When i use cout.tie(NULL), program doesn't print anything for my code, but if i print endl, program works fine

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#include <bits/stdc++.h>

using namespace std;

void scan_a_line_indefinitely()
{
    // scan line indefinitely
    string input_line;
    while(getline(cin,input_line)) 
    {
        cout << input_line ; **// doesn't print if i use this line**
        //cout << input_line << endl; **// if i use this line, it works fine**
    }

}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    scan_a_line_indefinitely();
    return 0;
}

someone please help me understand the problem with this code. i think the problem is with cout.tie() and cout.tie(), when i remove these, program works fine.

c++
2 Answers

std::cout will flush under these conditions:

  1. An input-stream which is tied to std::cout tries to read input.
    You removed the ties.

  2. iostreams are synchronized with stdio, thus effectively unbuffered.
    You disabled the synchronization.

  3. The buffer is full.
    That takes a bit longer.

  4. The program ends normally.
    That comes too late for you.

  5. There is a manual flush (stream.flush() which is called when streaming std::flush; stream << std::endl is equivalent to stream << stream.widen('\n') << std::flush).
    You have none of those.

So, fix any of them and you will see your output earlier.

user_32045510

If only iostreams are used you can add a manual flush to the output :

std::cout.flush();

Or

std::cout << /* the output */<< std::flush;

Also:

std::cout << std::endl is equivalent to std::cout << '\n' << std::flush

user_25047570
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