std::thread pass vector element by reference

I'm trying to figure out why the following works:

threaded thr[8] = { threaded(), threaded() ,threaded() ,threaded() ,threaded() ,threaded() ,threaded() ,threaded() };
std::vector<std::thread> vec;
for (int i = 0; i < threads; i++)
{
    vec.push_back(std::thread(&threaded::calc, &thr[i], i, num_samples));
}

And the following doesn't:

std::vector<threaded> thr;
std::vector<std::thread> vec;
for (int i = 0; i < threads; i++)
{
    thr.push_back(threaded());
    vec.push_back(std::thread(&threaded::calc, &thr[i], i, num_samples));
}

I tried with a std::ref instead of & - it still doesn't work. Here's threaded's definition:

struct threaded
{
    float elapsed1 = 0;
    float elapsed2 = 0;
    float res = 0;
    float res_jit = 0;
    void calc(int thread, int num_samples){//do something}
};

By doesn't work I mean, that when using vector and &, I get a memory access violation, when I try to use std::ref(thr[i]) instead of &, it doesn't want to compile with the following errors:

Error   C2672   'std::invoke': no matching overloaded function found        

and

Error   C2893   Failed to specialize function template 'unknown-type std::invoke(_Callable &&,_Types &&...)'        

If I use just thr[i] it works fine, but I want to modify the values of the threaded class, so I don't really want to pass a copy.