C++11多线程中std::call_once的使用

C++11中的std::call_once函数位于<mutex>头文件中。
在多线程编程中，有时某个任务只需要执行一次，此时可以用C++11中的std::call_once函数配合std::once_flag来实现。如果多个线程需要同时调用某个函数，std::call_once可以保证多个线程对该函数只调用一次。也可用在解决线程安全的单例模式。
template<class Callable, class... Args >
void call_once(std::once_flag& flag, Callable&& f, Args&&... args ); std::call_once: Executes the Callable object f exactly once, even if called from several threads. Each group of call_once invocations that receives the same std::once_flag object will meet the following requirements:
(1). Exactly one execution of exactly one of the functions (passed as f to the invocations in the group) is performed. It is undefined which function will be selected for execution. The selected function runs in the same thread as the call_once invocation it was passed to.
(2). No invocation in the group returns before the above-mentioned execution of the selected function is completed successfully, that is, doesn't exit via an exception.
(3). If the selected function exits via exception, it is propagated to the caller. Another function is then selected and executed.
下面是从其他文章中copy的测试代码，详细内容介绍可以参考对应的reference：
#include "call_once.hpp"
#include <iostream>
#include <thread>
#include <mutex>
#include <chrono>
#include <future>

/*
template< class Callable, class... Args >
void call_once( std::once_flag& flag, Callable&& f, Args&&... args );

Calls fn passing args as arguments, unless another thread has already executed
(or is currently executing) a call to call_once with the same flag.

If another thread is already actively executing a call to call_once with the same flag,
it causes a passive execution: Passive executions do not call fn but do not return until
the active execution itself has returned, and all visible side effects are synchronized at
that point among all concurrent calls to this function with the same flag.

If an active call to call_once ends by throwing an exception (which is propagated
to its calling thread) and passive executions exist, one is selected among these
passive executions, and called to be the new active call instead.

Note that once an active execution has returned, all current passive executions
and future calls to call_once (with the same flag) also return without becoming active executions.

The active execution uses decay copies of the lvalue or rvalue references of fn and args,
ignoring the value returned by fn.
*/

namespace call_once_ {
/////////////////////////////////////////////////////////
// reference: http://en.cppreference.com/w/cpp/thread/call_once namespace {
std::once_flag flag1, flag2;

void simple_do_once()
{
std::call_once(flag1, [](){ std::cout << "Simple example: called once\n"; });
}

void may_throw_function(bool do_throw)
{
if (do_throw) {
std::cout << "throw: call_once will retry\n"; // this may appear more than once
throw std::exception();
}
std::cout << "Didn't throw, call_once will not attempt again\n"; // guaranteed once
}

void do_once(bool do_throw)
{
try {
std::call_once(flag2, may_throw_function, do_throw);
}
catch (...) {
}
}
}

int test_call_once_1()
{
std::thread st1(simple_do_once);
std::thread st2(simple_do_once);
std::thread st3(simple_do_once);
std::thread st4(simple_do_once);
st1.join();
st2.join();
st3.join();
st4.join();

/*std::thread t1(do_once, true);
std::thread t2(do_once, true);
std::thread t3(do_once, false);
std::thread t4(do_once, true);
t1.join();
t2.join();
t3.join();
t4.join();*/

return 0;
}

///////////////////////////////////////////////////////////////
// reference: http://www.cplusplus.com/reference/mutex/call_once/ namespace {
int winner;
void set_winner(int x) { winner = x; }
std::once_flag winner_flag;

void wait_1000ms(int id) {
// count to 1000, waiting 1ms between increments:
for (int i = 0; i<1000; ++i)
std::this_thread::sleep_for(std::chrono::milliseconds(1));
// claim to be the winner (only the first such call is executed):
std::call_once(winner_flag, set_winner, id);
}
}

int test_call_once_2()
{
std::thread threads[10];
// spawn 10 threads:
for (int i = 0; i<10; ++i)
threads[i] = std::thread(wait_1000ms, i + 1);

std::cout << "waiting for the first among 10 threads to count 1000 ms...\n";

for (auto& th : threads) th.join();
std::cout << "winner thread: " << winner << '\n';

return 0;
}

////////////////////////////////////////////////////////////////////
// reference: http://www.modernescpp.com/index.php/thread-safe-initialization-of-a-singleton namespace {
/*constexpr*/const auto tenMill = 10000;

class MySingleton{
public:
static MySingleton& getInstance(){
std::call_once(initInstanceFlag, &MySingleton::initSingleton);
// volatile int dummy{};
return *instance;
}
private:
MySingleton() = default;
~MySingleton() = default;
MySingleton(const MySingleton&) = delete;
MySingleton& operator=(const MySingleton&) = delete;

static MySingleton* instance;
static std::once_flag initInstanceFlag;

static void initSingleton(){
instance = new MySingleton;
}
};

MySingleton* MySingleton::instance = nullptr;
std::once_flag MySingleton::initInstanceFlag;

std::chrono::duration<double> getTime(){

auto begin = std::chrono::system_clock::now();
for (size_t i = 0; i <= tenMill; ++i){
MySingleton::getInstance();
}
return std::chrono::system_clock::now() - begin;

};
}

int test_call_once_3()
{
auto fut1 = std::async(std::launch::async, getTime);
auto fut2 = std::async(std::launch::async, getTime);
auto fut3 = std::async(std::launch::async, getTime);
auto fut4 = std::async(std::launch::async, getTime);

auto total = fut1.get() + fut2.get() + fut3.get() + fut4.get();

std::cout << total.count() << std::endl;

return 0;
}

} // namespace call_once_
GitHub： https://github.com/fengbingchun/Messy_Test