how to pass multidimensional array in multithreading c++

Question

I was trying to create multithreading to handle 2 multi-dimensional arrays:

vector<thread> tt;  
for(int i=0;i<4;i++) tt.push_back(thread(th,arr1,arr2));

with the threaded function :

void th(int arr1[3][100][100], int arr2[100][100]) {
...
}

I also tried to pass by reference yet not get it work as well:

void th(int (&arr1)[3][100][100], int (&arr2)[100][100]) {
    ...
    }

Both of them gives me an "no type named 'type' in 'class std::result_of void(* (int **[])..." error. Can someone please show me how to correctly pass multidimensional arrays in multithreading?

Take out the multithreading and focus on getting your function declarations and calls right. — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Jan 2, 2015 at 17:50
tt.push_back(thread(th, std::ref(arr1), std::ref(arr2))); with std::ref and th taking references to arrays (so void th(int (&arr1)[3][100][100], int (&arr2)[100][100])) — Piotr Skotnicki
– Piotr Skotnicki, Commented Jan 2, 2015 at 18:03
ideone.com/RkR40P See how easy it is with std::array. Never use raw arrays — stefan
– stefan, Commented Jan 2, 2015 at 18:07
@PiotrS. Thanks Piotr. I tried ref indeed, however, got the same error. — ChuNan
– ChuNan, Commented Jan 2, 2015 at 18:07

user14717 · Accepted Answer · 2015-01-02 19:21:33Z

2

Your original function call smells strange to me, but still the following call compiles and runs just fine with g++-4.6.3 using the command

g++ -lpthread -std=c++0x -g multiArray.cc -o multiArray && ./multiArray

Then multiArray.cc has

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

void th(int ar1[3][100][100], int ar2[100][100])
{
    std::cout << "This works so well!\n";
}

int main()
{
    int ar1[3][100][100];
    int ar2[100][100];

    std::vector<std::thread> threads;
    for(int ii = 0; ii<4; ++ii)
    {
        threads.emplace_back(th, ar1,ar2);
    }


    for(auto & t : threads)
    {
        if(t.joinable())
        {
            t.join();
        }
    }
}

Let me clarify that this code is suspect; for instance, if you change the array sizes (without changing your array dimensions) this code compiles without problem.

edited Jan 2, 2015 at 19:21

answered Jan 2, 2015 at 19:15

user14717

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3 Comments

ChuNan Over a year ago

Thanks. Can we also pass by reference?

ChuNan Over a year ago

I assume it's faster if the array values are changed?

Piotr Skotnicki Over a year ago

@NickThompson a pointer to array drops its first dimension, a reference doesn't

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