Let's take a look at the following example. We have some base class that has virtual functions and must be initialized with reference to the std::ostream object:

#include <boost/noncopyable.hpp> 
#include <sstream> 
 
class tasks_processor: boost::noncopyable { 
    std::ostream& log_; 
 
protected: 
    virtual void do_process() = 0; 
 
public: 
    explicit tasks_processor(std::ostream& log) 
        : log_(log) 
    {} 
 
    void process() { 
        log_ << "Starting data processing"; 
        do_process(); 
    } 
};

We also have a derived class that has a std::ostream object and implements the do_process() function:

class fake_tasks_processor: public tasks_processor { 
    std::ostringstream logger_; 
 
    virtual void do_process() { 
        logger_ << "Fake processor processed...