Passing array as argument to std::thread

Question

I'm having difficulty passing an array of integers to a function using std::thread. It seems like thread doesn't like the array portion of it. What other way is there to pass an array to a threaded function?

#include <thread>
#include <ostream>
using namespace std;

void process(int start_holder[], int size){
  for (int t = 0; t < size; t++){
   cout << start_holder[t] << "\n";
  }
}
int main (int argc, char *argv[]){
  int size = 5;
  int holder_list[size] = { 16, 2, 77, 40, 12071};
  std::thread run_thread(process,holder_list,size); 
  //std::ref(list) doesnt work either
  //nor does converting the list to std::string then passing by std::ref
  run_thread.join();
}

If you're going to use standard library names for your variables, don't say using namespace std;. In fact, don't say it anyway. — juanchopanza
– juanchopanza, Commented Dec 8, 2015 at 21:25
Is this actually a problem with std::thread, or is it int list[size] = { 16, 2, 77, 40, 12071};, which doesn't work with Clang despite the VLA extension? — Qaz
– Qaz, Commented Dec 8, 2015 at 21:27
i think it's a problem with std::thread. should i use pthread instead and put the array in a struct? — Martin James
– Martin James, Commented Dec 8, 2015 at 21:28

CJCombrink · Accepted Answer · 2015-12-09 05:37:48Z

2

Since you are using C++ start using std::vector or std::list instead of the c-style arrays. There are many other container types as well. If you want a fixed size array use std::array instead (since C++11).

These containers have functions to get the size so you do not need to send it through as a separate argument.

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

void process(std::vector<int> start_holder){
    for(int t = 0; t < start_holder.size(); t++){
       std::cout << start_holder[t] << "\n";
    }
    // Or the range based for
    for(int t: start_holder) {
       std::cout << t << "\n";
    }
}

int main (int argc, char *argv[]){
    std::vector<int> holder_list{ 16, 2, 77, 40, 12071};
    std::thread run_thread(process, holder_list); 
    run_thread.join();
}

answered Dec 9, 2015 at 5:37

CJCombrink

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Comments

Krizz · Accepted Answer · 2015-12-08 21:47:54Z

1

Make size constant:

#include <thread>
#include <iostream>

void process(int* start_holder, int size){
  for (int t = 0; t < size; t++){
   std::cout << start_holder[t] << "\n";
  }
}

int main (int argc, char *argv[]){
  static const int size = 5;
  int holder_list[size] = { 16, 2, 77, 40, 12071};
  std::thread run_thread(process, holder_list, size); 
  run_thread.join();
}

If size is variable, int arr[size] is not a standard C++. It is a variable array extension to the language as your compiler says in the error and is not compatible with int* aka int [].

edited Dec 8, 2015 at 21:47

answered Dec 8, 2015 at 21:42

Krizz

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