std::thread:: thread

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std::thread
thread ( ) noexcept ;
(1) (since C++11)
thread ( thread && other ) noexcept ;
(2) (since C++11)
template < class F, class ... Args >
explicit thread ( F && f, Args && ... args ) ;
(3) (since C++11)
thread ( const thread & ) = delete ;
(4) (since C++11)

Constructs a new std::thread object.

1) Creates a new std::thread object which does not represent a thread.
2) Move constructor. Constructs the std::thread 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 After this call other no longer represents a thread of execution.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 a new std::thread object and associates it with a thread of execution. The new thread of execution starts executing:

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.
This overload participates in overload resolution only if std:: decay < F > :: type (until C++20) std:: remove_cvref_t < F > (since C++20) is not the same type as std::thread .

If any of the following conditions is satisfied, the program is ill-formed:

(until C++20)

If any of the following is false , the program is ill-formed:

(since C++20)
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::thread objects may represent the same thread of execution.

Parameters

other - another thread object to construct this thread 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:: thread :: id ( ) (i.e. joinable() is false ).
2) other. get_id ( ) equal to std::thread::id() and get_id() returns the value of other. get_id ( ) prior to the start of construction.
3) get_id() not equal to std::thread::id() (i.e. joinable() 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>
 
void f1(int n)
{
    for (int i = 0; i < 5; ++i)
    {
        std::cout << "Thread 1 executing\n";
        ++n;
        std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }
}
 
void f2(int& n)
{
    for (int i = 0; i < 5; ++i)
    {
        std::cout << "Thread 2 executing\n";
        ++n;
        std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }
}
 
class foo
{
public:
    void bar()
    {
        for (int i = 0; i < 5; ++i)
        {
            std::cout << "Thread 3 executing\n";
            ++n;
            std::this_thread::sleep_for(std::chrono::milliseconds(10));
        }
    }
    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(std::chrono::milliseconds(10));
        }
    }
    int n = 0;
};
 
int main()
{
    int n = 0;
    foo f;
    baz b;
    std::thread t1; // t1 is not a thread
    std::thread t2(f1, n + 1); // pass by value
    std::thread t3(f2, std::ref(n)); // pass by reference
    std::thread t4(std::move(t3)); // t4 is now running f2(). t3 is no longer a thread
    std::thread t5(&foo::bar, &f); // t5 runs foo::bar() on object f
    std::thread t6(b); // t6 runs baz::operator() on a copy of object b
    t2.join();
    t4.join();
    t5.join();
    t6.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';
}

Possible output: 

Thread 1 executing
Thread 2 executing
Thread 3 executing
Thread 4 executing
Thread 3 executing
Thread 1 executing
Thread 2 executing
Thread 4 executing
Thread 2 executing
Thread 3 executing
Thread 1 executing
Thread 4 executing
Thread 3 executing
Thread 2 executing
Thread 1 executing
Thread 4 executing
Thread 3 executing
Thread 1 executing
Thread 2 executing
Thread 4 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 2097 C++11 for overload (3) , F could be std::thread F is constrained
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]
  1. ↑ The move-constructibility is already indirectly required by std::is_constructible_v .

References

  • C++23 standard (ISO/IEC 14882:2024):
  • 33.4.3.3 thread constructors [thread.thread.constr]
  • C++20 standard (ISO/IEC 14882:2020):
  • 32.4.2.2 thread constructors [thread.thread.constr]
  • C++17 standard (ISO/IEC 14882:2017):
  • 33.3.2.2 thread constructors [thread.thread.constr]
  • C++14 standard (ISO/IEC 14882:2014):
  • 30.3.1.2 thread constructors [thread.thread.constr]
  • C++11 standard (ISO/IEC 14882:2011):
  • 30.3.1.2 thread constructors [thread.thread.constr]

See also

constructs new jthread object
(public member function of std::jthread )
C documentation for thrd_create
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