std::jthread runs a member function from another member function - c++

Question

Here is my code:
#include <iostream>
#include <zconf.h>
#include <thread>
class JT {
public:
std::jthread j1;
JT() {
j1 = std::jthread(&JT::init, this, std::stop_token());
}
void init(std::stop_token st={}) {
while (!st.stop_requested()) {
std::cout << "Hello" << std::endl;
sleep(1);
}
std::cout << "Bye" << std::endl;
}
};
void init_2(std::stop_token st = {}) {
while (!st.stop_requested()) {
std::cout << "Hello 2" << std::endl;
sleep(1);
}
std::cout << "Bye 2" << std::endl;
}
int main() {
std::cout << "Start" << std::endl;
JT *jt = new JT();
std::jthread j2(init_2);
sleep(5);
std::cout << "Finish" << std::endl;
}
Here is the output:
Start
Hello
Hello 2
Hello
Hello 2
Hello
Hello 2
Hello
Hello 2
Hello
Hello 2
Finish
Bye 2
Hello
The problem is I could get Bye 2 message but not Bye message.
I know the passed stop_token variable results in this problem but I do not know how to pass it to a member function inside another member function.

Answer 1

If I'm understanding the problem correctly (my understanding being that for std::jthread(&JT::init, this) jthread wants to call JT::init(std::stop_token st, this), which isn't going to work), you probably want to use std::bind_front to give it a Callable that works.
e.g.
JT() {
j1 = std::jthread(std::bind_front(&JT::init, this));
}

Answer 2

According to the useful comments, I have rewritten the class code as below:
class JT {
public:
std::jthread j1;
JT() {
j1 = std::jthread(&JT::init, this);
}
void init() {
auto st = j1.get_stop_token();
while (!st.stop_requested()) {
std::cout << "Hello" << std::endl;
sleep(1);
}
std::cout << "Bye" << std::endl;
}
};
You must get the stop_token on the fly through auto st = j1.get_stop_token();.
And the revised main function:
int main() {
std::cout << "Start" << std::endl;
JT *jt = new JT();
// auto jt = std::make_unique<JT>();
std::jthread j2(init_2);
sleep(5);
std::cout << "Finish" << std::endl;
delete jt;
}
You need to delete the class object directly or use RAII (like smart pointers).

Answer 3

The std::stop_token must be received as parameter by the JT::init function, during the thread construction. You can use either std::bind
j1 = std::jthread{ std::bind(&JT::init, this, std::placeholders::_1) };
or, more simpler, std::bind_front as in #Hasturkun answer.
Note
Obtaining the std::stop_token after the thread has been constructed will eventually result in missing the stop request, as demonstrated bellow:
#include <thread>
#include <iostream>
using namespace std::chrono_literals;
class JT {
public:
std::jthread j1;
JT() {
j1 = std::jthread(&JT::init, this);
}
~JT() {
j1.request_stop();
j1.join();
}
void init() {
auto st = j1.get_stop_token();
while (!st.stop_requested()) {
std::this_thread::sleep_for(1ms);
std::cout << "Hello" << std::endl;
}
std::cout << "Bye" << std::endl;
}
};
int main() {
std::cout << "Start" << std::endl;
for (int i = 0; i < 1000; i++) {
JT jt;
std::this_thread::sleep_for(5ms);
}
}
Which results in:
Start
Hello
Bye
Hello
Bye
Hello
Hello
Hello
Hello
Hello
Hello
....
and program never ending. I've tested on release with gcc 12.1.0 and msvc (VS 2019 16.11.5).