411 REFERENCE · Wikibooks
C++ Language/Std/Stl/Iterators/StreamIterators
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Like insert-iterators and reverse-iterators, using a "stream-iterator" changes the effect of STL's existing algorithms.
In this case, the iterator's = operator gets re-defined to have the effect of std::cout << source << " ";.
Additional information about stream-iterators (includes interactive examples)