std::jthread::jthread

jthread() noexcept;	(1)	(since C++20)
jthread( jthread&& other ) noexcept;	(2)	(since C++20)
template< class F, class... Args > explicit jthread( F&& f, Args&&... args );	(3)	(since C++20)
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 After this call other no longer represents a thread of execution.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(), `decay-copy`(std::forward<Args>(args))...)	(until C++23)
std::invoke(auto(std::forward<F>(f)), get_stop_token(), auto(std::forward<Args>(args))...)	(since C++23)

if the expression above is well-formed, otherwise starts executing:

std::invoke(`decay-copy`(std::forward<F>(f)), `decay-copy`(std::forward<Args>(args))...).	(until C++23)
std::invoke(auto(std::forward<F>(f)), auto(std::forward<Args>(args))...).	(since C++23)

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::remove_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::decay_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 exception may represent the error condition std::errc::resource_unavailable_try_again or another 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 exceptions 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 retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3476	C++20	overload (3) directly required (the decayed types of) `F` and the argument types to be move constructible	removed these requirements^[1]

↑ The move-constructibility is already indirectly required by std::is_constructible_v.

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
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Filesystem library (C++17)
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Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

Member functions
jthread::jthread
jthread::~jthread
jthread::operator=
Observers
jthread::joinable
jthread::get_id
jthread::native_handle
jthread::hardware_concurrency
Operations
jthread::join
jthread::detach
jthread::swap
Stop token handling
jthread::get_stop_source
jthread::get_stop_token
jthread::request_stop
Non-member functions
swap(std::jthread)