How to return value from std::thread - c++

Question

I have function that returns me a value. I want to use threads for doSth function and set returning value for variables above, here is an example:
#include <string>
#include <iostream>
#include <thread>
using namespace std;
// func for execution
int doSth(int number)
{
return number;
}
int main()
{
// some code ...
int numberOne; // no value now, but in thread I want to set a value from it
int numberTwo; // depending on function input value
thread t1(doSth, 1); // set numberOne = 1;
thread t2(doSth, 2); // set numberTwo = 2;
// wait them to execute
t1.join();
t2.join();
// now I should have numberOne = 1; numberTwo = 2
// some code ...
return 0;
}
How could I do it?

Answer 1

How to return value from std::thread
Besides std::async shown in other answers, you can use std::packaged_task:
std::packaged_task<int(int)> task{doSth};
std::future<int> result = task.get_future();
task(1);
int numberOne = result.get();
This allows separating creation of the task, and executing it in case that is needed.

Answer 2

Method 1: Using std::async (higher-level wrapper for threads and futures):
#include <thread>
#include <future>
#include <iostream>
int func() { return 1; }
int main(){
std::future<int> ret = std::async(&func);
int i = ret.get();
std::cout<<"I: "<<i<<std::endl;
return 0;
}
Method 2: Using threads and futures:
#include <thread>
#include <future>
#include <iostream>
void func(std::promise<int> && p) {
p.set_value(1);
}
int main(){
std::promise<int> p;
auto f = p.get_future();
std::thread t(&func, std::move(p));
t.join();
int i = f.get();
std::cout<<"I: "<<i<<std::endl;
return 0;
}

Answer 3

My prefered method is encapsulate the call in a specific method returning nothing (and managing error in the same way).
void try_doSth(int number, int* return_value, int* status)
{
try
{
*return_value = doSth(number);
*status = 0;
}
catch(const std::exception& e) { *status = 1; }
catch(...) { *status = 2; }
}
int r1,r2,s1,s2;
std::thread t1(try_doSth, 1, &r1, &s1);
std::thread t2(try_doSth, 2, &r2, &s2);