Documentation for Hash#merge and shallow copies. #228
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YorickPeterse
commented
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the Whoops, must have missed this one, do you want to commit this one Mr. Daloze? OK, I'll take care of it.
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eregon
commented
Hash#merge does not copy values, it just copies references, but that is transparent to the user.
So the same values are in the merged Hash:
> h = { a: 1 }
> v = [1, 2, 3]
> v.object_id
=> 2152765440
> h.merge({ key: v })[:key].object_id
=> 2152765440
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eregon
commented
@YorickPeterse So, thanks for the request, but unfortunately it is misleading as is. But the doc of Hash#merge could likely be improved, do not hesitate to make another request!
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- 14 0 hash.c
| @@ -1893,6 +1893,20 @@ rb_hash_update_block_i(VALUE key, VALUE value, VALUE hash) | ||
| * h2 = { "b" => 254, "c" => 300 } | ||
| * h1.merge!(h2) { |key, v1, v2| v1 } | ||
| * #=> {"a"=>100, "b"=>200, "c"=>300} | ||
| + * | ||
| + * Note that this method creates a shallow copy of the value in | ||
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| + * <i>other_hash</i>. This means that when for example | ||
| + * <code>Array#select!</code> is used on one of the values in | ||
| + * <i>other_hash</i> both the original object as well as the copy will | ||
| + * be modified. This is illustrated in the following example: | ||
| + * | ||
| + * original = { 'numbers' => [10, 20, 30] } | ||
| + * copy = {}.merge(original) | ||
| + * | ||
| + * copy['numbers'].select! { |number| number <= 20 } | ||
| + * | ||
| + * puts copy # => { 'numbers' => [10, 20] } | ||
| + * puts original # => { 'numbers' => [10, 20] } | ||
| */ | ||
| static VALUE | ||
The documentation of Hash#merge has been updated to clarify that this method
creates a shallow copy of the Hash specified in the method parameter.
Sidenote: if there is a particular set of standards I violated I'd love to hear so I can adjust the code to these standards.