Significance of ios::sync_with_stdio(false); cin.tie(NULL);

What do these lines do?

std::ios_base::sync_with_stdio(false);
std::cin.tie(nullptr);

ios_base::sync_with_stdio(false);

• Disables synchronization between C++ streams (cin, cout, etc.) and C stdio (scanf, printf) (stackoverflow.com).
• Default behavior: C++ and C I/O share buffers, allowing safe interleaving of printf/cout, etc.
• With false: C++ streams use their own buffers—much faster—but you must avoid mixing C and C++ I/O unless you re-enable sync (stackoverflow.com).
• This has no direct relationship to multi-threading, but synchronized C++ streams are thread-safe by design (stackoverflow.com).

cin.tie(nullptr);

• By default, cin is tied to cout, meaning every cin call flushes cout to ensure prompts are displayed before waiting for input (stackoverflow.com).
• cin.tie(nullptr) unties them: no automatic flush before input. This saves time when alternating cout and cin frequently (stackoverflow.com).
• Important caveat: Your output prompt might not appear before input unless you explicitly flush (e.g., cout << flush, endl, or until the buffer fills) (usaco.guide).

Why are they used together?

• Performance boost in scenarios with heavy I/O—like competitive programming—where speed matters and I/O volume is high.
• However, their primary purpose is to decouple behavior (not just speed). Speed gains are just a side effect.

When to use them

Summary

• sync_with_stdio(false): decouples C++ from C I/O buffers—speeds C++ streams.
• cin.tie(NULL): stops auto-flushing cout before cin—reduces unnecessary flush overhead.
• ⚠️ Use them wisely: avoid mixing I/O styles and manage flushing for prompts yourself.

Example

#include <iostream>
using namespace std;

int main() {
    ios::sync_with_stdio(false); // decouple I/O
    cin.tie(nullptr);               // no auto-flush

    cout << "Enter number: " << flush; // manual flush needed
    int x;
    cin >> x;

    cout << "You entered: " << x << "\n";
}