[Bug libstdc++/100612] New: std::jthread can't be initialized with a pointer to a member function taking a std::stop_token
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100612 Bug ID: 100612 Summary: std::jthread can't be initialized with a pointer to a member function taking a std::stop_token Product: gcc Version: 10.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: libstdc++ Assignee: unassigned at gcc dot gnu.org Reporter: jonathan.oconnor at protonmail dot com Target Milestone: --- Created attachment 50817 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50817&action=edit Sample source code showing error
[Bug libstdc++/100612] std::jthread can't be initialized with a pointer to a member function taking a std::stop_token
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100612
--- Comment #1 from Jonathan O'Connor
---
The std::jthread constructor does not support taking a pointer to a member
function that has, as a first argument, a std::stop_token.
In C++20, the new jthread class can accept a std::stop_token to aid in stopping
a thread cooperatively. Unfortunately, the library code in gcc 10.2.0 and all
later versions (according to godbolt.org) do not allow a member function taking
a stop_token to be used to initialize a jthread.
The problem is due to the std::jthread::_S_create() method which can't handle
my desired case.
In a private email discussion with Nico Josuttis, his opinion was this should
be allowed by the standard:
"well, my rough understanding is that as INVOKE is called
(see 20.9.2 Requirements [func.require] in the Standard),
you can always pass a member function with the object to call the member
function for as next argument.
That should work for both thread and jthread."
The fix should be relatively simple, involving a further if constexpr check for
the Callable being a member function. I'll try and make a patch, and append it
here.
#include
struct ThreadObj {
void withoutStopToken() {}
void withStopToken(std::stop_token st) {}
};
int main() {
ThreadObj obj;
// The following line causes an error. The other example compile fine.
std::jthread t1{&ThreadObj::withStopToken, &obj};
std::jthread t2{&ThreadObj::withoutStopToken, &obj};
return 0;
}
[Bug libstdc++/100612] std::jthread can't be initialized with a pointer to a member function taking a std::stop_token
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100612 --- Comment #5 from Jonathan O'Connor --- I was afraid you were going to say it's not a bug :-) That's why I reached out to Nico, who was on the committee, and was one of the people who proposed jthread. My view, as a user, is that jthread should be a drop in replacement for thread, with auto-joining and stop_token support. I've just reread the descripton of jthread's constructor here: https://en.cppreference.com/w/cpp/thread/jthread/jthread and it does seem to explicitly exclude the behaviour I would like. So, I have no problem with you closing this as not a bug. Sorry for wasting your time. I guess, I'll have to write a committee proposal. BTW, thanks for your all your work on g++ and the stdlib++. Much appreciated.
[Bug libstdc++/100612] std::jthread can't be initialized with a pointer to a member function taking a std::stop_token
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100612 --- Comment #7 from Jonathan O'Connor --- libstdc++. Woops, apologies! I feel somewhat vindicated by your non __STRICT_ANSI__ change! I'll now go away and write the proposal. Thanks for the encouragement.
[Bug libstdc++/100612] std::jthread can't be initialized with a pointer to a member function taking a std::stop_token
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100612 --- Comment #9 from Jonathan O'Connor --- I'd already figured out the workaround using a static member function calling the non-static member function. But the lambda version is nicer!
