Multiple threads to data to array C++ - c++

Question

I'm using for loop to create given number of threads, each one of them makes approximation of part of my integral, I want them to give that data back to array so later I can sum it up (if I think right, I can't just make sum += in each thread because they will collide), everything worked right, to the moment when I want to take that data from each thread, I get error:
calka.cpp:49:33: error: request for member 'get_future' in 'X', which is of non-class type 'std::promise<float>[(N + -1)]'
code:
#include <iostream> //cout
#include <thread> //thread
#include <future> //future , promise
#include <stdlib.h> //atof
#include <string> //string
#include <sstream> //stringstream
using namespace std;
// funkcja 4x^3 + (x^2)/3 - x + 3
// całka x^4 + (x^3)/9 - (x^2)/2 + 3x
void thd(float begin, float width, promise<float> & giveback)
{
float x = begin + 1/2 * width;
float height = x*x*x*x + (x*x*x)/9 - (x*x)/2 + 3*x ;
float outcome = height * width;
giveback.set_value(outcome);
stringstream ss;
ss << this_thread::get_id();
string output = "thread #id: " + ss.str() + " outcome" + to_string(outcome);
cout << output << endl;
}
int main(int argc, char* argv[])
{
int sum = 0;
float begin = atof(argv[1]);
float size = atof(argv[2]);
int N = atoi(argv[3]);
float end = begin + N*size;
promise<float> X[N-1];
thread t[N];
for(int i=0; i<N; i++){
t[i] = thread(&thd, begin, size, ref(X[i]));
begin += size;
}
future<float> wynik_ftr = X.get_future();
float wyniki[N-1];
for(int i=0; i<N; i++){
t[i].join();
wyniki[i] = wynik_ftr.get();
}
//place for loop adding outcome from threads to sum
cout << N;
return 0;
}

Answer 1

Don't use VLA - promise<float> X[N-1]. It is an extension of some compilers, so your code is not portable. Use std::vector instead.
It seems you want to split calculation of integral to N threads. You create N-1 background threads and one invocation of thd is executed from main thread. In main you join all results, so
you don't need to create wyniki as array to store a result per thread,
because you are gathering these results in serially manner - inside for loop in main function.
Therefore one float wyniki variable is sufficient.
Steps you have to do are:
prepare N promises
starts N-1 threads
call thd from main
join and add results from N-1 threads in for loop
join and add main thread result
Code:
std::vector<promise<float>> partialResults(N);
std::vector<thread> t(N-1);
for (int i = 0; i<N-1; i++) {
t[i] = thread(&thd, begin, size, ref(partialResults[i]));
begin += size;
}
thd(begin,size,ref(partialResults[N-1]));
float wyniki = 0.0f;
for (int i = 0; i<N-1; i++) {
t[i].join();
std::future<float> res = partialResults[i].get_future();
wyniki += res.get();
}
std::future<float> res = partialResults[N-1].get_future(); // get res from main
wyniki += res.get();
cout << wyniki << endl;