Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

libs/thread/example/once.cpp

// Copyright (C) 2001-2003
// William E. Kempf
//
// Permission to use, copy, modify, distribute and sell this software
// and its documentation for any purpose is hereby granted without fee,
// provided that the above copyright notice appear in all copies and
// that both that copyright notice and this permission notice appear
// in supporting documentation.  William E. Kempf makes no representations
// about the suitability of this software for any purpose.
// It is provided "as is" without express or implied warranty.

#include <boost/thread/thread.hpp>
#include <boost/thread/once.hpp>
#include <cassert>

int value=0;
boost::once_flag once = BOOST_ONCE_INIT;

void init()
{
    ++value;
}

void thread_proc()
{
    boost::call_once(&init, once);
}

int main(int argc, char* argv[])
{
    boost::thread_group threads;
    for (int i=0; i<5; ++i)
        threads.create_thread(&thread_proc);
    threads.join_all();
    assert(value == 1);
}