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std::jthread::jthread(3) C++ Standard Libary std::jthread::jthread(3) NAME std::jthread::jthread - std::jthread::jthread Synopsis jthread() noexcept; (1) (since C++20) jthread( jthread&& other ) noexcept; (2) (since C++20) template< class F, class... Args > (3) (since C++20) explicit jthread( F&& f, Args&&... args ); jthread( const jthread& ) = delete; (4) (since C++20) Constructs new std::jthread object. 1) Creates new std::jthread object which does not represent a thread. 2) Move constructor. Constructs the std::jthread object to represent the thread of execution that was represented by other. After this call other no longer represents a thread of execution. 3) Creates new std::jthread object and associates it with a thread of execution. The new thread of execution starts executing: std::invoke(decay-copy(std::forward<F>(f)), get_stop_token(), (until C++23) decay-copy(std::forward<Args>(args))...) std::invoke(auto(std::forward<F>(f)), get_stop_token(), (since C++23) auto(std::forward<Args>(args))...) if the expression above is well-formed, otherwise starts executing: std::invoke(decay-copy(std::forward<F>(f)), (until C++23) decay-copy(std::forward<Args>(args))...). std::invoke(auto(std::forward<F>(f)), (since C++23) auto(std::forward<Args>(args))...). The calls of decay-copy are evaluated (until C++23) The values produced by auto are materialized (since C++23) in the current thread, so that any exceptions thrown during evaluation and copying/moving of the arguments are thrown in the current thread, without starting the new thread. These overloads participate in overload resolution only if std::re- move_cvref_t<F> is not the same type as std::jthread. If any of the following is false, the program is ill-formed: * std::is_constructible_v<std::decay_t<F>, F> * (std::is_constructible_v<std::decay_t<Args>, Args> && ...) * std::is_invocable_v<std::decay_t<F>, std::decay_t<Args>...> || std::is_invocable_v<std::decay_t<F>, std::stop_token, std::de- cay_t<Args>...> The completion of the invocation of the constructor synchronizes with the beginning of the invocation of the copy of f on the new thread of execution. 4) The copy constructor is deleted; threads are not copyable. No two std::jthread objects may represent the same thread of execution. Parameters other - another std::jthread object to construct this std::jthread object with f - Callable object to execute in the new thread args - arguments to pass to the new function Postconditions 1) get_id() equal to std::jthread::id() (i.e. joinable() returns false) and get_stop_source().stop_possible() is false. 2) other.get_id() equal to std::jthread::id() and get_id() returns the value of other.get_id() prior to the start of construction. 3) get_id() not equal to std::jthread::id() (i.e. joinable() returns true), and get_stop_source().stop_possible() is true. Exceptions 3) std::system_error if the thread could not be started. The excep- tion may represent the error condition std::errc::resource_unavailable_try_again or an- other implementation-specific error condition. Notes The arguments to the thread function are moved or copied by value. If a reference argument needs to be passed to the thread function, it has to be wrapped (e.g. with std::ref or std::cref). Any return value from the function is ignored. If the function throws an exception, std::terminate is called. In order to pass return values or excep- tions back to the calling thread, std::promise or std::async may be used. Example // Run this code #include <chrono> #include <iostream> #include <thread> #include <utility> using namespace std::literals; void f1(int n) { for (int i = 0; i < 5; ++i) { std::cout << "Thread 1 executing\n"; ++n; std::this_thread::sleep_for(10ms); } } void f2(int& n) { for (int i = 0; i < 5; ++i) { std::cout << "Thread 2 executing\n"; ++n; std::this_thread::sleep_for(10ms); } } class foo { public: void bar() { for (int i = 0; i < 5; ++i) { std::cout << "Thread 3 executing\n"; ++n; std::this_thread::sleep_for(10ms); } } int n = 0; }; class baz { public: void operator()() { for (int i = 0; i < 5; ++i) { std::cout << "Thread 4 executing\n"; ++n; std::this_thread::sleep_for(10ms); } } int n = 0; }; int main() { int n = 0; foo f; baz b; std::jthread t0; // t0 is not a thread std::jthread t1(f1, n + 1); // pass by value std::jthread t2a(f2, std::ref(n)); // pass by reference std::jthread t2b(std::move(t2a)); // t2b is now running f2(). t2a is no longer a thread std::jthread t3(&foo::bar, &f); // t3 runs foo::bar() on object f std::jthread t4(b); // t4 runs baz::operator() on a copy of object b t1.join(); t2b.join(); t3.join(); std::cout << "Final value of n is " << n << '\n'; std::cout << "Final value of f.n (foo::n) is " << f.n << '\n'; std::cout << "Final value of b.n (baz::n) is " << b.n << '\n'; // t4 joins on destruction } Possible output: Thread 2 executing Thread 1 executing Thread 4 executing Thread 3 executing Thread 3 executing Thread 4 executing Thread 2 executing Thread 1 executing Thread 3 executing Thread 1 executing Thread 4 executing Thread 2 executing Thread 3 executing Thread 1 executing Thread 4 executing Thread 2 executing Thread 3 executing Thread 1 executing Thread 4 executing Thread 2 executing Final value of n is 5 Final value of f.n (foo::n) is 5 Final value of b.n (baz::n) is 0 Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct behavior overload (3) directly required (the decayed LWG 3476 C++20 types of) removed these F and the argument types to be move requirements^[1] constructible 1. The move-constructibility is already indirectly required by std::is_constructible_v. See also constructor constructs new thread object (public member function of std::thread) C documentation for thrd_create http://cppreference.com 2024.06.10 std::jthread::jthread(3)
NAME | Synopsis | Parameters | Postconditions | Exceptions | Notes | Example | Possible output: | See also
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