从线程输出简单的向量

Question

I am currently attempting to understand multi-threading in C++ and want to return separate vectors from multiple threads so that a number of for loops can be ran simultaneously, I currently have the following code, and am struggling with the documentation,

#include <thread>
#include <iostream>
#include <vector>
#include <future>

using namespace std;


vector<int> task1(vector<int> v) { 
    for(int i = 0; i<10; i++)
    {
        v[i] = i;
    }
}

vector<int> task2(vector<int> v) { 
    for(int i = 10; i<20; i++)
    {
        v[i] = i;
    }

}

int main () 
{
    packaged_task<vector<int>()> taskA{task1}; 
    packaged_task<vector<int>()> taskB{task2};

    future<vector<int>()> future = taskA.get_future();
    future<vector<int>()> future2 = taskB.get_future();

    thread t1{move(taskA)};
    thread t2{move(taskB)};

    t1.join();
    t2.join();

return 0;
}

The aim of this code is to just get two vectors - one with 0-9 and the other 10-19. If anyone has any pointers or simpler ways to perform this, they would be much appreciated.

Thanks for your time.

Answer 1

Async is probably the simplst. Your function needs to return a vector, not take one as an argument. I coded it as a lambda, but a regular function is just as good. Ask if you have questions.

#include <thread>
#include <future>
#include <vector>
int main() {
    using namespace std;
    using intvec = vector<int>;
    auto f= [](int start) {
        intvec s;
        for (int i = 0; i < 10; ++i)
            s.push_back(start + i);
        return s;
    };

    auto f1 = async(f, 0);
    auto f2 = async(f, 10);

    intvec s1 = f1.get();
    intvec s2 = f2.get();
}

... or if you prefer...

#include <thread>
#include <future>
#include <vector>
using namespace std;

vector<int> task1() { 
    vector<int> v;
    for(int i = 0; i<10; i++)
        { v.push_back(i);}
    return v;
}

vector<int> task2() { 
    vector<int> v;
    for(int i = 10; i<20; i++)
        { v.push_back(i);}
    return v;
}
int main() {

    auto f1 = async(task1);
    auto f2 = async(task2);

    vector<int> s1 = f1.get();
    vector<int> s2 = f2.get();
}