user02@FC4 Lesson02]$ cat -n first.cpp

   1  #include <iostream>
   2  using namespace std;
   3
   4  int main()
   5  {
   6     cout << "Rescued by C++!\n";

   7
   8     return 0;
   9  }

[user02@FC4 Lesson02]$ g++ -o first first.cpp
[user02@FC4 Lesson02]$ ./first
Rescued by C++!

---

endl flushes buffer is better than \n ?

   1  #include <iostream>
   2  using namespace std;
   3
   4  int main()
   5  {
   6     cout << "Rescued by C++!" << endl;
   7
   8     return 0;
   9  }

Only way to tell would be to check out the binary produced... I somehow doubt the latter would be better, and most likely, both produce exactly the same code.