the visual studio gives E0028 [closed] - c++

Question

Closed. This question needs debugging details. It is not currently accepting answers.
Edit the question to include desired behavior, a specific problem or error, and the shortest code necessary to reproduce the problem. This will help others answer the question.
Closed 2 years ago.
Improve this question
I have a problem , the visual studio gives me error E0028 (the expression must have a constant value) in 40 line, the code:
#include <string>
#include <thread>
#include <iostream>
#include <ctime>
#include <cstdio>
using namespace std;
void start(int i, int number_of_test, int numtodo, int numwas)
{
for (int i = numwas; i < numtodo; i++)
for (int j = 2; j < i; j++)
{
if (i % j == 0)
break;
else if (i == j + 1)
cout << i << " ";
}
cout << endl;
}
int main() {
int i = 1;
int threads;
cout << "threads";
cin >> threads;
int number;
cout << "number";
cin >> number;
int number_of_test = 0;
int numtodo = 0;
int numwas = 0;
while (i < threads + 1)
{
if (i < threads + 1) {
i = i + 1;
number_of_test = number / threads;
numtodo = number_of_test * i;
numwas = numtodo / number_of_test;
thread t[i](start, int(i), number_of_test, numtodo, numwas);
t[i].join();
}
}
}
I want to do threads in loop, for example not to write 128 threads, just to run the loop and it run a lot of threads. line 40 :thread t[i](start, int(i), number_of_test, numtodo, numwas); , error in t[I], I need to fix it , but I don't know how , I need to do a lot of threads (like t1, than t2 etc..)

Answer 1

This line does not make sense (and may be the cause for the problem):
thread t[i](start, int(i), number_of_test, numtodo, numwas);
You should create an array or vector to store the thread objects. Example:
#include <thread>
#include <iostream>
#include <vector>
//void start(...
int main() {
std::cout << "threads: ";
size_t threads;
std::cin >> threads;
std::vector<std::thread> t(threads); // create a vector to hold the thread objects
int number;
std::cout << "number: ";
std::cin >> number;
if (number < t.size()) return 1;
int number_of_test = 0;
int numtodo = 0;
int numwas = 0;
// start all threads
for (size_t i = 0; i < t.size(); ++i)
{
number_of_test = number / t.size();
numtodo = number_of_test * (i+1);
numwas = numtodo / number_of_test;
// start one thread and store it in its place in the vector:
t[i] = std::thread(start, i, number_of_test, numtodo, numwas);
}
// join all threads
for (auto& th : t) th.join();
}