Why doesn't this code using printf and cout have the expected output?

Question

I have the following code:

int main ()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);    

    for (int i = 0; i < 3; i++) {
        cout << i << " ";
        printf("%d ", i);
    }

    cout << endl;
    return 0;
}

The expected output of this code is:

0 0 1 1 2 2

but, instead, it prints:

0 1 2
0 1 2

This issue happens in GNU G++ 4.9.2 compiler

Answer 1

One possible explanation for this is that cout and printf use separate buffers. cout outputs on the terminal screen either when it is flushed using the endl command, or if the buffer is full (generally 512 bytes).

I am not sure about printf (so feel free to correct me if I'm wrong), but it also follows a similar behaviour. So what happens is that at the end of the program, both the buffers are flushed, and so the output you see is achieved.

I ran the code on my machine (GCC 4.8.1) along with the modification as below

cout << i << " . ";
printf("%d ", i);

The output I observed was 0 1 2 0 . 1 . 2 . which seems to indicate that printf flushes first in my case. I have no idea if it is by design (mentioned somewhere in the standard), or if it depends on the context.