C++11: std::call_once和std::one_flag

#include <iostream>
#include <thread>
#include <mutex>
#include <vector>

std::vector<int> vec;
std::mutex mutex;
std::once_flag flag;

static int val = 0;

void initialized_vec(std::vector<int>& vec)
{
for (int i = 0; i < 10; ++i) {
vec.push_back(val++);
}
}

void print_vec()
{
std::call_once(flag, initialized_vec, vec);
std::lock_guard<std::mutex> auto_lock(mutex);
for (const int& number : vec) {
std::cout << number << " ";
}
std::cout << std::endl;
}

int main()
{
std::thread threads[5];
for (int i = 0; i < 5; ++i) {
threads[i] = std::thread(print_vec);
}

for (std::thread& ref_thread : threads) {
ref_thread.join();
}

return 0;
}

#include <iostream>
#include <thread>
#include <mutex>

#include "Singleton.h"

void function(int& number)
{
number = 20;
}

int main()
{
int number{ 0 };
std::once_flag onceFlag;

std::call_once(onceFlag, function, number);
std::cout << number << std::endl;  //20! ref number

return 0;
}