template <class Fn, class... Args>
void call_once (once_flag& flag, Fn&& fn, Args&&...args);
Need to include header file ：<mutex>
parameter ：
（1）flag： yes std::once_falg object （ Define an object and pass it in ）,, Label belonging to control , same falg Only once （ See details below ）
（2）fn： Function objects that need to be executed only once ,,
（3）args： Pass to fn The parameters of the function ,, Pass it on if you have one , No, no delivery .
The following is explained through the procedure flag The role of ：
#include<iostream> #include<mutex> #include<thread> using namespace std; void
init() { cout << "data has inited" << endl; } void fun() { once_flag init_flag;
// Wrong usage call_once(init_flag, init); } int main() { thread t1(fun); thread
t2(fun); t1.join(); t2.join(); system("pause"); return 0; } Operation results ：
You can see the output twice “data has
inited”, that is because init_flag Is defined in fun Local variables in , Per thread fun In function init_flag It's different , All will be output twice ,, If init_flag It's a global variable, etc , It's the same variable , It will only output once ., The same as above flag Only once . It should be understood .
Correct demonstration of the above program after modification ：
#include<iostream> #include<mutex> #include<thread> using namespace std;
once_flag init_flag; void init() { cout << "data has inited" << endl; } void
fun() { call_once(init_flag, init); } int main() { thread t1(fun); thread
t2(fun); t1.join(); t2.join(); system("pause"); return 0; } Operation results ：
This time it's only output once ,, The description was executed only once !
purpose ：
std::call_once It is mainly used for the scenario of multi-threaded resource initialization only once , The typical example is the perfect solution to thread safety single column mode .
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