CODEWITHSUNDEEP
c++multithreading
Matteo Monti
Matteo Monti

Reputation: 8930

How tidy is thread id assignment?

I did the following experiment:

#include <iostream>
#include <thread>

void t()
{
    std :: cout << std :: this_thread :: get_id() << std :: endl;
}

void f()
{
    std :: cout << std :: this_thread :: get_id() << std :: endl;
}

int main()
{
    std :: cout << std :: this_thread :: get_id() << std :: endl;

    for(unsigned int i = 0; i < 1000; i++)
    {
        std :: thread d(t);
        d.join();

        std :: thread g(f);
        g.join();
    }
}

and noticed that the resulting list of thread_ids would always be the same. In other words, in this case the thread_id was continuously re-assigned whenever the thread was joined and re-opened.

Now, can I have any form of certainty that this will always happen? Or can it happen that a thread_id is assigned only once, then always different thread_ids get assigned in a random-like fashion?

Upvotes: 0

Views: 223

Answers (2)

Richard Hodges
Richard Hodges

Reputation: 69854

There's some documentation here: http://en.cppreference.com/w/cpp/thread/thread/id

emphasis is mine

The class thread::id is a lightweight, trivially copyable class that serves as a unique identifier of std::thread objects.

Instances of this class may also hold the special distinct value that does not represent any thread. Once a thread has finished, the value of std::thread::id may be reused by another thread.

This class is designed for use as key in associative containers, both ordered and unordered.

Upvotes: 0

wally
wally

Reputation: 11002

No, you cannot assume that it will always be the same. On VS2015 I get the following:

...
2444
18472
29912
25180
6612
29440
13220
4684
14004
12388
16424
26320
25948
28076
30904
6396
1160
4228
...

Upvotes: 3

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