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POMRI Internals: detailed explanation of rb_id2str
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copied!<p>In MRI, it appears that <code>rb_id2str()</code> is responsible for doing all of the work when you call <code>Symbol#to_s</code>. I was surprised to discover this is an extremely cryptic function for something that I assumed would be a fairly straight forward operation.</p> <p>I'm looking for a detailed explanation of what this function is doing. For reference, here is a link to the source in 1.9.3:</p> <p><a href="http://rxr.whitequark.org/mri/source/parse.y?v=1.9.3-p195#9950" rel="noreferrer">http://rxr.whitequark.org/mri/source/parse.y?v=1.9.3-p195#9950</a></p> <p>Some specific questions:</p> <p>What are the four major <code>if</code> blocks doing?</p> <ol> <li><code>if (id < tLAST_TOKEN)</code></li> <li><code>if (id < INT_MAX && rb_ispunct((int)id))</code></li> <li><code>if (st_lookup(global_symbols.id_str, id, &data))</code></li> <li><code>if (is_attrset_id(id))</code></li> </ol> <p>It would be great to get a generic overview of what each block of code inside the if statements does, but it doesn't need to be a line-by-line analysis.</p> <p>Finally, I'm curious about the memory/garbage collection implications of <code>to_s</code>: does calling <code>Symbol#to_s</code> create a new string that has to be garbage collected every time, or is there something like internal copy-on-write optimization that uses a reference to the interned representation of the symbol up until a mutation is made to the string?</p>

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