Ruby 2.4 adds compare by identity functionality

In Ruby, Object#equal? method is used to compare two objects by their identity, that is, the two objects are exactly the same or not. Ruby also has Object#eql? method which returns true if two objects have the same value.

For example:

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2str1 = "Sample string"
3str2 = str1.dup
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5str1.eql?(str2) #=> true
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7str1.equal?(str2) #=> false
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We can see that object ids of the objects are not same.

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2str1.object_id #=> 70334175057920
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4str2.object_id #=> 70334195702480
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In ruby, Set does not allow duplicate items in its collection. To determine if two items are equal or not in a Set ruby uses Object#eql? and not Object#equal?.

So if we want to add two different objects with the same values in a set, that would not have been possible prior to Ruby 2.4 .

Ruby 2.3

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2require 'set'
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4set = Set.new #=> #<Set: {}>
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6str1 = "Sample string" #=> "Sample string"
7str2 = str1.dup #=> "Sample string"
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9set.add(str1) #=> #<Set: {"Sample string"}>
10set.add(str2) #=> #<Set: {"Sample string"}>
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But with the new Set#compare_by_identity method introduced in Ruby 2.4, sets can now compare its values using Object#equal? and check for the exact same objects.

Ruby 2.4

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2require 'set'
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4set = Set.new.compare_by_identity #=> #<Set: {}>
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6str1 = "Sample string" #=> "Sample string"
7str2 = str1.dup #=> "Sample string"
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9set.add(str1) #=> #<Set: {"Sample string"}>
10set.add(str2) #=> #<Set: {"Sample string", "Sample string"}>
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Set#compare_by_identity?

Ruby 2.4 also provides the compare_by_identity? method to know if the set will compare its elements by their identity.

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2require 'set'
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4set1= Set.new #=> #<Set: {}>
5set2= Set.new.compare_by_identity #=> #<Set: {}>
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7set1.compare_by_identity? #=> false
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9set2.compare_by_identity? #=> true
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