Cpp threads is not working as expected when loops are involved

Question

C++ sleep_for method is not working as expected. I'm trying to write a program that prints from 1 to 10 but with some time gap between each print.

1 (after 1 sec) 2 (after 1 sec) 3 . . . and so on.

the code is:

#include <iostream>
#include <thread>
#include <chrono>


void produce()
{
    for(int i=0;i<10;i++)
    {
        std::cout<<i+1<<" ";
        std::this_thread::sleep_for(std::chrono::seconds(1));
    }
    std::cout<<"\n";
}


int main()
{
    std::cout<<"Starting the thread\n";
    std::thread thread1(produce);
    thread1.join();
    std::cout<<"Thread finished execution\n";
    return 0;
}

The output of the above program is:

wait for 10 seconds....then immediately print from 1 to 10. ( why is this happening ? )

If I change the produce method to:

void produce()
{
    for(int i=0;i<10;i++)
    {
        std::cout<<i+1<<" ";
        std::this_thread::sleep_for(std::chrono::seconds(1));
        std::cout<<"\n";
    }
    
}

Then, I'm getting the expected result ( all I did for put the cout inside the loop and it makes the program wait for a second before printing the next number.

Is there something that I don't understand about c++ threads or this is an actual issue ? Thanks in advance

Answer 1

This has nothing to do with threads, whatsoever.

  std::cout<<i+1<<" ";

Formatted output operations on a std::ostream are buffered. This is now sitting in cout's internal buffer, waiting for more stuff to be written, until it can all be written out in one big chunk, and/or until there's a newline.

  std::cout<<"\n";

Like this. As you've discovered by yourself.

Without a newline you need to explicitly flush it yourself:

  std::cout<<i+1<<" "<<std::flush;