same struct RString. Consider str = "foo" str2 = str str.object_id == str2.object_id #=> true I'm fairly sure that implies that str and str2 share everything including the RString object, so it couldn't really have one of str/str2 have a field that points to the other. I am not sure about 2) and 3) but I'd expect some copy-on-write mechanism based on a flag field in the RBasic struct, and taking substrings might be O(1) thanks to sharing too.
charliesome 15y ago
Here's a better explanation for #1: In Ruby, most classes (including String) are reference types. This is why `str2 = str` doesn't create a new string. It doesn't create a new RString either - str and str2 are both pointers to the same RString struct.
flgr 15y ago
AFAIK str2 = str will just share the RVALUE-Pointer. I think to actually see shared strings behavior, you would have to do str2 = str.clone or str2 = str[0..-1].