c++ - Why does in_avail() output zero even if the stream has some char?

Question

#include <iostream>
int main( )
{
   using namespace std;
   cout << cin.rdbuf()->in_avail() << endl;
   cin.putback(1);
   cin.putback(1);
   cout << cin.rdbuf()->in_avail() << endl;
   return 0;
} //compile by g++-4.8.1

I think this will output 0 and 2

but when I run the code, it output 0 and 0, why?

or if I change cin.putback(1); to int a; cin >> a; with input 12 12;

it still outputs 0 and 0

Answer 1

Apparently it's a bug/feature of some compiler implementations Insert the line

cin.sync_with_stdio(false);

somewhere near the beginning of code, and that should fix it

EDIT: Also remember that in_avail will always return 1 more than the number of chars in the input because it counts the end of input character.

EDIT2: Also as I just checked, putback does not work unless you have attempted to read something from the stream first, hence the "back" in "putback". If you want to insert characters into the cin, this thread will provide the answer: Injecting string to 'cin'