Vector get passed to a thread by val insted of by ref

[AmitM9S6]

I have a function :

void UpdateNotefications(vector <string> &Notefic)

where I change a vector...

I called it with a thread :

thread t1 (UpdateNotefications, Notefic);
			t1.detach();

though I see that inside the func, Notefic has a val, and outside, it is still empty (after the function is over)...

[Computergeek01]

You have to remember that this 'thread' constructor is a function of sorts itself, so if the ".detach()" member function returns a copy of the object then that copy is what gets forwarded to your thread and passed by reference.

[AmitM9S6]

So what can I do ?

[JLBorges]

To pass an argument to the thread function by reference, you have to wrap it.

// thread t1 (UpdateNotefications, Notefic);
thread t1 ( UpdateNotefications, std::ref(Notefic) ) ;

[AmitM9S6]

I have used that, but it still didnt passed it by ref...

[JLBorges]

> but it still didnt passed it by ref...

Before you jump to conclusions, check it out by writing a small test program.

#include <vector>
#include <thread>
#include <memory>
#include <functional>

void thread_fun( std::vector<int>& seq )
{
    std::cout << "thread: reference is to sequence at " << std::addressof(seq) << '\n' ;
    seq.insert( seq.end(), { 5, 6, 7, 8, 9 } ) ;
}

#include <iostream>

int main()
{
    std::vector<int> seq { 0, 1, 2, 3, 4 } ;
    std::cout << "main: sequence is at " << std::addressof(seq) << '\n' ;
    for( int v : seq ) std::cout << v << ' ' ;
    std::cout << '\n' ;

    std::thread t( &thread_fun, std::ref(seq) ) ;
    t.join() ;

    for( int v : seq ) std::cout << v << ' ' ;
    std::cout << '\n' ;
}

Output:

main: sequence is at 0x22fee4
0 1 2 3 4
thread: reference is to sequence at 0x22fee4
0 1 2 3 4 5 6 7 8 9

[AmitM9S6]

Thanks!