Simple example for std::call_once

call_once.cpp

#include <iostream>
#include <mutex>
#include <functional>

struct Object
{
    Object(int n) : n(n) {}

    void init() { std::call_once(initialized, [this] { std::cout << "init with n=" << n << std::endl; }); }

    void action_1() { init(); std::cout << "action 1" << std::endl; }
    void action_2() { init(); std::cout << "action 2" << std::endl; }
    void action_3() { init(); std::cout << "action 3" << std::endl; }

    int n{0};
    std::once_flag initialized;
};

int main(void)
{
    Object a(10);
    a.init();
    a.action_1();
    a.action_2();
    a.action_3();

    std::cout << "----------------" << std::endl;

    Object b(20);
    b.action_1();
    b.action_2();
    b.action_3();

    std::cout << "----------------" << std::endl;

    Object c(30);
    c.action_2();
    c.action_3();

    std::cout << "----------------" << std::endl;

    Object d(40);
    d.action_3();
}