CODEWITHSUNDEEP
c++multithreadingc++20
a_girl
a_girl

Reputation: 1205

How to pass a function with arguments and stop_condition in jthread?

I am trying to do something like this

#include <iostream>
#include <thread>
#include <chrono>

void doWork(char a, std::stop_token st = {}) {
    while (!st.stop_requested()) {
        std::cout << a << '\n';
    }
}

int main() {
    std::jthread t1{doWork, 'A'}, // error
                 t2{doWork, 'B'}; // error
    std::this_thread::sleep_for(std::chrono::milliseconds(50));
}

But it won't compile on gcc trunk with -std=c++2a:

/opt/compiler-explorer/gcc-trunk-20200219/include/c++/10.0.1/thread:537:20: error: static assertion failed: std::thread arguments must be invocable after conversion to rvalues

  537 |      static_assert(is_invocable_v<decay_t<_Callable>,

      |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  538 |       decay_t<_Args>...>,

      |       ~~~~~~~~~~~~~~~~~~

Is it even possible to do something like this?

I assumed that it should be possible, because I've seen a similar example with lambda (here):

//...
std::jthread t([&ready, &readyMutex, &readyCV] (std::stop_token st) {
    while (!stoken.stop_requested()) {
        //...
    }
});

Upvotes: 3

Views: 1194

Answers (1)

StoryTeller - Unslander Monica
StoryTeller - Unslander Monica

Reputation: 170153

You are almost correct in your implementation. The constraint on the invokable is that it accepts a std::stop_token as a first argument. So switching the order in the declaration of doWork will sort it out.

void doWork(std::stop_token st, char a) {
    while (!st.stop_requested()) {
        std::cout << a << '\n';
    }
}

The token comes from the library implementation, so its construction is not meant to worry you anyway.

Upvotes: 5

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