C++ changing vector in different thread - c++

Question

I'm trying to change a vector in a different thread, but the value of the vector is not changed. I thought that using std::ref will fix the issue but it didn't work.
This is the code that start the threads:
printf("tmp size: %d\n", tmp_size);
printf("before change");
printArray(tmp);
std::thread threads[1];
for(int i = 0; i < 1; i++){
threads[i] = std::thread(callback, std::ref(tmp));
}
for(int i = 0; i < 1; i++){
threads[i].join();
}
printf("after join: ");
printArray(tmp);
this is the callback:
void callback(std::vector<uint64_t> tmp){
tmp[0] = 1;
printf("inside callback");
printArray(tmp);
}
and the output is:
tmp size: 2
before change 0 0
inside callback 1 0
after join: 0 0
I was expecting that after the thread change the vector the values will be: inside callback: 1 0. Isn't it passed by reference?

Answer 1

You are passing a reference to the function, but then the function takes its parameter by value, giving it the value of the reference. Modifying the value of the reference does no good. You need to modify the reference. Here's a demonstration of how to do it correctly:
#include <vector>
#include <stdint.h>
#include <thread>
void callback(std::vector<uint64_t> &tmp)
{
tmp[0] += 1;
}
int main()
{
std::thread threads[1];
std::vector<uint64_t> tmp;
tmp.push_back(1);
for(int i = 0; i < 1; i++)
threads[i] = std::thread(callback, std::ref(tmp));
for(int i = 0; i < 1; i++)
threads[i].join();
printf("%d\n", (int) tmp[0]);
}

Answer 2

If you wanted the callback to change the vector, you would have to pass it by pointer or reference.
Your callback code has made a copy of it instead.
Another option that can sometimes be more thread-safe is if you were to "move" the vector into the thread and then move it back out when the thread finishes. Like so:
#include <thread>
#include <future>
#include <vector>
#include <iostream>
std::vector<int> addtovec(std::vector<int> vec, int add) {
for(auto &x: vec) {
x += add;
}
return vec;
}
std::ostream& operator<<(std::ostream& os, const std::vector<int> &v) {
os << '{';
bool comma = false;
for(const auto &x: v) {
if(comma) os << ',';
comma = true;
os << x;
}
os << '}';
return os;
}
int main() {
std::vector<int> a{1,2,3,9,8,7};
std::cout << "before: " << a << std::endl;
auto future = std::async(addtovec, std::move(a), 5);
std::cout << "after move: " << a << std::endl;
a = future.get();
std::cout << "after get: " << a << std::endl;
return 0;
}