Thread get_id() function in C++

Last Updated : 8 Jul, 2026

std::thread::get_id() is a member function of the std::thread class that returns the identifier of the associated thread. It is defined in the <thread> header and is commonly used in multithreaded programs.

  • Returns the identifier associated with a thread.
  • Useful for comparing threads, debugging, and logging.
C++ 
#include <iostream>
#include <thread>
using namespace std;

void task()
{
    cout << "Thread ID: "
         << this_thread::get_id() << endl;
}

int main()
{
    thread t(task);
    t.join();

    return 0;
}

Output
Thread ID: 140737347512000

Explanation: The program creates a thread and prints its unique thread ID using get_id(). The exact value varies between executions and systems.

Syntax

 thread_name.get_id();

  • Parameters: This function does not accept any parameters.
  • Return Value: Returns a std::thread::id identifying the associated thread. If no thread is associated, it returns a default-constructed std::thread::id.

Working of get_id()  

The get_id() function retrieves the identifier assigned to a thread by the C++ runtime.

  • Returns the ID of the associated thread.
  • Every active thread has a unique thread ID.
  • A default thread ID is returned for non-associated thread objects.
  • Thread IDs can be compared using comparison operators.
CPP 
#include <chrono>
#include <iostream>
#include <thread>
using namespace std;

// util function for thread creation
void sleepThread()
{
    this_thread::sleep_for(chrono::seconds(1));
}

int main()
{
    // creating thread1 and thread2

    thread thread1(sleepThread);
    thread thread2(sleepThread);

    thread::id t1_id = thread1.get_id();
    thread::id t2_id = thread2.get_id();

    cout << "ID associated with thread1= "
         << t1_id << endl;
    cout << "ID associated with thread2= "
         << t2_id << endl;

    thread1.join();
    thread2.join();

    return 0;
}

Output
ID associated with thread1= 140737347512000
ID associated with thread2= 140737213290176

Explanation: Two threads are created, and each thread object returns its unique identifier using get_id(). Since each thread is different, their IDs are also different.

Advantages

Using get_id() provides several benefits:

  • Helps during debugging and logging.
  • Enables comparison between different threads.
  • Supports better tracking of concurrent tasks.

Limitations

Despite its usefulness, get_id() has some limitations:

  • The numeric value of the ID is implementation-dependent.
  • A default thread object does not have a valid thread ID.
  • Thread IDs may be reused after a thread terminates.

Note: When compiling programs that use the <thread> library with g++, you may need to enable POSIX thread support: g++ -std=c++14 -pthread file.cpp

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