Ruby gsub doesn't change the content while setting the content does

Question

Could someone please explain the difference between the following two lines of code:

1. element.content.gsub!("#{i}", "#{a[i]}")
2. element.content = element.content.gsub("#{i}", "#{a[i]}")

In the following code:

a.each_index do |i|
  @doc.traverse do |element|
    if element.text?
      element.content = element.content.gsub("#{i}", "#{a[i]}")
    end
  end
end
puts @doc

The code as presented above does change @doc. While if I use line 1 with gsub! it doesn't have an effect on @doc. Does this have to do with how blocks handle their parameters? Shouldn't everything be passed by reference in Ruby unless explicitly copied using a method?

Answer 1

Checking http://nokogiri.org/Nokogiri/XML/Node.html:

static VALUE get_content(VALUE self) {
    xmlNodePtr node;
    xmlChar * content;
    Data_Get_Struct(self, xmlNode, node);
    content = xmlNodeGetContent(node);
    if(content) {
        VALUE rval = NOKOGIRI_STR_NEW2(content);
        xmlFree(content);
        return rval;
    }
    return Qnil;
}

A copy of the content is made, so any changes to it only affect that copy, and not the internal value of the node's contents.

Using 'element.content=' calls a separate method that does modify the internal value:

def content= string
    self.native_content = encode_special_chars(string.to_s)
end