C++多线程获取返回值方法详解

#include<iostream>
#include<thread>
#include<mutex>
#include<atomic>
using namespace std;

void func2(int x, int y,int* ans) {
  *ans= x + y;
}

int main()
{
  //计算（a+b）/(x+y)
  //用三个线程，一个线程计算a+b，另一个线程计算x+y
  int a, b, x, y;
  a = 10, b = 8, x = 2, y = 4;

  int* sum1 = new int(0);
  int* sum2 = new int(0);
  thread t1(func2, a, b, sum1);
  t1.join();
  thread t2(func2, x, y, sum2);
  t2.join();
  
  cout << (*sum1) / (*sum2) << endl;

  delete sum1;
  delete sum2;
  
  system("pause");
  return 0;
}

#include<iostream>
#include<thread>
#include<mutex>
#include<atomic>
#include<future>  //std::future std::promise
#include<utility>  //std::ref模板传参的时候使用

void func2(int x, int y,std::promise<int> &promiseObj) {
  promiseObj.set_value(x+y);
}

int main()
{
  //计算（a+b）/(x+y)
  //用三个线程，一个线程计算a+b，另一个线程计算x+y
  int a, b, x, y;
  a = 10, b = 8, x = 2, y = 4;

  int sum1, sum2;
  //声明一个类
  std::promise<int> promiseObj;
  //将future和promise关联
  std::future<int> futureObj = promiseObj.get_future();
  //模板传参的时候使用ref，否则传参失败
  std::thread t1(func2, a, b, ref(promiseObj));
  t1.join();
  //获取值
  sum1 = futureObj.get();
  std::cout << "sum1=" << sum1 << std::endl;

  //不能直接复用上面的future和promise
  std::promise<int> promiseObj2;
  std::future<int> futureObj2 = promiseObj2.get_future();

  std::thread t2(func2, x, y, ref(promiseObj2));
  t2.join();
  sum2 = futureObj2.get();
  std::cout << "sum2=" << sum2 << std::endl;

  std::cout << "sum1/sum2=" << sum1 / sum2 << std::endl;
  
  std::system("pause");
  return 0;
}