Creating thread does not work due to arguments being references - c++

Question

I'm currently trying to make my first steps in writing multi-threaded code in CPP and I'm getting a error message that I don't understand at all....
Consider the two functions:
void add_vector(std::vector<int> & vec, int & store){
store = std::accumulate(vec.begin(), vec.end(), 0);
}
int parallel_matrix_sum(std::vector<std::vector<int>> const & mat){
std::vector<std::thread> threads;
std::vector<int> tmp(l);
for (int k =0; k<l; k++){
threads.push_back(std::thread(add_vector, std::cref(mat[k]), std::ref(tmp[k])));
}
for(auto && thread: threads){
thread.join();
}
return std::accumulate(tmp.begin(), tmp.end(), 0);
}
The error I get is
error: attempt to use a deleted function
.
.
.
in instantiation of function template specialization 'std::thread::thread<void (&)(std::vector<int> &, int &), std::reference_wrapper<const std::vector<int>>, std::reference_wrapper<int>, void>' requested here
threads.push_back(std::thread(add_vector, std::cref(mat[k]), std::ref(tmp[k])));
^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/type_traits:1916:5: note: '~__nat' has been explicitly marked deleted here
~__nat() = delete;
Upon googling this issue, all the information points to the fact that std::thread needs to copy the arguments, which cannot be done for references.. The suggested solution seems to be to wrap the arguments in a std::ref call. I am already doing this and it is still not working. Does anybody have some suggestions?

Answer 1

I did some playing. I got this to work:
#include <iostream>
#include <thread>
#include <vector>
void add_vector(const int & a, const int & b, int & c) {
c = a + b;
}
int main() {
std::vector<std::thread> threads;
int a = 5;
int b = 10;
int c = 0;
std::cout << "start threads.\n";
threads.push_back(std::thread(add_vector, a, b, std::ref(c) ));
std::cout << "Join.\n";
for (std::thread &thread: threads) {
thread.join();
}
std::cout << "Done. c == " << c << "\n";
}
This is with g++ using --std=c++17.
I updated this with info from Innocent Bystander. Passing by reference can work if you wrap in std::ref() as shown.
I'm going to try a fresh example with vectors.
#include <iostream>
#include <thread>
#include <vector>
void add_vector(const std::vector<int> & vec, int &result) {
result = 0;
for (const int &value: vec) {
result += value;
}
}
int main() {
std::vector<std::thread> threads;
std::vector<int> values;
std::vector<int> results;
values.push_back(10);
values.push_back(15);
results.push_back(0);
std::cout << "start threads.\n";
threads.push_back(std::thread(add_vector, values, std::ref(results[0]) ));
std::cout << "Join.\n";
for (std::thread &thread: threads) {
thread.join();
}
std::cout << "Done. c == " << results[0] << "\n";
}
This above with vector works. I didn't wrap the first args. I did wrap the final one.
The end answer: take a look at the very first argument to your method. It isn't listed as const.