{"id":7042,"date":"2014-05-17T00:24:16","date_gmt":"2014-05-17T00:24:16","guid":{"rendered":"https:\/\/unknownerror.org\/index.php\/2014\/05\/17\/confusion-with-super-collection-of-common-programming-errors\/"},"modified":"2014-05-17T00:24:16","modified_gmt":"2014-05-17T00:24:16","slug":"confusion-with-super-collection-of-common-programming-errors","status":"publish","type":"post","link":"https:\/\/unknownerror.org\/index.php\/2014\/05\/17\/confusion-with-super-collection-of-common-programming-errors\/","title":{"rendered":"Confusion with super-Collection of common programming errors"},"content":{"rendered":"

tl;dr:<\/strong> super<\/code> behaves in unexpected ways, and variables matter, not just objects.<\/p>\n

When super<\/code> is called, it’s not called with the object that was passed in.<\/p>\n

It’s called with the variable that is called options<\/code> at the time of the call. For example, with the following code:<\/p>\n

class Parent\n  def to_xml(options)\n    puts \"#{self.class.inspect} Options: #{options.inspect}\"\n  end\nend\n\nclass OriginalChild < Parent\n  def to_xml(options)\n    options.merge!(:methods  => [ :murm_case_name, :murm_type_name ])\n    super\n  end\nend\n\nclass SecondChild < Parent\n  def to_xml(options)\n    options = 42\n    super\n  end\nend\n\nbegin\n  parent_options, original_child_options, second_child_options = [{}, {}, {}]\n  Parent.new.to_xml(parent_options)\n  puts \"Parent options after method called: #{parent_options.inspect}\"\n  puts\n  OriginalChild.new.to_xml(original_child_options)\n  puts \"Original child options after method called: #{original_child_options.inspect}\"\n  puts\n  second_child_options = {}\n  SecondChild.new.to_xml(second_child_options)\n  puts \"Second child options after method called: #{second_child_options.inspect}\"\n  puts\nend\n<\/code><\/pre>\n
Which produces the output<\/p>\n
Parent Options: {}\nParent options after method called: {}\n\nOriginalChild Options: {:methods=>[:murm_case_name, :murm_type_name]}\nOriginal child options after method called: {:methods=>[:murm_case_name, :murm_type_name]}\n\nSecondChild Options: 42\nSecond child options after method called: {}\n<\/code><\/pre>\n
You can see that with SecondChild<\/code> the super method is called with the variable options<\/code> which refers to a Fixnum<\/code> of value 42<\/code>, not with the object that was originally referred to by options<\/code>.<\/p>\n
With using options.merge!<\/code>, you’d modify the hash object that was passed to you, which means that the object referred to by the variable original_child_options<\/code> is now modified, as can be seen in the Original child options after method called: {:methods=>[:murm_case_name, :murm_type_name]}<\/code> line.<\/p>\n
(Note: I changed options<\/code> to 42 in SecondChild, rather than call Hash#merge<\/code>, because I wanted to show it wasn’t merely a case of side effects on an object)<\/p>\n","protected":false},"excerpt":{"rendered":"
tl;dr: super behaves in unexpected ways, and variables matter, not just objects. When super is called, it’s not called with the object that was passed in. It’s called with the variable that is called options at the time of the call. For example, with the following code: class Parent def to_xml(options) puts “#{self.class.inspect} Options: #{options.inspect}” […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-7042","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/posts\/7042","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/comments?post=7042"}],"version-history":[{"count":0,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/posts\/7042\/revisions"}],"wp:attachment":[{"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/media?parent=7042"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/categories?post=7042"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/tags?post=7042"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}