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A lot of developers know the dangers of concatenation with Strings and objects. In Java we had the StringBuffer.append vs. + (and now StringBuilder) knowledge transfer.

Ruby has the same issue, and people have talked about it before.

We ran into this issue in one of our projects, and I remember Dave Thomas talk about a problem that was fixed by moving from string concatenation to putting the contents on an array.

I think this benchmark says it all:

require 'benchmark'
Benchmark.bm do |x|
        x.report do
                a = 'foo'
                100000.times { a += ' foo' }
        end
        x.report do
                a = 'foo'
                100000.times { a << ' foo' }
        end
end

Output

dion@stewart [~]$ ruby t.rb
      user     system      total        real
 13.790000  25.180000  38.970000 ( 40.102451)
  0.060000   0.000000   0.060000 (  0.064342)

So, favour << unless you really want to copy strings around.