What's equivalent of fflush( stin ) in C++?

[Dufresne]

Hi!
In C language we can empty input buffer by calling function
fflush( stdin ); after scanf(); , but i doesn't work when we use it after cin.
So how can we empty input buffer in C++ ?

[Bazzy]

cin.sync();

[Dufresne]

I didn't give the result that i want.
For following code , it enters infinite loop if i enter a character instead of a number.

#include <iostream>
#include <cstdlib>

using namespace std ;

int main( void )
{
    int number ; 
    cout << "Enter number > " ;
    cin >> number ;
    
    while( number <0 || number > 0 )
    {
      cin.sync(); //  infine loop if user enters a character
      cout << "\nError!Enter again > " ;
      cin >> number ;
    }
    cout << "\nNumber is " << number << endl;
    
    system( "PAUSE" );
    return  ( 0 ) ;
}

[Bazzy]

For that you should use getline and stringstreams: http://www.cplusplus.com/forum/articles/6046/

[Duthomhas]

I am soon to post an article on this... it is pretty big since the subject is fairly in-depth... but the simple answer is just

You can't and/or shouldn't fflush() stdin.

Remember, fflush() is not defined on input streams! It does not matter that it always worked for you in the past -- it is non-standard and will break at some point.


What typically is asked is simply to get rid of pending input before the end of the line. For that, simply use one of:

// C++
#include <iostream>
#include <limits>
using namespace std;

cin.ignore( numeric_limits <streamsize> ::max(), '\n' );

/* C */
#include <stdio.h>

int c;
do c = getch(); while ((c != EOF) && (c != '\n'));

Completely clearing the user input is a different matter, and is typically not recommended except in special circumstances (which I will endeavor to touch upon in my upcoming article), since it requires special conditions and programming.

Unfortunately, the istream::sync() method, I've learned, isn't very clearly defined and has little use.

I just married so it may be a week before I can post the article. In lieu of seeking to do things like flush the input, I heartily recommend you use getline() and stringstreams as Bazzy suggests. The user always expects to press ENTER after every input, so your input routines should match -- read a line of input, then parse it properly. Don't use formatted input methods directly from the user.

Here is an example:

#include <iostream>
#include <sstream>
#include <string>
using namespace std;

int main()
  {
  string name;
  unsigned age;

  cout << "What is your name? " << flush;
  getline( cin, name );

  cout << "How old are you? " << flush;
  {
    string s;
    getline( cin, s );
    if (!(istringstream( s ) >> age))  //yoinks! fixed this
      age = 0;
  }

  cout << "Hello, " << name << "!\nYou are " << age << " years old.\n";
  return 0;
  }

Hope this helps.

[Dufresne]

For that you should use getline and stringstreams: http://www.cplusplus.com/forum/articles/6046/

Thanks Bazzy. This article is useful.

@Duoas
Thank you very much Duoas.Your answer also helped a lot.

// C++
#include <iostream>
#include <limits>
using namespace std;

cin.ignore( numeric_limits <streamsize> ::max(), '\n' );

This code works the way i want. But

#include <iostream>
#include <sstream>
#include <string>
using namespace std;

int main()
  {
  string name;
  unsigned age;

  cout << "What is your name? " << flush;
  getline( cin, name );

  cout << "How old are you? " << flush;
  {
    string s;
    getline( cin, s );
    if (!(ostringstream( s ) >> age))
      age = 0;
  }

  cout << "Hello, " << name << "!\nYou are " << age << " years old.\n";
  return 0;
  }

This one gives compile error on Dev-C++. I get error.

18 no match for 'operator>>' in 'ostringstream(((const std::basic_string<char, std::char_traits<char>, std::allocator<char> >&)((const std::basic_string<char, std::char_traits<char>, std::allocator<char> >*)(&s))), (std::_Ios_Openmode)16u) >> age'

[Duthomhas]

Sorry, I wrote ostringstream instead of istringstream. I fixed the error above.