Boost C++ Application Development Cookbook - Second Edition

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As you may see, base_from_member has an integer as a second template argument. This is done for cases when we need multiple base_from_member classes of the same type:

class fake_tasks_processor2     : boost::base_from_member<std::ostringstream, 0>     , boost::base_from_member<std::ostringstream, 1>     , public tasks_processor {     typedef boost::base_from_member<std::ostringstream, 0> logger0_t;    typedef boost::base_from_member<std::ostringstream, 1> logger1_t;     virtual void do_process() {         logger0_t::member << "0: Fake processor2 processed!";         logger1_t::member << "1: Fake processor2 processed!";     } public:     fake_tasks_processor2()         : logger0_t()         , logger1_t()         , tasks_processor(logger0_t::member)     {} };

The boost::base_from_member ...