C++ reference: ios::rdstate

ios::rdstate
iostate rdstate ( ) const;

ios

cplusplus.com

Get control state.
Returns the current stream control state flags.

Parameters.

none

Return Value.
An object of type ios_base::iostate that can contain any combination of the following state flag member constants:

badbit (critical error in stream buffer)
eofbit (End-Of-File reached while extracting)
failbit (failure extracting from stream)
goodbit (no error condition, represents the absence of the above bits)

These are static member constants inherited from ios_base.
To check for a specific state other than goodbit you should check only if the corresponding bit is set using the & bitwise operator with its specific bitmask, because more than one error state could be set at the same time.

Example.

// getting state of stream object
#include <iostream>
#include <fstream>
using namespace std;

int main () {
  ifstream is;
  is.open ("test.txt");
  if ( (is.rdstate() & ifstream::failbit ) != 0 )
    cerr << "Error opening 'test.txt'\n";
  return 0;
}

Basic template member declaration ( basic_ios<charT,traits> ):

iostate rdstate () const;

See also.
fail, good, bad, eof, clear,
ios class