multithreading - Changing shared variables from multiple threads c++ -

i wondering if there way achieve changing shared/global variables multiple threads in c++

imagine code:

#include <vector> #include <thread>  void pushanother(int x);  std::vector<int> myarr;  void main() {     myarr.push_back(0);      std::thread t1(pushanother, 2);      t1.join(); }  void pushanother(int x) {     myarr.push_back(x); } 

in particular case, code (barring lack of join on thread), surprisingly ok.

this because constructor of std::thread causes memory fence operation , first thread not modify or read state of vector after fence.

in effect have transferred control of vector second thread.

however, modifying code represent more normal situation requires explicit locks:

#include <vector> #include <thread> #include <mutex>  void pushanother(int x);  // mutex handle contention of shared resource std::mutex m;  // shared resource std::vector<int> myarr;  auto push_it(int i) -> void {     // take lock...     auto lock = std::unique_lock<std::mutex>(m);      // modify/read resource     myarr.push_back(i);      // ~lock implicitly releases lock }  int main() {      std::thread t1(pushanother, 2);      push_it(0);      t1.join(); }  void pushanother(int x) {     push_it(x); }