thread Class

Defines an object that's used to observe and manage a thread of execution within an application.

Syntax

class thread;

Remarks

You can use a thread object to observe and manage a thread of execution within an application. A thread object that's created by using the default constructor isn't associated with any thread of execution. A thread object that's constructed by using a callable object creates a new thread of execution and calls the callable object in that thread. Thread objects can be moved but not copied, which is why a thread of execution can be associated with only one thread object.

Every thread of execution has a unique identifier of type thread::id. The function this_thread::get_id returns the identifier of the calling thread. The member function thread::get_id returns the identifier of the thread that's managed by a thread object. For a default-constructed thread object, the thread::get_id method returns an object that has a value that's the same for all default-constructed thread objects and different from the value that's returned by this_thread::get_id for any thread of execution that could be joined at the time of the call.

Members

Public Classes

Public Constructors

Public Methods

Public Operators

Requirements

Header: <thread>

Namespace: std

detach

Detaches the associated thread. The operating system becomes responsible for releasing thread resources on termination.

void detach();

Remarks

After a call to detach, subsequent calls to get_id return id.

If the thread associated with the calling object isn't joinable, the function throws a system_error that has an error code of invalid_argument.

If the thread associated with the calling object is invalid, the function throws a system_error that has an error code of no_such_process.

get_id

Returns a unique identifier for the associated thread.

id get_id() const noexcept;

Return value

An id object that uniquely identifies the associated thread, or id() if no thread is associated with the object.

hardware_concurrency

Static method that returns an estimate of the number of hardware thread contexts.

static unsigned int hardware_concurrency() noexcept;

Return value

An estimate of the number of hardware thread contexts. If the value can't be computed or isn't well defined, this method returns 0.

Microsoft specific

hardware_concurrency returns the number of logical processors, which corresponds to the number of hardware threads that can execute simultaneously. It takes into account the number of physical processors, the number of cores in each physical processor, and simultaneous multithreading on each single core.

Before Windows 11 and Windows Server 2022, applications were limited by default to a single processor group, having at most 64 logical processors. This limited the number of concurrently executing threads to 64. For more information, see Processor Groups.

Starting with Windows 11 and Windows Server 2022, processes and their threads have processor affinities that by default span all processors in the system and across multiple groups on machines with more than 64 processors. The limit on the number of concurrent threads is now the total number of logical processors in the system.

id class

Provides a unique identifier for each thread of execution in the process.

class thread::id {
    id() noexcept;
};

Remarks

The default constructor creates an object that doesn't compare equal to the thread::id object for any existing thread.

All default-constructed thread::id objects compare equal.

join

Blocks until the thread of execution associated with the calling object completes.

void join();

Remarks

If the call succeeds, subsequent calls to get_id for the calling object return a default thread::id that doesn't compare equal to the thread::id of any existing thread; if the call doesn't succeed, the value that's returned by get_id is unchanged.

joinable

Specifies whether the associated thread is joinable.

bool joinable() const noexcept;

Return value

true if the associated thread is joinable; otherwise, false.

Remarks

A thread object is joinable if get_id() != id().

native_handle

Returns the implementation-specific type that represents the thread handle. The thread handle can be used in implementation-specific ways.

native_handle_type native_handle();

Return value

native_handle_type is defined as a Win32 HANDLE cast as void *.

thread::operator=

Associates the thread of a specified object with the current object.

thread& operator=(thread&& Other) noexcept;

Parameters

Other
A thread object.

Return value

*this

Remarks

The method calls detach if the calling object is joinable.

After the association is made, Other is set to a default-constructed state.

swap

Swaps the object state with that of a specified thread object.

void swap(thread& Other) noexcept;

Parameters

Other
A thread object.

thread constructor

Constructs a thread object.

thread() noexcept;
template <class Fn, class... Args>
explicit thread(Fn&& F, Args&&... A);

thread(thread&& Other) noexcept;

Parameters

F
An application-defined function to execute on the thread.

A
A list of arguments to be passed to F.

Other
An existing thread object.

Remarks

The first constructor constructs an object that's not associated with a thread of execution. The value returned by get_id for the constructed object is thread::id().

The second constructor constructs an object that's associated with a new thread of execution. It executes the pseudo-function INVOKE defined in <functional>. If not enough resources are available to start a new thread, the function throws a system_error object that has an error code of resource_unavailable_try_again. If the call to F terminates with an uncaught exception, terminate is called.

The third constructor constructs an object that's associated with the thread that's associated with Other. Other is then set to a default-constructed state.

See also

Header Files Reference
<thread>