NAME¶

std::thread::thread - std::thread::thread

Synopsis¶

thread() noexcept; (1) (since C++11)
thread( thread&& other ) noexcept; (2) (since C++11)
template< class Function, class... Args > (3) (since C++11)
explicit thread( Function&& f, Args&&... args );
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 After this call other no longer represents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¶

// Run this code

#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 constrained
ambiguous with the move constructor

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):