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copied!<p>This is extremely easy to test:</p> <pre><code>class MyCache def initialize @hash = {} end def []=(key, value) puts "Cache key '#{key}' written" @hash[key] = value end def [](key) puts "Cache key '#{key}' read" @hash[key] end end </code></pre> <p>Now simply try the <code>||=</code> syntax:</p> <pre><code>cache = MyCache.new cache["my key"] ||= "my value" # cache value was nil (unset) # Cache key 'my key' read # Cache key 'my key' written cache["my key"] ||= "my value" # cache value is already set # Cache key 'my key' read </code></pre> <p>So we can conclude that no assignment takes place if the cache key already exists.</p> <p>The following extract from the Rubyspec <a href="http://github.com/rubyspec/rubyspec/blob/master/language/variables_spec.rb" rel="noreferrer">shows</a> that this is <strong>by design</strong> and should not be dependent on the Ruby implementation:</p> <pre><code>describe "Conditional operator assignment 'obj.meth op= expr'" do # ... it "may not assign at all, depending on the truthiness of lhs" do m = mock("object") m.should_receive(:foo).and_return(:truthy) m.should_not_receive(:foo=) m.foo ||= 42 m.should_receive(:bar).and_return(false) m.should_not_receive(:bar=) m.bar &&= 42 end # ... end </code></pre> <p>In the same file, there is a similar spec for <code>[]</code> and <code>[]=</code> that mandates identical behaviour.</p> <p>Although the Rubyspec is still a work in progress, it has become clear that the major Ruby implementation projects intend to comply with it.</p>

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