Boost C++ Application Development Cookbook - Second Edition

How to do it...

1. We have to add variables for storing information that defaults were initialized and a variable for storing the defaults:

#include <boost/thread/once.hpp>struct postprocessor {    typedef std::vector<std::string> answer_t;private:    mutable boost::once_flag default_flag_;    mutable answer_t default_;

Variables are mutable because we are going to modify them inside const member functions.

2. Let's initialize our variables:

public:    postprocessor()        : default_flag_(BOOST_ONCE_INIT)        , default_()    {}

3. Finally, let's change the act function:

    // Concurrent calls on the same variable are safe.    answer_t act(const std::string& in) const {        answer_t ret;        if (in.empty()) {            // Extremely rare condition. boost::call_once(default_flag_, [this]() ...