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std::thread::thread

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

Constructs new thread object.

1) Creates new thread object which does not represent a thread.

2) Move constructor. Constructs the 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 After this call other no longer represents a thread of execution.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 new std::thread object and associates it with a thread of execution. The new thread of execution starts executing /*INVOKE*/(std::move(f_copy), std::move(args_copy)...), where

/*INVOKE*/ performs the INVOKE operation specified in Callable, which can be performed by std::invoke (since C++17), and
f_copy is an object of type std::decay<Function>::type and constructed from std::forward<Function>(f), and
args_copy... are objects of types std::decay<Args>::type... and constructed from std::forward<Args>(args)....

Constructions of these objects are executed in the context of the caller, so that any exceptions thrown during evaluation and copying/moving of the arguments are thrown in the current thread, without starting the new thread. The program is ill-formed if any construction or the INVOKE operation is invalid.

This constructor does not participate in overload resolution if std::decay<Function>::type is the same type as thread.

The completion of the invocation of the constructor synchronizes-with (as defined in std::memory_order) 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

#include <iostream>
#include <utility>
#include <thread>
#include <chrono>
 
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	constructor taking a Callable object might be ambiguous with the move constructor	constrained

References

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]

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