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# std::thread::thread

From cppreference.com
< cpp‎ | thread‎ | thread

thread(); | (1) | (since C++11)
thread( thread&& other ); | (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 After this call `other` no longer represents a thread of execution.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

    std::invoke(decay_copy(std::forward<Function>(f)), decay_copy(std::forward<Args>(args))...);

where `decay_copy` is defined as

    template <class T>
    std::decay_t<T> decay_copy(T&& v) { return std::forward<T>(v); }

Except that the calls to `decay_copy` are evaluated 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 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.

This constructor does not participate in overload resolution if std::decay_t<Function> is the same type as std::thread. | (since C++14)

4) The copy constructor is deleted; threads are not copyable. No two `std::thread` objects may represent the same thread of execution.

### [edit] 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

### [edit] 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)

### [edit] Exceptions

1-2)
`noexcept` specification:
noexcept
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.

### [edit] 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.

### [edit] Example

Run this code

    #include <iostream>
    #include <utility>
    #include <thread>
    #include <chrono>
    #include <functional>
    #include <atomic>

    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));
        }
    }

    int main()
    {
        int n = 0;
        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
        t2.join();
        t4.join();
        std::cout << "Final value of n is " << n << '\n';
    }

Possible output:

    Thread 1 executing
    Thread 2 executing
    Thread 1 executing
    Thread 2 executing
    Thread 1 executing
    Thread 2 executing
    Thread 1 executing
    Thread 2 executing
    Thread 2 executing
    Thread 1 executing
    Final value of n is 5

### [edit] References

- C++11 standard (ISO/IEC 14882:2011):

  - 30.3.1.2 thread constructors [thread.thread.constr]

### [edit] See also

C documentation for thrd_create

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