Ruby Keyword: defined?
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defined? nil ⇒ "nil" defined? self ⇒ "self" defined? true ⇒ "true" defined? false ⇒ "false" defined? "asdf" ⇒ "expression" defined? a = 3 ⇒ "assignment" # but doesn't actually set a to 3. defined? printf ⇒ "method" printf = 3 defined? printf ⇒ "local-variable" # nil if there is no local or method with that name defined? $foo ⇒ "global-variable" # nil if $foo has not been assigned to defined? Object ⇒ "constant" # nil if there is no constant with that name defined? super ⇒ "super" # nil if there is no super method. defined? yield ⇒ "yield" # nil if no block was passed to this method
defined? is most often used to determine whether a constant or global variable is defined. For example ruby-1.8.7 didn't have a RUBY_ENGINE constant, so jruby was often detected using:
defined?(RUBY_ENGINE) && RUBY_ENGINE == "jruby"
Another idiomatic use is to safely call a super-method:
super if defined?(super)
It can be used to detect whether a block was passed to this method, though
block_given? is usually preferred.
yield if defined?(yield)
Although the return value of
defined? is a string, this is usually ignored and only the truthiness of the result considered.
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