[Algorithms] Configuring C++ Standard I/O

"Let's prevent bottlenecks in the I/O process when there are many test-case inputs and outputs."

When doing CP contests, the opening lines of other participants' C++ solutions always have something like this.

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

#What happens on each line?

ios_base::sync_with_stdio(false);

Disable I/O buffer synchronization between the cin stream and C's stdio tools (scanf, gets, etc.).

By default, C/C++ programs share the standard I/O buffer and perform synchronization (which is why you can mix scanf and cin), but this setting makes it skip that work. (Each gets its own independent I/O buffer.)

The overhead removed in this process gives cin a speed boost.

std::cin.tie(NULL);

Untie cin and cout.

By default, the standard I/O buffer is flushed before every cin (they are tied together).
For example, given a test case and code like the following,

2
apple
banana

int t;
string str;

int main() {
  cin >> t;
  while (t--) {
    cin >> str;
    cout << "I like " << str << "\n";
  }
}

it executes in the order:

1. flush
2. cin >> t
3. flush
4. cin >> str
5. flush (prints "I like apple")
6. cin >> str
7. flush (prints "I like banana")

As the input size grows, the number of flushes increases linearly.
Let's see how this changes after untie.

1. cin >> t
2. cin >> str
3. cin >> str
4. flush (prints "i like apple\nI like banana")

Much more relieving. Let's use the code below as a template.

#include <bits/stdc++.h>
using namespace std;
#define FAST_IO ios_base::sync_with_stdio(0),cin.tie(0)

int main() {
  FAST_IO;
  cout << "도움이 되셨으면 좋겠습니다.\n";
}