std::jthread::jthread
From cppreference.com
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 After this call other no longer represents a thread of execution.3) Creates new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))...)
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(until C++23) |
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(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))...).
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(until C++23) |
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(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.
See also
constructs new thread object (public member function of std::thread)
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C documentation for thrd_create
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